migrating-gtkaction.sgml

This GTK+ version 2.12.3. GTK+ is a multi-platform toolkit for creating graphical user interfaces.

<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 #GtkItemFactory, 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 #GtkAction mechanism.
  </para>

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

    <para>
      A #GtkAction 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 #GtkAction 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 #GtkAction object will emit an
      "activate" signal.
    </para>

    <para>
      #GtkActionGroup is simply a group of #GtkAction 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>
      #GtkUIManager 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>
      #GtkUIManager 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
      #GtkUIManager 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
      gnome_app_fill_menu() or gnome_app_fill_toolbar(), 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 gnome_app_fill_menu() 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 #GtkActionEntry structures
	  for each group.  This will allow you to create the necessary
	  #GtkActionGroup 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 #GtkUIManager.  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
	  #GtkAction and #GtkUIManager.
	</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 
         #GtkIconFactory, add it to the list of default factories, then 
         create a #GtkIconSet for each of your own icons. Add the sets to 
         the factory, and use the id in the #GtkActionEntry 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 gnome_app_fill_menu() or similar.  The menu hierarchy i
        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",
    normal_quality_callback, NULL, NULL, GNOME_APP_PIXMAP_FILENAME, "normal-quality.png",
    0, 0, NULL },
  { GNOME_APP_UI_ITEM, "_Low Quality", "Display images in low quality, fast mode",