migrating-gtkaction.sgml

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<chapter id="gtk-migrating-GtkAction">
  <chapterinfo>
    <author>
      <firstname>Federico</firstname>
      <surname>Mena-Quintero</surname>
      <affiliation>
	<address>
	  <email>[e-mail]</email>
	</address>
      </affiliation>
    </author>
  </chapterinfo>

  <title>Migrating from old menu and toolbar systems to GtkAction</title>

  <para>
    Prior to GTK+ 2.4, there were several APIs in use to create menus
    and toolbars.  GTK+ itself included <link
    linkend="GtkItemFactory">GtkItemFactory</link>, which was
    historically used in the GIMP; libgnomeui provided the gnome-ui
    set of macros; libbonoboui provided a complex mechanism to do menu
    merging across embedded components.  GTK+ 2.4 includes a system
    for creating menus and toolbars, with merging of items, based
    around the <link linkend="GtkAction">GtkAction</link> mechanism.
  </para>

  <section id="actions-and-action-groups">
    <title>Actions and Action Groups</title>

    <para>
      A <link linkend="GtkAction">GtkAction</link> represents an
      operation that the user can perform from the menus and toolbars
      of an application.  It is similar to "verbs" in other menu
      systems.  A <link linkend="GtkAction">GtkAction</link> has a
      name, which is its identifier, and it can have several widgets
      that represent it in the user interface.  For example, an action
      for <symbol>EditCopy</symbol> can have a menu item as well as a
      toolbar button associated to it.  If there is nothing selected
      in the document, the application can simply de-sensitize the
      <symbol>EditCopy</symbol> action; this will cause both the menu
      item and the toolbar button to be de-sensitized automatically.
      Similarly, whenever the user selects the menu item or the
      toolbar button associated to the <symbol>EditCopy</symbol>
      action, the corresponding <link
      linkend="GtkAction">GtkAction</link> object will emit an
      "activate" signal.
    </para>

    <para>
      <link linkend="GtkActionGroup">GtkActionGroup</link> is simply a
      group of <link linkend="GtkAction">GtkAction</link> objects.  An
      application may want to have several groups:  one for global
      actions such as "new document", "about", and "exit"; then one
      group for each open document with actions specific to the
      document, such as "cut", "copy", "paste", and "print".
    </para>

    <para>
      Normal actions are simply commands, such as
      <symbol>FileSave</symbol> or <symbol>EditCopy</symbol>.  Toggle
      actions can be active or inactive, such as
      <symbol>FormatBold</symbol> or <symbol>ViewShowRulers</symbol>.
      Radio actions define a set of items for which one and only one
      can be active at a time, for example, {
      <symbol>ViewHighQuality</symbol>,
      <symbol>ViewNormalQuality</symbol>,
      <symbol>ViewLowQuality</symbol> }.
    </para>
  </section>

  <section id="ui-manager">
    <title>User Interface Manager Object</title>

    <para>
      <link linkend="GtkUIManager">GtkUIManager</link> is an object
      that can construct menu and toolbar widgets from an XML
      description.  These widgets are in turn associated to
      corresponding actions and action groups.
    </para>

    <para>
      <link linkend="GtkUIManager">GtkUIManager</link> supports
      merging of menus and toolbars for applications that have
      multiple components, each with separate sets of commands.  For
      example, a word processor that can embed images may want to have
      toolbar buttons for Bold and Italic when the cursor is on a text
      block, but Crop and Brightness/Contrast buttons when the cursor
      is on an image.  These actions, which change depending on the
      state of the application, can be merged and de-merged from a
      <link linkend="GtkUIManager">GtkUIManager</link> as appropriate.
    </para>
  </section>

  <section id="migrating-gnomeuiinfo">
    <title>Migrating from GnomeUIInfo</title>

    <para>
      Prior to GTK+ 2.4, some applications used the GnomeUIInfo
      mechanism from
      <filename>&lt;libgnomeui/gnome-app-helper.h&gt;</filename> to
      define their menus and toolbars.  With it, a program decleres an
      array of <structname>GnomeUIInfo</structname> structures, which
      contain information for menu or toolbar items such as their
      label, icon, and accelerator key.  Then, one calls
      <function>gnome_app_fill_menu()</function> or
      <function>gnome_app_fill_toolbar()</function>, or one of the
      related functions, to create the appropriate widgets based on
      these structures.
    </para>

    <para>
      A downside of this API is that the same structures are used to
      pass back pointers to the widgets that got created.  This means
      that the structures cannot simply be kept around if the program
      requires multiple instances of the user interface (e.g. several
      windows); each new invocation of
      <function>gnome_app_fill_menu()</function> would overwrite the
      widget fields of the structures.
    </para>

    <para>
      Another disadvantage is that there is no automatic way to
      synchronize the state of related controls.  If there are toolbar
      toogle buttons for "Bold", "Italic", "Underline", and also
      corresponding menu items under "Format/Bold", etc., one has to
      synchronize their toggled states by hand whenever the user
      selects any one of them.
    </para>

    <para>
      Finally, there is no way to do menu and toolbar merging for
      applications that require embedded components.
    </para>

    <para>
      To convert an application that uses GnomeUIInfo into the new
      GtkAction mechanism, you need to do several things:
    </para>

    <orderedlist>
      <listitem>
	<para>
	  Separate your existing GnomeUIInfo entries into normal
	  actions, toggle actions, and radio actions, and then create
	  a separate array of <link
	  linkend="GtkActionEntry">GtkActionEntry</link> structures
	  for each group.  This will allow you to create the necessary
	  <link linkend="GtkActionGroup">GtkActionGroup</link>
	  objects.  Note that this does not describe the actual
	  "shape" that your menus and toolbars will have; it simply
	  defines the set of commands that will appear in them.
	</para>
      </listitem>
      <listitem>
	<para>
	  Create an XML description of your menus and toolbars for use
	  with <link linkend="GtkUIManager">GtkUIManager</link>.  This
	  defines the actual shape of the menus and toolbars.
	</para>
      </listitem>
      <listitem>
	<para>
	  Port the code that uses gnome-app and gnome-app-helper to
	  <link linkend="GtkAction">GtkAction</link> and <link
	  linkend="GtkUIManager">GtkUIManager</link>.
	</para>
      </listitem>
      <listitem>
       <para>
         If your GnomeUIInfo entries use GNOME_APP_PIXMAP_DATA or 
         GNOME_APP_PIXMAP_FILENAME for pixmaps, you have to create a 
         <link linkend="GtkIconFactory">GtkIconFactory</link>, add it 
         to the list of default factories, then create a 
         <link linkend="GtkIconSet">GtkIconSet</link> for each of your 
         own icons. Add the sets to the factory, and use the id in the 
         <link linkend="GtkActionEntry">GtkActionEntry</link> like a 
         regular GTK+ stock id.
       </para>
      </listitem>
    </orderedlist>

    <example id="gnomeuiinfo-example">
      <title>GnomeUIInfo Example</title>

      <para>
	The following code shows a declaration of a simple menu bar to
	be used with <function>gnome_app_fill_menu()</function> or
	similar.  The menu hierarchy looks like this:
      </para>

      <itemizedlist>
	<listitem>
	  <para><guimenu>File</guimenu></para>
	  <simplelist>
	    <member><guimenuitem>Open</guimenuitem></member>
	    <member><guimenuitem>&mdash;</guimenuitem></member>
	    <member><guimenuitem>Exit</guimenuitem></member>
	  </simplelist>
	</listitem>

	<listitem>
	  <para><guimenu>View</guimenu></para>
	  <simplelist>
	    <member><guimenuitem>Zoom In</guimenuitem></member>
	    <member><guimenuitem>Zoom Out</guimenuitem></member>
	    <member><guimenuitem>&mdash;</guimenuitem></member>
	    <member><guimenuitem>[ ] Full Screen</guimenuitem></member>
	    <member><guimenuitem>&mdash;</guimenuitem></member>
	    <member><guimenuitem>( ) High Quality</guimenuitem></member>
	    <member><guimenuitem>( ) Normal Quality</guimenuitem></member>
	    <member><guimenuitem>( ) Low Quality</guimenuitem></member>
	  </simplelist>
	</listitem>
      </itemizedlist>

      <programlisting>
static GnomeUIInfo file_menu_items[] = {
  { GNOME_APP_UI_ITEM, "_Open", "Open a file",
    open_callback, NULL, NULL, GNOME_APP_PIXMAP_STOCK, GTK_STOCK_OPEN,
    'o', GDK_CONTROL_MASK, NULL },
  { GNOME_APP_UI_SEPARATOR },
  { GNOME_APP_UI_ITEM, "E_xit", "Exit the program",
    exit_callback, NULL, NULL, GNOME_APP_PIXMAP_STOCK, GTK_STOCK_QUIT,
    'q', GDK_CONTROL_MASK, NULL},
  { GNOME_APP_UI_ENDOFINFO }
};

static GnomeUIInfo view_radio_items[] = {
  { GNOME_APP_UI_ITEM, "_High Quality", "Display images in high quality, slow mode",
    high_quality_callback, NULL, NULL, GNOME_APP_PIXMAP_FILENAME, "high-quality.png",
    0, 0, NULL },
  { GNOME_APP_UI_ITEM, "_Normal Quality", "Display images in normal quality",