path: root/doc/devel/evolution-plugin-manual.xml

<?xml version='1.0'?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
                  "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [

<!ENTITY Evolution "<application>Evolution</application>">
<!ENTITY GNOME "<application>GNOME</application>">
<!ENTITY eclipse "<application>Eclipse</application>">
<!ENTITY Camel "<application>Camel</application>">
<!ENTITY EPlugin "<application>EPlugin</application>">
<!ENTITY e-popup-reference SYSTEM "e-popup.xml">
<!ENTITY e-menu-reference SYSTEM "e-menu.xml">
<!ENTITY e-config-reference SYSTEM "e-config.xml">
<!ENTITY e-event-reference SYSTEM "e-event.xml">
<!ENTITY e-plugin-reference SYSTEM "e-plugin.xml">

<!ENTITY em-popup-reference SYSTEM "em-popup.xml">
<!ENTITY em-format-reference SYSTEM "em-format.xml">

<!ENTITY em-popups SYSTEM "em-popups.xml">
<!ENTITY em-menus SYSTEM "em-menus.xml">
<!ENTITY em-configs SYSTEM "em-configs.xml">
<!ENTITY em-events SYSTEM "em-events.xml">

<!ENTITY ecal-popups SYSTEM "ecal-popups.xml">

<!ENTITY es-events SYSTEM "es-events.xml">
<!ENTITY es-menus SYSTEM "es-menus.xml">

]>
<?xml-stylesheet href="sdocbook.css" type="text/css"?>

<book lang="en">
  <!-- DocBook file was created by LyX 1.3
  See http://www.lyx.org/ for more information -->
  <bookinfo>
    <title>
      &Evolution; Plugin Development Manual
    </title>
    
    <authorgroup>
      <corpauthor>
    Novell, Inc.
      </corpauthor>
      <author>
    <firstname>Michael</firstname><surname>Zucchi</surname>
      </author>
    </authorgroup>

    <copyright>
      <year>2004</year>
      <holder>Novell, Inc.</holder>
    </copyright>

  </bookinfo>

  <preface id="preface">
    <title>Preface</title>

    <para>
      This document is work-in-progress.  Its structure and design is still as
      fluid as the underlying strucutre and design of some parts of EPlugin.
      There's no guarantee it will be updated at regular intervals,
      particularly this version.
    </para>
    <para>
      The API documentation is currently generated using the Linux kernel-doc
      script.  The stylesheets used to generate the HTML you're seeing seems to
      have bugs which duplicates some sections.  It is also ugly and difficult
      to navigate.
    </para>

    <sect1>
      <title>Conventions</title>
      <para>
    The following conventions are used in the manual ... (insert details
    here).
      </para>
      <sect2>
    <title>XML Annotation</title>
    <para>
      XML definitions are annotated with BNF-style markers to indicate
      alternative (|), multiples (* or +), and optional (?) items.  If no
      annotation is present then the item must be present once.
    </para>
    <variablelist>
      <varlistentry>
        <!-- is symbol the right one here? -->
        <term><symbol>|</symbol></term>
        <listitem>
          <simpara>Indicates an alternative option.  Only one of the items
        separated by <symbol>|</symbol> is to be chosen.
          </simpara>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><symbol>*</symbol></term>
        <listitem>
          <simpara>Following an item, <symbol>*</symbol> indicates the item
        may occur any number of times, including no times (0 or more
        multiple).
          </simpara>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><symbol>+</symbol></term>
        <listitem>
          <simpara>Following an item, <symbol>+</symbol> indicates the item
        must occur at least once, but may occur more than ones (1 or
        more multiple).
          </simpara>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term><symbol>?</symbol></term>
        <listitem>
          <simpara>Following an item, <symbol>?</symbol> indicates the item
        may occur at most once, if present (0 or 1 times).
          </simpara>
        </listitem>
      </varlistentry>
    </variablelist>
      </sect2>
    </sect1>

  </preface>

  <part id="informational">
    <title>
      EPlugin
    </title>
    <chapter>
      <title>
    Introduction
      </title>
      <para>
    This book aims to be a comprehensive technical manual for the
    development of plugins for &Evolution;, a personal information manager
    for &GNOME;.
      </para>
      <para>
    Up-to, and including, &Evolution; version 2.0, &Evolution; contained
    limited extensibility interfaces. There were only two ways to extend
    &Evolution;; by implementing a new top-level component, or by
    implementing a &Camel; provider. When implementing a top-level component,
    there was still little integration, and in effect it was merely a more
    complex way of writing a separate &GNOME; application. &Camel; providers
    were only designed to be e-mail storage backends, so were of limited
    use for general extensibility. Despite this, both mechanisms were used
    for example for the Exchange Connector, although the system made the
    integration clumsy and difficult.
      </para>
      <para>
    This lack of extensibility has severaly stifled external developer
    contributions by forcing any extensions to be considered as core
    features. &Evolution; being a commercial product, it has tight usability
    and quality requirements that limits the ability to experiment with
    the core feature set in this way. As a result, very few lines of code
    or new features have been implemented by external contributors.
      </para>
      <para>
    One of the major goals for the 2.2 release was to implement an
    extensibility system, given the working name of EPlugin, which must
    provide a frame-work for both providing extensibility hooks, and for
    extending the functionality of &Evolution;.
      </para>
      <sect1>
    <title>
      Plugin System
    </title>
    <para>
      Any plugin system will generally have a number of goals:
    </para>
    <itemizedlist>
      <listitem>
        <simpara>
          Provide a language independent invocation mechanism
        </simpara>
      </listitem>
      <listitem>
        <simpara>
          Allow extension of parts of the user interface and processing elements
        </simpara>
      </listitem>
      <listitem>
        <simpara>
          Require minimal extra or foreign code to implement in the core application
        </simpara>
      </listitem>
      <listitem>
        <simpara>
          Require minimal interface code to implement the extensions
        </simpara>
      </listitem>
      <listitem>
        <simpara>
          Not to impact performance or increase resource usage unduly
        </simpara>
      </listitem>
      <listitem>
        <simpara>
          Versioning
        </simpara>
      </listitem>
      <listitem>
        <simpara>
          Be able to be extended itself fairly easily.
        </simpara>
      </listitem>
    </itemizedlist>
    <para>
      EPlugin manages to fulfill these goals in most cases. EPlugin isn't a
      single object or interface in itself, although there is an object
      titled EPlugin, it is a synergistic
      <footnote><simpara>I've always wanted to use
          <emphasis>synergistic</emphasis> in a sentence since
          I read it on the back of the Commodore 64 Users
          Guide.</simpara></footnote>
      collection of integrated and
      continually evolving objects which work together to achieve these
      goals (and that will definitly be the end of the MarketSpeak). It
      consists of a loader to invoke extension callbacks, hooks to resolve
      these callbacks, targets to identify context, and managers which are
      used by the core code to provide functionality and merging points for
      the extensions.
    </para>
    <para>
      EPlugin's design was inspired and influenced by the &eclipse;
      project. It aims at a lower target however, so it was able more easily
      implemented in a practical time-frame.
    </para>
    <para>
      &EPlugin; was chosen as an approach to the problem of adding
      scriptability to &Evolution;.  Instead of just linking to Perl, or
      Python, or even Mono by itself an approach was taken which focuses on
      the application end of the system.  So instead of making every part
      of the application export its functionality and have to deal with
      whatever script engine is present, EPlugin addresses the hooking part
      of the equation in a language-independent manner.  It also attemps to
      do it in a way which doesn't impact on the application development
      either.
    </para>
    <para>
      The EPlugin world is awash with its own language. The next few
      sections will introduce the basic plugin nomenclature and high-level
      view of this world.
    </para>

      </sect1>

      <sect1>
    <title>
      Loaders
    </title>
    <para>
      The core of EPlugin is a light-weight object loader and callback
      invocation system. Because of the varied calling conventions of
      different languages, and to reduce the overhead of the plugin system
      itself, all callbacks only receive and return a single argument. By
      using structures to pass complex arguments, native C plugins require
      no extra overhead, and marshalling details are moved into the plugin
      implementation itself where required. It also simplifies memory
      management issues significantly. For example, the C plugin handler
      merely loads a shared library using GModule, and resolves a symbol by
      name; and is so all of 50 lines of code, total. The loaders are the
      only modules which need to interace with non-native code or
      conventions.
    </para>
    <para>
      The other task of the plugin core is to load XML definitions of the
      plugins. Extension hooks are registered with the plugin core before
      the plugins are scanned, and are automatically instantiated to load
      each definition appropriately as they are encountered.
    </para>
    <para>
      At each layer, a level of indirection is used so that new loaders and
      new hooks can be added transparently, and extend the plugin
      definition freely with any information they require.
    </para>
      </sect1>

      <sect1>
    <title>
      Hooks
    </title>
    <para>
      The hooks
      <footnote>
        <para>A hook is something you can hang your stuff on.</para>
      </footnote>
      which are registered with the loader provide meta-data for
      the management implementation layer for extending it at
      run-time. Their primary
      functions are to load the detail of the XML plugin definition, map it
      to the implementation, and marshal the implementation callbacks to
      the common plugin interface. How they do this depends on the
      implementation itself, and ranges from registering factory methods to
      simply adding the items directly.
    </para>
    <para>
      In most cases the physical object need not be loaded until the
      callback is invoked, since the plugin definitions provide enough
      contextual information to build the interface or determine when they
      need to be invoked.
    </para>
      </sect1>

      <sect1>
    <title>
      Managers
    </title>
    <para>
      Managers
      <footnote>
        <para>
          Unlike real managers, these are the ones that do the heavy lifting.
        </para>
      </footnote>
      provide tools for the core code to extend itself at specific
      points, and in many cases are the objects used directly in the code
      to implement core features. In other cases they simply provide the
      hooks with an entry point into &Evolution;. For example, for the main
      menu hook, the manager is a thin layer to BonoboUI. On the other
      hand, EPopup is a complete implementation of a popup menu management
      system which was already used in &Evolution; 2.0. Some managers are
      one-off objects used as constructors for other objects, others are
      view-dependent, and some are static objects, such as the Event
      routers.
    </para>
      </sect1>

      <sect1>
    <title>
      Items
    </title>
    <para>
      Each manager uses a number of items to describe the object they
      control or create. The items are added to each manager instance from
      the plugins or from core code. The items from all of these sources
      are then merged together when required and processed accordingly. For
      example, menu items are merged into a tree of GtkMenus. Events on the
      other hand are simply ordered and then invoked in the order of their
      priority. Items are part of the manager implementation, and in
      &EPlugin; they are all extensible objects too, which the hooks use to
      perform mapping to the plugin. Items may be extended by code hooking
      into the implementation, either the plugin hooks, or the core code.
    </para>
      </sect1>

      <sect1>
    <title>
      Targets
    </title>
    <para>
      Targets
      <footnote><para>Think of a target as the target of
          interest.</para></footnote>
      are view or component specific context objects. They contain
      enough information to be used as stand-alone contexts to implement
      callbacks for both core functions and plugin hooks. For example for
      the mail view, a select target contains a folder and a list of
      selected messages. An attachment (part) target contains the
      &Camel; representation of the part and the mime-type for
      that part. Targets are part of the manager implementation and are
      extended by subclassing the manager.
    </para>
      </sect1>
    </chapter>

    <chapter id="plugin-loaders">
      <title>
    Plugin Loaders
      </title>
      <para>
    Plugin loaders implement a hool to a new language, or loading system in the
    plugin system.  The actual binding of new languages to the plugin system or
    other parst of &Evolution;s api's are beyond the scope of this
    document, some languages make this easier than others.
      </para>
      <sect1 id="plugin-loaders-base">
    <title>
      Base Plugin
    </title>
    <para>
      The <link linkend="API-struct--EPlugin">EPlugin base class</link>
      is an abstract class which provides the basic services for plugin
      implementations. The main services are:
      <itemizedlist>
        <listitem><simpara>Resolve plugin type and instantiate an EPlugin
        object to represent and manage it.</simpara></listitem>
        <listitem><simpara>Load the base structure of the XML plugin
        definition files.</simpara></listitem>
        <listitem><simpara>Resolve plugin hook types and instantiate a
        EPluginHook to represent and manage it.</simpara></listitem>
        <listitem><simpara>Provide a simple, language-independent api for
        invoking plugin callbacks</simpara></listitem>
        <listitem><simpara>Provide I18N context for plugins.</simpara></listitem>
        <listitem><simpara>Some simple static helper methods to simplify each
        implementing class.</simpara></listitem>
      </itemizedlist>
      See the <xref linkend="REF-EPlugin"/> for
      these details.
    </para>

    <sect2 id="plugin-loaders-definition">
      <title>Definition of a Plugin</title>
      <para>
        The base plugin XML definition.  Subclasses of EPlugin extend this
        basic structure with additional parameters or elements as they
        require.
      </para>
      <para>
        Note that there may be any number of <sgmltag>e-plugin</sgmltag>
        elements in a given plugin file, this may be used to simplify
        distribution of plugin packages.
      </para>
      <programlisting>
        <![CDATA[
<?xml version="1.0">
<e-plugin-list>
  <e-plugin
    id="unique id"
    type="loader type"
    domain="translation domain" ?
    name="plugin name"
    ...>
    <description>long description</description> ?
    <author name="real name"? email="email addr"?/> *
    <hook
      class="hook class"
      ...>
    ...
    </hook> +
  </e-plugin> +
</e-plugin-list>]]></programlisting>
      <variablelist>
        <varlistentry>
          <term><parameter>id</parameter></term>
          <listitem>
        <simpara>
          A unique string identifying this plugin.  By convention this
          will follow the java-like class namespace system.
          e.g. <constant>com.ximian.evolution.test-plugin</constant>
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>type</parameter></term>
          <listitem>
        <simpara>
          The type name of the plugin loader.  Currently <link
            linkend="plugin-loaders-lib">shlib</link> and <link
            linkend="plugin-loaders-mono">mono</link> are the only
          supported values.  If no known handler is registered for this
          type, the plugin definition is silently ignored.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>domain</parameter></term>
          <listitem>
        <simpara>
          The translation domain for this plugin, as passed to the
          <function>dcgettext</function> call of the gettext package.
          If not supplied then the default application domain is used
          (i.e. &quot;evolution&quot;).  This is used to translate
          translatable strings for display.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>name</parameter></term>
          <listitem>
        <simpara>
          A short name for the plugin.  &quot;Bob's Wonder
          Extender&quot; might be suitable.  This value will be
          translated.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>description</parameter></term>
          <listitem>
        <simpara>
          A longer description of the plugin's purpose. This value will be
          translated.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>author</parameter></term>
          <listitem>
        <simpara>
          One of the authors who wrote the plugin.  Either
          <parameter>name</parameter> or <parameter>email</parameter>
          may be ommitted.  This element may occur multiple times to
          indicate multiple authors.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>hook</parameter></term>
          <listitem>
        <simpara>
          This is a list of all of the hooks that this plugin wishes to
          hook into.  See the <link linkend="plugin-hooks">Plugin
            Hooks</link> section for the details of the basic hook
          types defined.
        </simpara>
        <simpara>
          The hook <parameter>class</parameter> is resolved using the
          registered hook types, and if none can be found, or a version
          mismatch occurs, then the hook is silently ignored.
        </simpara>
          </listitem>
        </varlistentry>
      </variablelist>
    </sect2>
      </sect1>

      <sect1 id="plugin-loaders-lib">
    <title>
      Shared Library Loader
    </title>
    <para>
      The shared library loader <link
        linkend="API-struct--EPluginLib">EPluginLib</link> implements a
      concrete EPlugin type which loads GNU shared libraries via the
      GModule api.  It simply resolves symbols directly from the loaded
      shared object and invokes them expecting a function signature of
      <type>EPluginLibFunc</type>.
    </para>
    <para>
      To manage plugin lifecycle, the function
      <function>e_plugin_lib_enable</function>
      will be invoked which allows the plugin to initialise itself.  Its
      signature should match <type>EPluginLibEnableFunc</type>, and it will
      be called with <parameter>enable=1</parameter>.  If the enable
      function returns non-zero it is assumed to have failed intialisation
      and will not be invoked further.
    </para>

    <sect2 id="plugin-loaders-lib-definition">
      <title>Definition</title>
      <para>The shared library loader only requires one extra parameter in
        the base plugin definition.
      </para>
      <programlisting>
        <![CDATA[
<e-plugin
  ...
  type="shlib"
  location="/full/path/name.so"
  load-on-startup ?
  ...
  <hook class="...">
   ...
</e-plugin>]]></programlisting>
      <variablelist>
        <varlistentry>
          <term><parameter>type</parameter></term>
          <listitem>
        <simpara>
          The type name of the shared library plugin is
          <constant>shlib</constant>.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>location</parameter></term>
          <listitem>
        <simpara>
          The location parameter contains
          the full path-name of a shared object to load.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>load-on-startup</parameter></term>
          <listitem>
        <simpara>
          If provided, indicates that the plugin should be loaded and
          enabled when the application is first started.  Normally the
          plugin is only loaded when necessary, i.e. when a callback is
          first invoked.  Only use this option if you have defined an
          <function>e_plugin_lib_enable</function> callback in your
          module, and only if absolutely necessary.
        </simpara>
          </listitem>
        </varlistentry>
      </variablelist>
    </sect2>

    <sect2 id="plugin-loaders-lib-invocation">
      <title>Invocation</title>
      <simplesect>
        <title>Function specification
        </title>
        <para>Where a function spec is required in a plugin
          hook definition, it should simply be the full name of an
          exported symbol in the shared object.
        </para>
      </simplesect>
      <simplesect>
        <title>Callback signature</title>
        <funcsynopsis><funcprototype>
        <funcdef>void * <function>function</function></funcdef>
        <paramdef>EPlugin * <parameter>ep</parameter></paramdef>
        <paramdef>void * <parameter>data</parameter></paramdef>
          </funcprototype></funcsynopsis>
        <variablelist>
          <varlistentry>
        <term><function>function</function></term>
        <listitem>
          <simpara>
            The callback function.
          </simpara>
        </listitem>
          </varlistentry>
          <varlistentry>
        <term><parameter>ep</parameter></term>
        <listitem>
          <simpara>
            The container EPlugin representing this plugin.
          </simpara>
        </listitem>
          </varlistentry>
          <varlistentry>
        <term><parameter>data</parameter></term>
        <listitem>
          <simpara>
            Hook context data.  It is part of the hook's api to specify
            the type of this pointer.
          </simpara>
        </listitem>
          </varlistentry>
          <varlistentry>
        <term><returnvalue>return value</returnvalue></term>
        <listitem>
          <simpara>
            Return data.  It is part of the hook's api to specify the
            type of this pointer.
          </simpara>
        </listitem>
          </varlistentry>
        </variablelist>
      </simplesect>
    </sect2>
      </sect1>

      <sect1 id="plugin-loaders-mono">
    <title>
      Mono Assembly Loader
    </title>
    <para>
      The mono assembly loader <link
        linkend="API-struct--EPluginMono">EPluginMono</link> implements a
      concrete EPlugin type which loads C# assemblies using Mono.  Apart
      from loading the assembly, it can optionally instantiate a class to
      implement the callback or invoke static methods directly.
    </para>

    <sect2 id="plugin-loaders-mono-definition">
      <title>Definition</title>
      <para>The mono assembly loader needs the name of the assembly and
        optionally the name of the class for handling the callbacks.
      </para>
      <programlisting>
        <![CDATA[
<e-plugin
  ...
  type="mono"
  location="/full/path/name.dll"
  handler="PluginClass" ?
  ...
  <hook class="...">
   ...
</e-plugin>]]></programlisting>
      <variablelist>
        <varlistentry>
          <term><parameter>type</parameter></term>
          <listitem>
        <simpara>
          The type name of a mono assembly plugin is
          <constant>mono</constant>.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>location</parameter></term>
          <listitem>
        <simpara>
          The location parameter contains
          the full path-name of an assembly to load.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>handler</parameter></term>
          <listitem>
        <simpara>
          If supplied, the handler contains the fully qualified name of
          the class which handles all callbacks for this plugin.  If a
          handling class is used, then the function specifications
          become relative to this class.
        </simpara>
        <simpara>
          This class will be
          instantiated once upon the first callback invocation, and
          remain active for the life of the plugin (or application).
        </simpara>
          </listitem>
        </varlistentry>
      </variablelist>
    </sect2>

    <sect2 id="plugin-loaders-mono-invocation">
      <title>Invocation</title>
      <simplesect>
        <title>Function specification
        </title>
        <para>If no <parameter>handler</parameter> class is specified, then
          the function specification must match a static method in the
          assembly.  This is passed to <function>mono_method_desc_new</function>
          and <function>mono_method_desc_search_in_image</function>,
          typically <function>FunctionName(intptr)</function>.
        </para>
        <para>
          If the handler is specified, then the function specification is
          relative to the handler class.  This is passed to
          <function>mono_method_desc_new</function> and
          <function>mono_method_desc_search_in_class</function>, typically
          <function>:MethodName(intptr)</function>.
        </para>
      </simplesect>
      <simplesect>
        <title>Callback signature</title>
        <funcsynopsis><funcprototype>
        <funcdef>IntPtr <function>function</function></funcdef>
        <paramdef>IntPtr <parameter>data</parameter></paramdef>
          </funcprototype></funcsynopsis>
        <variablelist>
          <varlistentry>
        <term><function>function</function></term>
        <listitem>
          <simpara>
            The callback method.
          </simpara>
        </listitem>
          </varlistentry>
          <varlistentry>
        <term><parameter>data</parameter></term>
        <listitem>
          <simpara>
            The hook context data.  This is a pointer to unmanaged data, and it is up-to the plugin to interpret this
            data right now, although some helper binding classes are
            planned.  FIXME: hook-up when they and doco are done.
          </simpara>
        </listitem>
          </varlistentry>
          <varlistentry>
        <term><returnvalue>return value</returnvalue></term>
        <listitem>
          <simpara>
            The callback return data.  It is up to the hook's api to
            define the type of this pointer.  It may be a simple boxed
            value type, or a memory pointer allocated in unmanaged memory (e.g. a
            GObject handle or a CamelObject cobject value).
          </simpara>
        </listitem>
          </varlistentry>
        </variablelist>
      </simplesect>
    </sect2>
      </sect1>

    </chapter>

    <chapter id="plugin-hooks">
      <title>
    Plugin Hooks
      </title>
      <para>
    This chapter will introduce the available plugin hook types. A given
    plugin can hook into any of these hooks any number of times. Some refer
    to specific instances of objects and others are implicitly defined.
      </para>
      <para>
    By design, there is considerable similarity and orthogonality amongst
    all of the various hook types and management objects.
      </para>
      <sect1 id="plugin-hooks-popup">
    <title>
      Popup Menus
    </title>
    <para>
      The popup menu hook lets you hook into any of the context menus in
      &Evolution;, by name and context. Complex, dynamic, and multi-level
      menus are created on the fly by merging the items for a given menu as
      it is being shown. Each component provides its own context targets to
      self-describe the situation under which the menu is invoked. Plugins
      and core code alike are then invoked at the user's direction. The
      popup manager and all context data lives as long as the menu and
      until a choice is made, simplifying memory management.
    </para>
    <para>
      The menu is merged from multiple plugins and core application code by
      using a simple lexiographical sort of an absolute path to the menu
      item. This merged list is then scanned and expanded into a tree of
      menus. Individual items can be hidden or inactive based on the target
      and a simple mask which is defined by the component itself. A rich
      collection of menu item types are possible, from simple, to
      checkboxes or images. The popup code is simple, and easy to use, and
      simplifies the use of popup menu's in the core application anyway,
      that they are pluggable is a free-bonus.
    </para>
    <sect2>
      <title>Defining a popup hook</title>
      <para>
        Not sure if this fits here as such.  Probably temporary placeholder.
      </para>
      <programlisting>
        <![CDATA[
<hook class="com.ximian.evolution.mail.popup:1.0">
 <menu id="menuid" target="targettype">
  <item
   type="item | toggle | radio | image | submenu | bar"
   active ?
   path="foo/bar"
   label="menu text"
   icon="icon name" ?
   visible="target mask" ?
   enable="target mask" ?
   activate="function spec"/> *
 </menu> *
</hook>]]></programlisting>
      <!-- this is all too bloody verbose, is there a better way? -->
      <para>
        <emphasis>Need to define menu tag</emphasis>
      </para>
      <variablelist>
        <varlistentry>
          <term><parameter>type</parameter></term>
          <listitem>
        <simpara>
          The menu item type.  The type maps directly to the
          corresponding EPopupItem types.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>active</parameter></term>
          <listitem>
        <simpara>
          If present, then radio or toggle menu items are active when
          first shown.  After the first instantiation, they will
          remember their active state.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>path</parameter></term>
          <listitem>
        <simpara>
          A '/' separated path used to position the item within menu
          and in the right submenu.  Each menu and plugin should
          define how its menu's are layed out so other plugins can
          determine what value to use here.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>label</parameter></term>
          <listitem>
        <simpara>
          The text to be displayed on the menu item.  This will be
          translated based on the plugin translation domain.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>icon</parameter></term>
          <listitem>
        <simpara>
          The name of a gnome-icon-theme standard icon, or the full
          path-name of an icon image to use as menu item icon.  This
          will be blank if not supplied.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>visible</parameter></term>
          <listitem>
        <simpara>
          A comma separated list of mask enumeration values used to
          define when this item is shown.  What values are valid
          depend on the menu hook class of the menu being hooked
          onto.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>enable</parameter></term>
          <listitem>
        <simpara>
          A comma separated list of mask enumeration values used to
          define when this item is enabled.  What values are valid
          depend on the menu hook class of the menu being hooked
          onto.  This is currently unimplemented.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>activate</parameter></term>
          <listitem>
        <simpara>
          A plugin-type specific function specification.  This
          function will be resolved and called when the menu item is
          activated.
        </simpara>
          </listitem>
        </varlistentry>
      </variablelist>
    </sect2>
    <sect2>
      <title>Merging Plugin Items</title>
      <para>
        A very simple algorithm is used to form the menu by merging the
        plugin's menu items with the system menu items for a given menu.
        What follows is a simple example of how this is done.  It will be
        demonstrated using a simplified menu from the message-list, as used
        in the &Evolution; Mail component, and a simple plugin which adds a
        single menu item and menu separator into the middle of the menu,
        when appropriate.
      </para>
      <para>
        When the application wishes to show a specific popup menu, it
        creates a new EPopup object with a unique menu id to manage it.  It
        adds all of the items it wishes to add to the menu (see "Builtin
        Items" in the following diagrams).  The application then asks for
        the menu to be created.  The menu building
        process adds all of the menu items from all plugins that target
        this specific menu into a flat list, discarding those which don't
        match the current Target qualifications.  The result is then
        sorted using a simple ASCII sort, and then a menu built from the
        remaining items.  This is probably best described by some diagrams.
      </para>
      <para>
        The following two diagrams show how a popup menu is automatically
        customised depending on the context.  On the left of each diagram
        are all of the menu items which apply to the example menu.  The
        menu label, with the qualifiers listed underneath, with the menu
        item path along-side.  On the right-hand side of each diagram is
        the result of:
      </para>
      <itemizedlist>
        <listitem><simpara>Selecting items based on the target
        qualifiers</simpara></listitem>
        <listitem><simpara>Sorting the remaining items based on their
        path.</simpara></listitem>
        <listitem><simpara>Building this sorted list into a
        menu.</simpara></listitem>
      </itemizedlist>

      <para>
        The actual list of target qualifiers are defined by the application
        itself.  Generally a specific menu will have only one possible
        target, and a list of matching target qualifiers.  The example
        shows how a plugin can insert a menu item anwhere it wishes in the
        menu system.  Submenus are also supported, and they work in exactly
        the same manner, with / characters used to separate submenu paths.
        A submenu must sort into the position immediately before the
        definition of its items.
      </para>

      <figure id="e-popup-merge-1">
        <title>Merging a menu with many items selected.</title>
        <mediaobject>
          <imageobject>
        <imagedata fileref="images/e-popup-merge-1.pic" format="PIC" />
          </imageobject>
          <imageobject>
        <imagedata fileref="images/e-popup-merge-1.eps" format="EPS"/>
          </imageobject>
          <imageobject>
        <imagedata fileref="images/e-popup-merge-1.png" format="PNG"/>
          </imageobject>
          <textobject>
        <phrase>Showing how a display list of menu items is selected
          and then sorted for display.</phrase>
          </textobject>
        </mediaobject>
      </figure>

      <para>
        The first diagram shows when the target qualifiers are
        <parameter>many</parameter>, and
        <parameter>mark_unread</parameter>.  The menu items which operate
        on only one selected message are not shown.  Similarly for those
        able to be marked as unread (i.e. they are currently read).
      </para>

      <figure id="e-popup-merge-2">
        <title>Merging a menu with one item selected.</title>
        <mediaobject>
          <imageobject>
        <imagedata fileref="images/e-popup-merge-2.pic" format="PIC"/>
          </imageobject>
          <imageobject>
        <imagedata fileref="images/e-popup-merge-2.eps" format="EPS"/>
          </imageobject>
          <imageobject>
        <imagedata fileref="images/e-popup-merge-2.png" format="PNG"/>
          </imageobject>
          <textobject>
        <phrase>Showing how a display list of menu items is selected
          and then sorted for display with different qualifiers.</phrase>
          </textobject>
        </mediaobject>
      </figure>

      <para>
        This diagram shows when the target qualifiers are
        <parameter>one</parameter>, and
        <parameter>mark_read</parameter>.  The menu items which operate
        on only many selected messages are not shown.  Similarly for those
        able to be marked as read.
      </para>
    </sect2>
      </sect1>

      <sect1 id="plugin-hooks-menu">
    <title>
      Main menus
    </title>
    <para>
      The main menu hook lets you hook into various main menus in
      &Evolution;, based on the current active view (component). The system
      works by piggy-backing on the existing use of the BonoboUI menu
      system used by all of the &Evolution; components. Bonobo handles the
      menu merging and user input, and the hook resolves the verb being
      invoked and redirects it to the plugin. Each view defines a single
      target which describes the appropriate context. For the Mail view,
      this is the current folder and currently selected message(s).
    </para>
    <para>
      Each view keeps track of its own manager object. When it is
      (de)activated, it also (de)activates the management object which
      dynamically adds and removes the menu items from the
      BonoboUIContainer via a supplied BonoboUI XML definition file
      &lt;perhaps it should embed the bonobouixml&gt;. If the target
      changes, the view lets the manager know, and it updates the
      visibility and sensitivity of objects appropriately, allowing
      reasonably dynamic user-interfaces to be managed automatically. The
      plugin itself isn't loaded until the menu item in question is invoked
    </para>
    <para>
      Simple menu items and toggle menu items are supported currently.
      Also, because actual menu display is driven by BonoboUI, then toolbar
      items can also be added using this mechanism.
    </para>
    <sect2>
      <title>Defining a menu hook</title>
      <para>
        Not sure if this fits here as such.  Probably temporary placeholder.
      </para>
      <programlisting>
        <![CDATA[
<hook class="com.ximian.evolution.mail.bonoboMenu:1.0">
 <menu id="menuid" target="targettype"
  <ui file="/path/to/bonobo-ui-menu-definition.xml"> +
  <item
   type="item | toggle | radio"
   active ?
   path="/commands/FooBar"
   verb="FooBar"
   visible="target mask" ?
   enable="target mask" ?
   activate="function spec"/> *
 </menu> *
</hook>]]></programlisting>
      <para>
        <emphasis>Need to define menu tag</emphasis>
      </para>
      <variablelist>
        <varlistentry>
          <term><parameter>ui</parameter></term>
          <listitem>
        <simpara>
          The <parameter>ui</parameter> element contains a filename of the
          BonoboUI XML menu definition to load when the view is activated.  Any number of
          <parameter>ui</parameter> elements may be defined, and they
          are all loaded.
        </simpara>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><parameter>type</parameter></term>
          <listitem>
        <simpara>
          The menu item type.  The type maps directly to the
          corresponding EMenuItem types.
          <parameter>radio</parameter> is currently not implemented.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>active</parameter></term>
          <listitem>
        <simpara>
          If present, then radio or toggle menu items are active when
          first shown.  After the first instantiation, they will
          remember their active state.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>path</parameter></term>
          <listitem>
        <simpara>
          The BonoboUI element path corresponding to this menu item.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>verb</parameter></term>
          <listitem>
        <simpara>
          The BonoboUI verb corresponding to the item to be listened to.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>visible</parameter></term>
          <listitem>
        <simpara>
          A comma separated list of mask enumeration values used to
          define when this item is shown.  What values are valid
          depend on the menu hook class of the menu being hooked
          onto.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>enable</parameter></term>
          <listitem>
        <simpara>
          A comma separated list of mask enumeration values used to
          define when this item is sensitive.  What values are valid
          depend on the menu hook class of the menu being hooked
          onto.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>activate</parameter></term>
          <listitem>
        <simpara>
          A plugin-type specific function specification.  This
          function will be resolved and called when the menu item is
          activated.  The funciton's parameters will depend on the type
          of menu item being invoked.
        </simpara>
          </listitem>
        </varlistentry>
      </variablelist>
    </sect2>
    <sect2>
      <title>Merging Plugin Items</title>
      <para>
        Merging is performed by BonoboUI, and the source of the menu data
        is defined by the <parameter>ui</parameter> file.
      </para>
    </sect2>
      </sect1>

      <sect1 id="plugin-hooks-config">
    <title>
      Configuration Pages and Wizards
    </title>
    <para>
      Configuration pages are somewhat more complex than any of the other
      types of hookable object. This is reflected in the complexity of the
      items and callbacks involved.
    </para>
    <para>
      Essentially, the EConfig object is used in combination to both
      instrument existing windows and building new content.  Each
      configuration window comprises of several basic elements with some
      minor variations allowed.  It consists of a number of pages in a
      specific order, each containing a number of titled sections in a specific
      order, each containing a number of items.  The variations are that
      the top-level widget may be a GtkNotebook or a GnomeDruid; and each
      section may instrument a GtkBox, or a GtkTable.  The definition of
      the available hooks will define what form they take.
    </para>
    <para>
      The EConfig manager uses the description of all the items supplied to
      it to build the complete window.  It can also drive various aspects
      of the UI, such as navigating through a druid or handling
      instant-apply vs. modify-and-save dialogues.
    </para>
    <figure id="e-config-flow">
      <title>Event and Data Flow in EMConfig</title>
      <mediaobject>
        <imageobject>
          <imagedata fileref="images/e-config-flow.pic" format="PIC"/>
        </imageobject>
        <imageobject>
          <imagedata fileref="images/e-config-flow.eps" format="EPS"/>
        </imageobject>
        <imageobject>
          <imagedata fileref="images/e-config-flow.png" format="PNG"/>
        </imageobject>
        <textobject>
          <phrase>The flow of information and control signals in the
        configuration management object.</phrase>
        </textobject>
      </mediaobject>
    </figure>

    <sect2>
      <title>Defining a configuration page hook</title>
      <para>
        Not sure if this fits here as such.  Probably temporary placeholder.
      </para>
      <programlisting>
        <![CDATA[
<hook class="com.ximian.evolution.mail.config:1.0">
 <group
  id="window id"
  target="targettype"
  check="function spec"?
  commit="function spec"?
  abort="function spec"?>
  <item
   type="book | druid | page | page_start | page_finish | section | section_table | item"
   path="/absolute/path"
   label="name" | factory="function  spec"
  /> *
 </menu> *
</hook>]]></programlisting>

      <sect3>
        <title>Group Element Properties</title>

        <variablelist>
          <varlistentry>
        <term><parameter>id</parameter></term>
        <listitem>
          <simpara>
            The name of the configuration window to which this hook
            applies.
          </simpara>
        </listitem>
          </varlistentry>
          <varlistentry>
        <term><parameter>target</parameter></term>
        <listitem>
          <simpara>
            The type of target this configuration window applies too.
            This will normally be tied directly to the specific
            configuration window itself.
          </simpara>
        </listitem>
          </varlistentry>
          <varlistentry>
        <term><parameter>check</parameter></term>
        <listitem>
          <simpara>
            A callback which will be invoked to validate the
            configuration or a specific page of the configuration.  It
            will be invoked with a
            <link
              linkend="API-struct--EConfigHookPageCheckData">EConfigHookPageCheckData</link>
            structure, and is expected to return a non-NULL value if
            the page validates.
          </simpara>
          <simpara>
            The callback will be expected to handle all
            <parameter>pageid</parameter>'s present in the
            configuration window, and should return
            <constant>TRUE</constant> for pages it does not recognise.
            If <parameter>pageid=&quot;&quot;</parameter> (an empty
            string), then the <parameter>check</parameter> function
            should validate all settings.  See also <xref
              linkend="API-e-config-add-page-check" />.
          </simpara>
        </listitem>
          </varlistentry>
          <varlistentry>
        <term><parameter>commit</parameter></term>
        <listitem>
          <simpara>
            A callback which will be invoked to commit the
            configuration data, if the configuration page isn't an
            instant-apply one.  This callback can write any
            configuration changes to permanent storage.  It is not used
            for instant-apply windows.
          </simpara>
        </listitem>
          </varlistentry>
          <varlistentry>
        <term><parameter>abort</parameter></term>
        <listitem>
          <simpara>
            A callback which will be invoked to abort the configuration
            process.  This callback is called when the
            <guibutton>Cancel</guibutton> button is pressed on stateful
            configuration windows.
          </simpara>
        </listitem>
          </varlistentry>
        </variablelist>
      </sect3>
      <sect3>
        <title>Item Element Properties</title>

        <variablelist>
          <varlistentry>
        <term><parameter>type</parameter></term>
        <listitem>
          <simpara>
            The menu item type.  The type maps directly to the
            corresponding EConfigItem types.  Only one of
            <parameter>book</parameter> and <parameter>druid</parameter>
            may be supplied for the entire configuration page, and this
            will usually already be defined by the application.
          </simpara>
        </listitem>
          </varlistentry>
          <varlistentry>
        <term><parameter>path</parameter></term>
        <listitem>
          <simpara>
            The path to the configuration item in question.  This is a
            simple string that when sorted using an ASCII sort will place
            the items in the right order.  That is, sections before items
            before pages before the root object.
          </simpara>
        </listitem>
          </varlistentry>
          <varlistentry>
        <term><parameter>label</parameter></term>
        <listitem>
          <simpara>
            The textual label of this item.  This may only be supplied
            for the section and page types.  For sections it will be the
            section frame text.  For pages this will be the druid page
            title or the notebook tab text.  If a
            <parameter>factory</parameter>is supplied then this value is
            not used.  This will be translated based on the plugin
            translation domain.
          </simpara>
        </listitem>
          </varlistentry>
          <varlistentry>
        <term><parameter>factory</parameter></term>
        <listitem>
          <simpara>
            If supplied, the factory method used to create the GtkWidget
            elements for this configuration item.  Factories may be
            supplied for any of the item types.  If no
            <parameter>label</parameter> is set then the
            <parameter>factory</parameter> must be set.
          </simpara>
        </listitem>
          </varlistentry>
        </variablelist>
      </sect3>
    </sect2>
    <sect2>
      <title>Generating Configuration Pages</title>
      <para>
        Configuration items essentially spam 3 dimensions, but are
        merged in a similar fashion to the way Popup items are merged.  The
        main difference is that there are no target qualifiers used to
        select which items are shown, it is up to the item factory to
        either create or not create the item as it sees fit.  The EConfig
        manager takes care of the rest, including removing un-used sections
        or pages.
      </para>
      <para>
        All items for a given configuration screen are converted into a
        list and sorted based on the <parameter>path</parameter>.  The
        configuration builder then goes through each item, creating
        container widgets or calling factories as required.  If a given
        page or section is empty, then it is removed automatically.  This
        process isn't only a one-off process.  For certain complex
        configuration screens, items or even pages and sections need to be
        dynamic based on a previous setting.  EConfig supports this mode of
        operation too, in which case it re-builds the configuration screen
        the same way, and automatically destroys the old widgets
        <footnote><simpara>In most cases, in some cases additional manual
        processing is required in the factory
        callback.</simpara></footnote> and even re-orders pages and
        sections where appropriate to make the user-interface consistent.
      </para>
      <para>
        The following few examples some of the flexibility of the EConfig
        system.
      </para>
      <figure id="e-config-build-1">
        <title>The application defined, unaltered configuration page.</title>
        <mediaobject>
          <!-- do we need to build our own eps for the image? -->
          <imageobject>
        <imagedata fileref="images/e-config-build-1.png" format="PNG"/>
          </imageobject>
          <textobject>
        <phrase>Shows the original HTML Mail settings page.</phrase>
          </textobject>
        </mediaobject>
      </figure>
      <para>
        First we have the original configuration window.  This is defined
        by the application, the application uses EConfig to build this
        window, and in the process EConfig instruments the sections that
        the application defines.  This allows plugins to add new
        pages/sections/items anywhere on the page - to a granularity as
        defined by the application.  For example the application may at
        minimum merely define the top-level notebook or druid object and a
        number of pages.  When the pages are created the application could
        add as much content as it wants, which would still allow plugins to
        extend the user interface, but only by adding options to the end of
        each page.  At the other end of the scale the application could
        enumerate every single item (i.e. row) in every section on every
        page, allowing plugins to put new items anywhere in the display.
      </para>
      <figure id="e-config-build-2">
        <title>A plugin adding a new section to an existing page.</title>
        <mediaobject>
          <!-- do we need to build our own eps for the image? -->
          <imageobject>
        <imagedata fileref="images/e-config-build-2.png" format="PNG"/>
          </imageobject>
          <textobject>
        <phrase>Shows the HTML Mail settings page with a new section
          and item added by a plugin.</phrase>
          </textobject>
        </mediaobject>
      </figure>
      <para>
        In this case the plugin has merely added a new section on the
        bottom of the HTML Mail settings page.  When the factory is called
        the plugin has a parent GtkTable (in this case, it could be a VBox)
        and borderless frame already defined, and it just has to
        instantiate its own control widgets, add them to the table, and
        return one of the widgets.  The returned widget is used later if
        the window needs to be reconfigured, although this particular configuration
        page is static so it isn't needed.
      </para>
      <figure id="e-config-build-3">
        <title>A plugin inserting a new page for its settings.</title>
        <mediaobject>
          <!-- do we need to build our own eps for the image? -->
          <imageobject>
        <imagedata fileref="images/e-config-build-3.png" format="PNG"/>
          </imageobject>
          <textobject>
        <phrase>Shows the plugin adding a new page for its setting as
          an alternative.</phrase>
          </textobject>
        </mediaobject>
      </figure>
      <para>
        And finally we have exactly the same plugin, which has exactly the
        same code.  But a small change to the plugin definition allows the
        plugin to add an arbitrary new page (in an arbitrary position) into
        the whole window.  If this was a druid, then new druid pages can
        also be inserted at arbitrary locations, and page navigation (in a
        strictly linear manner) is automatically controlled by EConfig as
        per <xref linkend="e-config-flow"/>.
      </para>
      <para>
        In practice, EConfig provides more than it takes the application to
        use - generally little or no extra application code is required to
        use it.  It also
        <footnote><simpara>Or it will - the code needs some
        tweaking.</simpara></footnote> enforces and simplifies HIG
        compliance.  And as a side-benefit to the application it
        transparently provides extension hooks for
        external code to provide a seamlessly integrated user experience.
      </para>
    </sect2>
      </sect1>

      <sect1 id="plugin-hooks-event">
    <title>
      Events
    </title>
    <para>
      No extensibility framework would be complete without an event
      system. Events are used to reflect changes in internal state of the
      application, and track actions by the user. They can contain any
      information and additionally can be filtered based on the information
      itself. Special targets are used, as in the other plugin hooks, to
      hold this information.
    </para>
    <para>
      Event managers are defined to contain the different event types that
      a given component can export. Only one event manager object is
      instantiated for each component, and each plugin listening to events
      from that component are registered on that event manager directly.
    </para>
    <para>
      Events handlers have priorities, and can swallow events, allowing
      some level of complexity of event routing. This feature might not
      prove useful and may be removed in the future if it isn't.
    </para>
    <sect2>
      <title>Defining an event hook</title>
      <para>
        Not sure if this fits here as such.  Probably temporary placeholder.
      </para>

      <programlisting>
        <![CDATA[
<hook class="com.ximian.evolution.mail.events:1.0">
 <event
   target="target name"
   id="event name"
   type="pass | sink" ?
   priority="signed integer" ?
   enable="target mask" ?
   handle="function spec"/> *
</hook>]]></programlisting>
      <variablelist>
        <varlistentry>
          <term><parameter>target</parameter></term>
          <listitem>
        <simpara>
          The target type of the event listener.  This will normally
          match in a 1:1 relationship to the event
          <parameter>id</parameter> itself.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>id</parameter></term>
          <listitem>
        <simpara>
          The name of the event to listen to.  By convention the names
          will be of the form
          <constant>target.event</constant>.
          e.g. <constant>folder.changed</constant>, or
          <constant>message.read</constant>, etc.  Although they are
          just simple case-sensitive strings.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>type</parameter></term>
          <listitem>
        <simpara>
          The event listener type.  The type maps directly to the corresponding
          corresponding EEventItem types.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>priority</parameter></term>
          <listitem>
        <simpara>
          A signed integer specifying the priority of this event
          listener.  0 (zero) should be used normally, although positive
          and negative integers in the range -128 to 127 may aslo be
          used.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>enable</parameter></term>
          <listitem>
        <simpara>
          A comma separated list of mask enumeration values used to
          qualify when this event listener is invoked.  What values are valid
          depend on the event hook class.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>handle</parameter></term>
          <listitem>
        <simpara>
          A plugin-type specific function specification.  This
          function will be resolved and called when an event is routed to
          this listener.
        </simpara>
          </listitem>
        </varlistentry>
      </variablelist>
    </sect2>
      </sect1>

      <sect1 id="plugin-hooks-format">
    <title>
      Mail Formatter
    </title>
    <para>
      The mail formatter plugin will invoke plugin code to format any part
      of an email based on mime-type. There are several formatters used
      internally by the mailer for different contexts, and each can be
      hooked into separately, providing extensible mail formatting for
      everything from the primary mail display, to printing, to reply
      quoting and more. If you are implementing a handler for a given
      mime-type, each formatter appropriate for the data-type should be
      hooked into, so that it displays properly in all contexts.
    </para>
    <para>
      Since the management object in this case is the same formatting
      object as used by the core mail display engine, a plugin may override
      or reimplement complete new functionality seamlessly.
    </para>
    <para>
      This plugin hook isn't strictly part of the core functionality as it
      is provided only by the mail component. It however demonstrates that the
      plugin system is extensible itself.
    </para>
    <sect2>
      <title>Defining a formatter hook</title>
      <para>
        Not sure if this fits here as such.  Probably temporary placeholder.
      </para>

      <programlisting>
        <![CDATA[
<hook class="com.novell.evolution.mail.format:1.0">
 <group id="formatter type">
  <item
    flags="handler flags"
    mime_type="major/minor"
    format="function spec"/> +
 </group> +
</hook>]]></programlisting>
      <variablelist>
        <varlistentry>
          <term><parameter>id</parameter></term>
          <listitem>
        <simpara>
          The actual formatter this applies to.  e.g. EMFormat for the
          base formatter class, EMFormatHTML for HTML output to a
          GtkHTML object, etc.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>flags</parameter></term>
          <listitem>
        <simpara>
          Flags to define whether this is an attachment or inline
          content.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>id</parameter></term>
          <listitem>
        <simpara>
          The name of the event to listen to.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>mime_type</parameter></term>
          <listitem>
        <simpara>
          The type of object this handler formats.
        </simpara>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><parameter>format</parameter></term>
          <listitem>
        <simpara>
          A plugin-type specific function specification.  This
          function will be invoked to format objects of the specified
          <parameter>mime_type</parameter>.
        </simpara>
          </listitem>
        </varlistentry>
      </variablelist>
    </sect2>

    <sect2>
      <title>The formatting process</title>
      <para>
        The formatting process is driven by the <link
          linkend="API-struct--EMFormat">EMFormat</link> object, although
        there are different subclasses of this object used for different
        purposes.  These behave quite differently so each must be explained
        separately.  There is the basic formatter type which converts a
        CamelMimeMessage into a stream of data, and there is a HTML
        <link linkend="API-struct--EMFormatHTML">formatter type</link>
        which uses a GtkHTML object to parse the content
        and may request further information required to complete the
        formatting.
      </para>
      <para>
        A basic formatter goes through the following steps:
      </para>
      <orderedlist>
        <listitem>
          <simpara>Outputs pre-amble information.  e.g. Flag-For-Followup
        status.</simpara>
        </listitem>
        <listitem>
          <simpara>Invokes <methodname>format_message</methodname> to begin the message
        formatting. <methodname>format_message</methodname> displays the message header, then
        looks up the content object.</simpara>
        </listitem>
        <listitem>
          <simpara>Using the mime-type of the content object (whether
        supplied or calculated), a handler is looked up from a
        per-class table to process the type.</simpara>
        </listitem>
        <listitem>
          <simpara>If no handler exists, then the data is formatted as an
        attachment.</simpara>
        </listitem>
        <listitem>
          <simpara>If a handler exists, then it is invoked to display that
        type.  Depending on whether the data is to be displayed
        'inline' or not, the data may also get an attachment expander
        and button.</simpara>
        </listitem>
        <listitem>
          <simpara>The handler transforms the part's data, if need be, and
        writes the appropriate format output to a stream.</simpara>
        </listitem>
      </orderedlist>
      <para>
        For conglomerate types, the formatting process is continued
        recursively, until all parts have been displayed, as appropriate.
      </para>
      <para>
        A HTML formatter goes through the same basic steps, but has
        additional features and requirements.  It uses multiple
        threads.  At least one other thread is used for all of the
        Camel message content operations since some of them may block on
        remote I/O.  This also simplifies cancellation processing.  Also,
        because it has access to a full HTML rendering object, references
        to embedded content (images, buttons, etc.) are also processed.
      </para>
      <para>
        Most format handlers don't need to know about all the fiddly
        details however.  If they are just outputting HTML content with no
        out of band references, they work identical to the basic format
        handlers with the exception they cannot call any Gtk GUI code
        because of threading issues.  This can still be done by using an
        IFRAME.  If they want to embed an icon or other image,
        they simply need to insert the HTML IMG tag reference in their
        format handler, and setup a callback to handle it when GtkHTML
        requests it.  EMFormat has some helper classes to make this
        only a few lines of code, including generation of the IMG SRC URL.
        IFRAMEs work identically to IMG tags, and  similar process is
        involved with embedding custom widgets using the OBJECT tag.
        EMFormatHTML takes care of calling the right callbacks for the
        right embedded reference from the right thread.
      </para>
      <para>
        Since format handlers are chained off a given type, then a plugin
        can also inherit formatting behaviour as well as override it.  This
        gives much greater flexibility since the plugin need only implement
        its behaviour in specific situations.  e.g. an OpenPGP message
        handler could fall-back to the normal text-formatter if it doesn't
        detect the ASCII armour in a text/plain part.  Or another handler
        may disable itself based on configuration or state.
      </para>
      <para>
        All format handlers for all types must also be fully re-entrant
        code (more or less write-once global and static variables) if
        they call any other formatting functions.
      </para>
    </sect2>
      </sect1>
    </chapter>
  </part>

  <part>
    <title>
      &Evolution; Hook Points.
    </title>
    <partintro>
      <para>
    This section enumerates all of the published hook points and target
    types available in each component in &Evolution;.
      </para>

      <simplesect>
    <title>Table Format</title>
    <informaltable>
      <tgroup cols="2">
        <colspec colnum="1" colname="field" colwidth="1*"/>
        <colspec colnum="2" colname="value" colwidth="4*"/>
        <tbody>
          <row>
        <entry>Id</entry>
        <entry>The hook point id.</entry>
          </row>
          <row>
        <entry>Target</entry>
        <entry>The target which this hook uses for its context data.
          Targets are described in a following section.
        </entry>
          </row>
          <row>
        <entry>Items</entry>
        <entry>If appropriate and defined, specifies identifying path
          names of items which make up the hook.  e.g. popup menu
          items, and configuration pages.  These item specifications
          allow the plugin writer to position their items
          appropriately.
        </entry>
          </row>
        </tbody>
      </tgroup>
    </informaltable>
      </simplesect>

    </partintro>
    <chapter id="mail-hooks">
      <title>
    Mail Hooks
      </title>
      <para>
    <emphasis>
      Need to find out the right docbook to mark-up most of this
      text.
    </emphasis>
      </para>

      <sect1 id="mail-hooks-popup">
    <title>Popup menus</title>

    <para>
      The mail popup menu class is
      <interfacename>org.gnome.evolution.mail.popup:1.0</interfacename>.
    </para>
    <para>
      The plugin callback data will be the target matching the plugin
      menu itself, and the callback returns no value.
    </para>

    &em-popups;

    <sect2>
      <title>Internal popup menus</title>
      <para>
        The following popup menus are defined, but they are used with no
        target, and so provide no useful context if they were to be hooked
        onto.
      </para>
      <para>
        <interfacename>com.ximian.mail.messagelist.popup.drop</interfacename>
        is used for the ASK drop type on the message list.
      </para>
      <para>
        <interfacename>com.ximian.mail.storageset.popup.drop</interfacename>
        is used for the ASK drop type on the folder tree.
      </para>
    </sect2>

    <sect2>
      <title>Mail Popup Targets</title>

      <para>
        <emphasis>
          Not sure if this needs to explain the qualifier meanings, or
          leave it to the in-line comment stuff in the enumeration
          definition.  Maybe it just needs a direct link to the
          enumeration.
        </emphasis>
      </para>

      <sect3 id="mail-hooks-popup-EMPopupTargetFolder">
        <title>Folder Target</title>

        <para>
          This target is used to define actions on a folder context.
          Normally associated with the folder tree.
        </para>

        <informaltable>
          <tgroup cols="2">
        <colspec colnum="1" colname="field" colwidth="1*"/>
        <colspec colnum="2" colname="value" colwidth="4*"/>
        <tbody valign="top">
          <row>
            <entry>Name</entry>
            <entry><constant>folder</constant></entry>
          </row>
          <row>
            <entry>Structure</entry>
            <entry>
              <link
            linkend="API-struct--EMPopupTargetFolder">EMPopupTargetFolder</link>
            </entry>
          </row>
          <row>
            <entry>Qualifiers</entry>
            <entry>
              <simplelist>
            <member><constant>folder</constant> = <constant>EM_POPUP_FOLDER_FOLDER</constant></member>
            <member><constant>store</constant> = <constant>EM_POPUP_FOLDER_STORE</constant></member>
            <member><constant>inferiors</constant> = <constant>EM_POPUP_FOLDER_INFERIORS</constant></member>
            <member><constant>delete</constant> = <constant>EM_POPUP_FOLDER_DELETE</constant></member>
            <member><constant>select</constant> = <constant>EM_POPUP_FOLDER_SELECT</constant></member>
              </simplelist>
            </entry>
          </row>
        </tbody>
          </tgroup>
        </informaltable>
      </sect3>

      <sect3 id="mail-hooks-popup-EMPopupTargetSelect">
        <title>Selection Target</title>

        <para>This target is used to define context for actions associated
          with a selection of mail messages from a specific folder.</para>

        <informaltable>
          <tgroup cols="2">
        <colspec colnum="1" colname="field" colwidth="1*"/>
        <colspec colnum="2" colname="value" colwidth="4*"/>
        <tbody valign="top">
          <row>
            <entry>Name</entry>
            <entry><constant>select</constant></entry>
          </row>
          <row>
            <entry>Structure</entry>
            <entry>
              <link
            linkend="API-struct--EMPopupTargetSelect">EMPopupTargetSelect</link>
            </entry>
          </row>
          <row>
            <entry>Qualifiers</entry>
            <entry>
              <simplelist>
            <member><constant>one</constant> = <constant>EM_POPUP_SELECT_ONE</constant></member>
            <member><constant>many</constant> = <constant>EM_POPUP_SELECT_MANY</constant></member>
            <member><constant>mark_read</constant> = <constant>EM_POPUP_SELECT_MARK_READ</constant></member>
            <member><constant>mark_unread</constant> = <constant>EM_POPUP_SELECT_MARK_UNREAD</constant></member>
            <member><constant>delete</constant> = <constant>EM_POPUP_SELECT_DELETE</constant></member>
            <member><constant>undelete</constant> = <constant>EM_POPUP_SELECT_UNDELETE</constant></member>
            <member><constant>mailing_list</constant> = <constant>EM_POPUP_SELECT_MAILING_LIST</constant></member>
            <member><constant>resend</constant> = <constant>EM_POPUP_SELECT_EDIT</constant></member>
            <member><constant>mark_important</constant> = <constant>EM_POPUP_SELECT_MARK_IMPORTANT</constant></member>
            <member><constant>mark_unimportant</constant> = <constant>EM_POPUP_SELECT_MARK_UNIMPORTANT</constant></member>
            <member><constant>flag_followup</constant> = <constant>EM_POPUP_SELECT_FLAG_FOLLOWUP</constant></member>
            <member><constant>flag_completed</constant> = <constant>EM_POPUP_SELECT_FLAG_COMPLETED</constant></member>
            <member><constant>flag_clear</constant> = <constant>EM_POPUP_SELECT_FLAG_CLEAR</constant></member>
            <member><constant>add_sender</constant> = <constant>EM_POPUP_SELECT_ADD_SENDER</constant></member>
            <member><constant>mark_junk</constant> = <constant>EM_POPUP_SELECT_MARK_JUNK</constant></member>
            <member><constant>mark_nojunk</constant> = <constant>EM_POPUP_SELECT_MARK_NOJUNK</constant></member>
            <member><constant>folder</constant> = <constant>EM_POPUP_SELECT_FOLDER</constant></member>
              </simplelist>
            </entry>
          </row>
        </tbody>
          </tgroup>
        </informaltable>
      </sect3>

      <sect3 id="mail-hooks-popup-EMPopupTargetURI">
        <title>URI Target</title>

        <para>This target defines context for operations on a URI, normally
          displayed inline somewhere in the message view.</para>

        <informaltable>
          <tgroup cols="2">
        <colspec colnum="1" colname="field" colwidth="1*"/>
        <colspec colnum="2" colname="value" colwidth="4*"/>
        <tbody valign="top">
          <row>
            <entry>Name</entry>
            <entry><constant>uri</constant></entry>
          </row>
          <row>
            <entry>Structure</entry>
            <entry><link
            linkend="API-struct--EMPopupTargetURI">EMPopupTargetURI</link></entry>
          </row>
          <row>
            <entry>Qualifiers</entry>
            <entry>
              <simplelist>
            <member><constant>http</constant> = <constant>EM_POPUP_URI_HTTP</constant></member>
            <member><constant>mailto</constant> = <constant>EM_POPUP_URI_MAILTO</constant></member>
            <member><constant>notmailto</constant> = <constant>EM_POPUP_URI_NOT_MAILTO</constant></member>
              </simplelist>
            </entry>
          </row>
        </tbody>
          </tgroup>
        </informaltable>
      </sect3>

      <sect3 id="mail-hooks-popup-EMPopupTargetPart">
        <title>Message Part Target</title>

        <para>This target defines context for operations on messages, or
          individual message parts.  The same target is used for inline
          images or other content which can be encapsulated in a MIME part
          (i.e. anything).</para>

        <informaltable>
          <tgroup cols="2">
        <colspec colnum="1" colname="field" colwidth="1*"/>
        <colspec colnum="2" colname="value" colwidth="4*"/>
        <tbody valign="top">
          <row>
            <entry>Name</entry>
            <entry><constant>part</constant></entry>
          </row>
          <row>
            <entry>Structure</entry>
            <entry><link
            linkend="API-struct--EMPopupTargetPart">EMPopupTargetPart</link></entry>
          </row>
          <row>
            <entry>Qualifiers</entry>
            <entry>
              <simplelist>
            <member><constant>message</constant> = <constant>EM_POPUP_PART_MESSAGE</constant></member>
            <member><constant>image</constant> = <constant>EM_POPUP_PART_IMAGE</constant></member>
              </simplelist>
            </entry>
          </row>
        </tbody>
          </tgroup>
        </informaltable>
      </sect3>

      <sect3 id="mail-hooks-popup-EMPopupTargetAttachments">
        <title>Attachments Target</title>

        <para>This target is used to define context for operations on the
          mail composer attachment bar.</para>

        <informaltable>
          <tgroup cols="2">
        <colspec colnum="1" colname="field" colwidth="1*"/>
        <colspec colnum="2" colname="value" colwidth="4*"/>
        <tbody valign="top">
          <row>
            <entry>Name</entry>
            <entry><constant>attachments</constant></entry>
          </row>
          <row>
            <entry>Structure</entry>
            <entry><link
            linkend="API-struct--EMPopupTargetAttachments">EMPopupTargetAttachments</link></entry>
          </row>
          <row>
            <entry>Qualifiers</entry>
            <entry>
              <simplelist>
            <member><constant>one</constant> = <constant>EM_POPUP_ATTACHMENTS_ONE</constant></member>
            <member><constant>many</constant> = <constant>EM_POPUP_ATTACHMENTS_MANY</constant></member>
              </simplelist>
            </entry>
          </row>
        </tbody>
          </tgroup>
        </informaltable>
      </sect3>

    </sect2>
      </sect1>

      <sect1 id="mail-hooks-menu">
    <title>Main menus</title>

    <para>
      The mail popup menu class is
      <interfacename>org.gnome.evolution.mail.bonobomenu:1.0</interfacename>.
    </para>
    <para>
      The plugin callback data will be the target matching the plugin
      menu itself, and the callback returns no value.
    </para>

    &em-menus;

    <sect2>
      <title>Mail Menu Targets</title>

      <sect3 id="mail-hooks-menu-EMMenuTargetSelect">
        <title>Message Selection Target</title>

        <para>This target is used to define context for operations on a
          selection of messages in the view's message list.</para>

        <informaltable>
          <tgroup cols="2">
        <colspec colnum="1" colname="field" colwidth="1*"/>
        <colspec colnum="2" colname="value" colwidth="4*"/>
        <tbody valign="top">
          <row>
            <entry>Name</entry>
            <entry><constant>select</constant></entry>
          </row>
          <row>
            <entry>Structure</entry>
            <entry>
              <link
            linkend="API-struct--EMMenuTargeSelect">EMMenuTargetSelect</link>
            </entry>
          </row>
          <row>
            <entry>Qualifiers</entry>
            <entry>
              <simplelist>
            <member><constant>one</constant> = <constant>EM_MENU_SELECT_ONE</constant></member>
            <member><constant>many</constant> = <constant>EM_MENU_SELECT_MANY</constant></member>
            <member><constant>mark_read</constant> = <constant>EM_MENU_SELECT_MARK_READ</constant></member>
            <member><constant>mark_unread</constant> = <constant>EM_MENU_SELECT_MARK_UNREAD</constant></member>
            <member><constant>delete</constant> = <constant>EM_MENU_SELECT_DELETE</constant></member>
            <member><constant>undelete</constant> = <constant>EM_MENU_SELECT_UNDELETE</constant></member>
            <member><constant>mailing_list</constant> = <constant>EM_MENU_SELECT_MAILING_LIST</constant></member>
            <member><constant>resend</constant> = <constant>EM_MENU_SELECT_EDIT</constant></member>
            <member><constant>mark_important</constant> = <constant>EM_MENU_SELECT_MARK_IMPORTANT</constant></member>
            <member><constant>mark_unimportant</constant> = <constant>EM_MENU_SELECT_MARK_UNIMPORTANT</constant></member>
            <member><constant>flag_followup</constant> = <constant>EM_MENU_SELECT_FLAG_FOLLOWUP</constant></member>
            <member><constant>flag_completed</constant> = <constant>EM_MENU_SELECT_FLAG_COMPLETED</constant></member>
            <member><constant>flag_clear</constant> = <constant>EM_MENU_SELECT_FLAG_CLEAR</constant></member>
            <member><constant>add_sender</constant> = <constant>EM_MENU_SELECT_ADD_SENDER</constant></member>
            <member><constant>mark_junk</constant> = <constant>EM_MENU_SELECT_MARK_JUNK</constant></member>
            <member><constant>mark_nojunk</constant> = <constant>EM_MENU_SELECT_MARK_NOJUNK</constant></member>
            <member><constant>folder</constant> = <constant>EM_MENU_SELECT_FOLDER</constant></member>
              </simplelist>
            </entry>
          </row>
        </tbody>
          </tgroup>
        </informaltable>
      </sect3>
    </sect2>
      </sect1>

      <sect1 id="mail-hooks-config">
    <title>Config Windows and Druids</title>

    <para>
      The mail config class is
      <interfacename>org.gnome.evolution.mail.config:1.0</interfacename>.
    </para>

    &em-configs;

    <sect2>
      <title>Mail Config Targets</title>

      <sect3 id="mail-hooks-config-EMConfigTargetAccount">
        <title>Account Target</title>

        <para>The account target is used for configuring accounts, and so
          has a pointer to the EAccount being configured.  This is a copy
          of the actual account object, and is copied to the original once
          the data is ready to commit.</para>

        <informaltable>
          <tgroup cols="2">
        <colspec colnum="1" colname="field" colwidth="1*"/>
        <colspec colnum="2" colname="value" colwidth="4*"/>
        <tbody valign="top">
          <row>
            <entry>Name</entry>
            <entry><constant>account</constant></entry>
          </row>
          <row>
            <entry>Structure</entry>
            <entry>
              <link
            linkend="API-struct--EMConfigTargetAccount">EMConfigTargetAccount</link>
            </entry>
          </row>
          <row>
            <entry>Items</entry>
            <entry>Define some of the items available and where they fit
              in the gui</entry>
          </row>
        </tbody>
          </tgroup>
        </informaltable>
      </sect3>

      <sect3 id="mail-hooks-config-EMConfigTargetPrefs">
        <title>Preferences Target</title>

        <para>The preferences target is used for global preferences.  As
          such it just contains a pointer to the global configuration store
          - a GConfClient.</para>
        
        <informaltable>
          <tgroup cols="2">
        <colspec colnum="1" colname="field" colwidth="1*"/>
        <colspec colnum="2" colname="value" colwidth="4*"/>
        <tbody valign="top">
          <row>
            <entry>Name</entry>
            <entry><constant>prefs</constant></entry>
          </row>
          <row>
            <entry>Structure</entry>
            <entry>
              <link
            linkend="API-struct--EMConfigTargetPrefs">EMConfigTargetPrefs</link>
            </entry>
          </row>
        </tbody>
          </tgroup>
        </informaltable>
      </sect3>
      
      <sect3 id="mail-hooks-config-EMConfigTargetFolder">
        <title>Folder Target</title>

        <informaltable>
          <tgroup cols="2">
        <colspec colnum="1" colname="field" colwidth="1*"/>
        <colspec colnum="2" colname="value" colwidth="4*"/>
        <tbody valign="top">
          <row>
            <entry>Name</entry>
            <entry><constant>folder</constant></entry>
          </row>
          <row>
            <entry>Structure</entry>
            <entry>
              <link
            linkend="API-struct--EMConfigTargetFolder">EMConfigTargetFolder</link>
            </entry>
          </row>
        </tbody>
          </tgroup>
        </informaltable>
      </sect3>
      
    </sect2>
      </sect1>

      <sect1 id="mail-hooks-event">
    <title>Events</title>

    <para>
      The mail event class is
      <interfacename>org.gnome.evolution.mail.events:1.0</interfacename>.
    </para>

    &em-events;

    <sect2>
      <title>Mail Event Targets</title>

      <sect3 id="mail-hooks-event-EMEventTargetFolder">
        <title>Folder Target</title>
        
        <informaltable>
          <tgroup cols="2">
        <colspec colnum="1" colname="field" colwidth="1*"/>
        <colspec colnum="2" colname="value" colwidth="4*"/>
        <tbody valign="top">
          <row>
            <entry>Name</entry>
            <entry><constant>folder</constant></entry>
          </row>
          <row>
            <entry>Structure</entry>
            <entry>
              <link
            linkend="API-struct--EMEventTargetFolder">EMEventTargetFolder</link>
            </entry>
          </row>
          <row>
            <entry>Qualifiers</entry>
            <entry>List qualifiers</entry>
          </row>
        </tbody>
          </tgroup>
        </informaltable>
      </sect3>
    </sect2>
      </sect1>

      <sect1 id="mail-hooks-format">
    <title>Formatters</title>

    <para>
      The mail formatter hook class is
      <interfacename>com.novell.evolution.mail.format:1.0</interfacename>.
    </para>

    <sect2>
      <title>Base Formatter</title>

      <para>
        The EMFormat class is the base class for all formatting types.
        It should only be used to define compound and complex types which
        do not rely on outputting any textual information, or rely on any
        screen or print output differences.
      </para>

      <informaltable>
        <tgroup cols="2">
          <colspec colnum="1" colname="field" colwidth="1*"/>
          <colspec colnum="2" colname="value" colwidth="4*"/>
          <tbody valign="top">
        <row>
          <entry>Name</entry>
          <entry><constant>EMFormat</constant></entry>
        </row>
        <row>
          <entry>Target</entry>
          <entry>
            <link
              linkend="mail-hooks-format-EMFormatHookTarget">EMFormatHookTarget</link>
          </entry>
        </row>
          </tbody>
        </tgroup>
      </informaltable>
    </sect2>

    <sect2>
      <title>HTML Formatter</title>

      <para>
        The EMFormatHTML class is the base class for most formatting types
        which generate HTML output.  It renders output to a GtkHTML
        object.  It uses a fairly complex multi-thread approach to the
        formatting to ensure the user-interface is not blocked for
        processing.  GtkHTML is used in a limited way by this class for
        HTML parsing and resolution of embedded objects.  Embedded objects
        and Widgets may not be used from formatters which hook onto this
        entry point.
      </para>

      <informaltable>
        <tgroup cols="2">
          <colspec colnum="1" colname="field" colwidth="1*"/>
          <colspec colnum="2" colname="value" colwidth="4*"/>
          <tbody valign="top">
        <row>
          <entry>Name</entry>
          <entry><constant>EMFormatHTML</constant></entry>
        </row>
        <row>
          <entry>Target</entry>
          <entry>
            <link
              linkend="mail-hooks-format-EMFormatHookTarget">EMFormatHookTarget</link>
          </entry>
        </row>
          </tbody>
        </tgroup>
      </informaltable>

      <para>
        <emphasis>This section needs a huge amount of
          explanation, and/or more detail needs to be added to another
          section about the formatter class</emphasis>
      </para>
    </sect2>

    <sect2>
      <title>HTML Display Formatter</title>

      <para>
        The EMFormatHTMLDisplay class is a subclass of EMFormatHTML, and is
        used as a mail display widget.  As such, it has access to all of
        the facilities of GtkHTML, such as embedded widgets.  Like the
        EMFormatHTML class, this uses a complex multi-thread architecture.
      </para>

      <informaltable>
        <tgroup cols="2">
          <colspec colnum="1" colname="field" colwidth="1*"/>
          <colspec colnum="2" colname="value" colwidth="4*"/>
          <tbody valign="top">
        <row>
          <entry>Name</entry>
          <entry><constant>EMFormatHTMLDisplay</constant></entry>
        </row>
        <row>
          <entry>Target</entry>
          <entry>
            <link
              linkend="mail-hooks-format-EMFormatHookTarget">EMFormatHookTarget</link>
          </entry>
        </row>
          </tbody>
        </tgroup>
      </informaltable>

      <para>
        <emphasis>This section needs a huge amount of
          explanation, and/or more detail needs to be added to another
          section about the formatter class</emphasis>
      </para>
    </sect2>

    <sect2>
      <title>HTML Print Formatter</title>

      <para>
        The EMFormatHTMLPrint class is a subclass of EMFormatHTML, and is
        used as a mail printing widget.  It cannot access embedded
        widgets.  For most purposes you would normally only connect to the
        EMFormatHTML hook, and generate generic HTML output which could be
        printed or shown on-screen if it isn't overriden by the display
        formatter.
      </para>

      <informaltable>
        <tgroup cols="2">
          <colspec colnum="1" colname="field" colwidth="1*"/>
          <colspec colnum="2" colname="value" colwidth="4*"/>
          <tbody valign="top">
        <row>
          <entry>Name</entry>
          <entry><constant>EMFormatHTMLPrint</constant></entry>
        </row>
        <row>
          <entry>Target</entry>
          <entry>
            <link
              linkend="mail-hooks-format-EMFormatHookTarget">EMFormatHookTarget</link>
          </entry>
        </row>
          </tbody>
        </tgroup>
      </informaltable>

      <para>
        <emphasis>This section needs a huge amount of
          explanation, and/or more detail needs to be added to another
          section about the formatter class</emphasis>
      </para>
    </sect2>

    <sect2>
      <title>Mail Quote Formatter</title>
      <para>
        The EMFormatQuote class is a subclass of EMFormat, and is
        used as generator for quoted mail content and for
        inline-forwarding.  This formatter converts message objects into
        a pure HTML stream, which is not parsed directly, but normally fed
        to the message composer.
      </para>
      <informaltable>
        <tgroup cols="2">
          <colspec colnum="1" colname="field" colwidth="1*"/>
          <colspec colnum="2" colname="value" colwidth="4*"/>
          <tbody valign="top">
        <row>
          <entry>Name</entry>
          <entry><constant>EMFormatQuote</constant></entry>
        </row>
        <row>
          <entry>Target</entry>
          <entry>
            <link
              linkend="mail-hooks-EMFormatHookTarget">EMFormatHookTarget</link>
          </entry>
        </row>
          </tbody>
        </tgroup>
      </informaltable>
    </sect2>

    <sect2 id="mail-hooks-format-EMFormatHookTarget">
      <title>Mail Formatter Targets</title>

      <para>There is only one target for all mail formatters, and it is
        implied automatically for all formatter hooks.</para>

      <informaltable>
        <tgroup cols="2">
          <colspec colnum="1" colname="field" colwidth="1*"/>
          <colspec colnum="2" colname="value" colwidth="4*"/>
          <tbody valign="top">
        <row>
          <entry>Structure</entry>
          <entry>
            <link
              linkend="API-struct--EMFormatHookTarget">EMFormatHookTarget</link>
          </entry>
        </row>
        <row>
          <entry>Flags</entry>
          <entry>
            <simplelist>
              <member><constant>inline</constant> = <constant>EM_FORMAT_HANDLER_INLINE</constant></member>
              <member><constant>inline_disposition</constant> =
            <constant>EM_FORMAT_HANDLER_INLINE_DISPOSITION</constant></member>
              </simplelist>
          </entry>
        </row>
          </tbody>
        </tgroup>
      </informaltable>

    </sect2>
      </sect1>
    </chapter>

    <chapter id="contacts-hooks">
      <title>
    Contacts Hooks
      </title>
      <para>
    Hooks available in the the contacts component.
      </para>

      <sect1 id="contacts-hooks-popup">
    <title>Popup menus</title>

    <para>
      The contacts popup menu class is
      <interfacename>org.gnome.evolution.addressbook.popup:1.0</interfacename>.
    </para>

    <!-- TODO: define the entity once it exists 
    &eab-popups; -->

    <sect2>
      <title>Calendar Popup Targets</title>
      <para>TBD</para>
    </sect2>
      </sect1>
      
      <sect1 id="contacts-hooks-menu">
    <title>Main menus</title>

    <para>
      The addressbook menu class is
      <interfacename>org.gnome.evolution.addressbook.bonobomenu:1.0</interfacename>.
    </para>

    <!-- TODO: define the entity once it exists 
    &eab-menus; -->

    <sect2>
      <title>Contacts Menu Targets</title>
      <para>TBD</para>
    </sect2>
      </sect1>

      <sect1 id="contacts-hooks-config">
    <title>Config Windows and Druids</title>
    <para>
      The addressbook config class is
      <interfacename>org.gnome.evolution.addressbook.config:1.0</interfacename>.
    </para>
    
    <!-- TODO: define the entity once it exists
    &eab-configs; -->
    
    <sect2>
      <title>Contacts Config Targets</title>
      <para>TBD</para>
    </sect2>
      </sect1>

      <sect1 id="contacts-hooks-event">
    <title>Events</title>

    <para>
      None defined.
    </para>
      </sect1>
    </chapter>

    <chapter id="calendar-hooks">
      <title>
    Calendar Hooks
      </title>
      <para>
    Hooks available in the the calendar component.
      </para>

      <sect1 id="calendar-hooks-popup">
    <title>Popup menus</title>

    <para>
      The calendar popup menu class is
      <interfacename>org.gnome.evolution.calendar.popup:1.0</interfacename>.
    </para>

    &ecal-popups;

    <sect2>
      <title>Calendar Popup Targets</title>
      <para>TBD</para>
    </sect2>
      </sect1>
      
      <sect1 id="calendar-hooks-menu">
    <title>Main menus</title>

    <para>
      The calendar menu class is
      <interfacename>org.gnome.evolution.calendar.bonobomenu:1.0</interfacename>.
    </para>
    
    <!-- TODO: define the entity once it exists
    &ecal-menus; -->

    <sect2>
      <title>Calendar Menu Targets</title>
      <para>TBD</para>
    </sect2>
      </sect1>

      <sect1 id="calendar-hooks-config">
    <title>Config Windows and Druids</title>
    <para>
      The calendar config class is
      <interfacename>org.gnome.evolution.calendar.config:1.0</interfacename>.
    </para>

    <!-- TODO: define the entity once it exists
    &ecal-configs; -->
    
    <sect2>
      <title>Calendar Config Targets</title>
      <para>TBD</para>
    </sect2>
      </sect1>

      <sect1 id="calendar-hooks-event">
    <title>Events</title>

    <para>
      None defined.
    </para>
      </sect1>
    </chapter>

    <chapter id="shell-hooks">
      <title>
    Shell Hooks
      </title>

      <sect1 id="shell-hooks-menu">
    <title>Main menus</title>

    <para>
      The mail menu class is
      <interfacename>org.gnome.evolution.shell.bonobomenu:1.0</interfacename>.
    </para>
    <para>
      The plugin callback data will be the target matching the plugin
      menu itself, and the callback returns no value.
    </para>
    &es-menus;
      </sect1>
      <sect1 id="shell-hooks-event">
    <title>Events</title>

    <para>
      The shell event class is
      <interfacename>org.gnome.evolution.shell.events:1.0</interfacename>.
    </para>

    &es-events;
      </sect1>

    </chapter>

  </part>

  <part id="reference">
    <title>
      Reference
    </title>
    <partintro>
      <para>
    This section of the book is a detailed API reference of the
    objects and methods that implement the core plugin system and hooks.
      </para>
      <para>
    It contains the detailed information required for all uses of the
    plugin system. That is, implementors
    of new hook types, application developers providing hook points, and
    plugin developers.
      </para>
    </partintro>
    <chapter id="REF-EPlugin">
      <title>
    EPlugin
      </title>
      <para>
    The EPlugin object manages the loading and invocation of physical
    plugin definitions and plugin binaries.  The base EPlugin class is an
    abstract class which loads plugin definitons, resolving hooks, and
    provides an api for invoking callbacks.
      </para>
      <para>
    The EPluginLib object is a concrete derived class of EPlugin which
    handles loading shared libraries using the GModule interface.
      </para>
      &e-plugin-reference;
    </chapter>

    <chapter>
      <title>
    EPopup
      </title>
      <para>
    The EPopup object manages a single popup menu.  It is used to
    application code as a convenience function for building dynamic popup
    menus based on a specific context.
      </para>
      <para>
    The EPopupHook object is loaded by
    the &EPlugin; system, and is used to provide dynamic extension to the
    application context menus.
      </para>
      &e-popup-reference;

      <!-- this looks like bum here, not sure where else to put it though -->
      &em-popup-reference;
    </chapter>

    <chapter>
      <title>
    EMenu
      </title>
      <para>
    The EMenu object manages the menus for a given view or component.  It
    is used by application code to allow the plugin system an entry point
    to current application view.  It may also be used by the application as
    a convenience function to dynamically alter the menu system based on
    user context.
      </para>
      <para>
    The EMenuHook object is loaded by the &EPlugin; system, and is used to
    provide dynamic extension to the application menus.
      </para>
      &e-menu-reference;
    </chapter>

    <chapter>
      <title>
    EConfig
      </title>
      <para>
    The EConfig object manages the building of dynamic configuration pages
    to configure specific application objects.  The same basic object can
    be used to fully drive a wizard-like druid object, or to drive a
    note-book of configuration options.  It is used by application code to
    provide the core controller in a model-view-controller implementation
    of a UI window.
      </para>
      <para>
    The EConfigHook object is loaded by the &EPlugin; system, and is used hook
    in additional configuration items into configuration windows or druids
    dynamically.
      </para>
      &e-config-reference;
    </chapter>

    <chapter>
      <title>
    EEvent
      </title>
      <para>
    The EEvent object manages broadcast of events for a given component or
    application.  It is used by application code to provide the plugin
    system with an entry point for user and system state events.
      </para>
      <para>
    The EEventHook object is loaded by the &EPlugin; system, and is used hook
    event listeners into dynamically loaded event handlers.
      </para>
      &e-event-reference;
    </chapter>

    <chapter>
      <title>
    EMFormat
      </title>
      <para>
    The EMFormat object drives the formatting of MIME message content for
    display, print, and replying.  EMFormatHTML is an implementation of
    EMFormat which writes its output to a GtkHTML instance.
      </para>
      <para>
    The EMFormatHook object is loaded by the &EPlugin; system, and is used hook
    event listeners into dynamically loaded event handlers.
      </para>
      &em-format-reference;
    </chapter>
  </part>

</book>