<page xmlns="http://projectmallard.org/1.0/" type="topic" id="howto_write_a_plugin">
  <info>
    <link type="guide" xref="index#advanced" />
    <title type="sort">3. Writing New Plugins</title>
    <link type="next" xref="preferences" />
    <desc>
      Extending <app>Accerciser</app> with your desired functionalities
    </desc>
    <credit type="author">
      <name>Eitan Isaacson</name>
      <email>eitan@ascender.com</email>
    </credit>
    <credit type="author">
      <name>Peter Parente</name>
      <email>pparent@us.ibm.com</email>
    </credit>
    <credit type="author">
      <name>Aline Bessa</name>
      <email>alibezz@gmail.com</email>
    </credit>
    <license>
      <p>Creative Commons Share Alike 3.0</p>
    </license>
  </info>
  <title> Writing Plugins for <app>Accerciser</app></title>
  <p>
    Extending <app>Accerciser</app> with new plugins became much simpler
    since <link href="https://wiki.gnome.org/Apps/Accerciser/PluginTutorial">this tutorial</link>
    was written. Given that it provides a very detailed explanation about the topic,
    this section consists basically of its original content - only simple editions
    and updates were done.
  </p>
  <p>
    <app>Accerciser</app> supports three basic types of plugins:
  </p>
  <list>
    <item>
      <p>
        Base plugins - These plugins are derived from the Plugin base class. They do not provide
        a visible interface, but could provide additional functionality to <app>Accerciser</app>.
      </p>
    </item>
    <item>
      <p>
        Console plugins - These plugins provide simple console output into a text area in a plugin
        tab. Not to be confused with the packaged <link xref="ipython_plugin">IPython Console Plugin</link>.
      </p>
    </item>
    <item>
      <p>
        Viewport plugins - The majority of <app>Accerciser</app> default plugins. 
        They provide a custom graphical interface in a tab. 
      </p>
    </item>
  </list>

  <section id="base_plugin">
    <title>Creating a Base Plugin</title>
    <p>
      We will create a simplified version of the <link xref="quick_select_plugin">Quick Select Plugin</link>. 
      This plugin will select the last focused accessible when pressing <keyseq><key>ctrl</key><key>alt</key><key>e</key></keyseq>.
    </p>
    <p>
      First off, the import lines we will use are: 
    </p>
    <code>
      from accerciser.plugin import Plugin
      import gtk
      import pyatspi
    </code>
    <p>
      Next we will derive a new class from the Plugin base class, 
      and assign some mandatory class attributes: 
    </p>
    <code>
      class FocusSelect(Plugin):
        plugin_name = 'Focus Select'
        plugin_description = 'Allows selecting last focused accessible.'
    </code>
    <p>
      We will now override the init method, in which we will set a global key
      action for selecting the last focused accessible item, register an event
      listener for the "focus" event, and set the <cmd>last_focused</cmd> instance
      variable to <cmd>None</cmd>. 
    </p>
    <code>
      def init(self):
        pyatspi.Registry.registerEventListener(self.accEventFocusChanged, 'focus')
        self.global_hotkeys = [('Inspect last focused accessible',
                                self.inspectLastFocused,
                                gtk.keysyms.e,
                                gtk.gdk.CONTROL_MASK | gtk.gdk.MOD1_MASK)]
        self.last_focused = None
    </code>
    <p>
      Notice that the <cmd>global_hotkeys</cmd> instance variable is a list of
      tuples. Each tuple is a global hotkey action, composed by an action description,
      a desired method to call, a key symbol of keypress, and a key modifier mask.
    </p>
    <p>
      In the "focus" event callback, we assign the <cmd>last_focused</cmd> instance
      variable with the accessible item that has just emitted the "focus" event. 
    </p>
    <code>
      def accEventFocusChanged(self, event):
        if not self.isMyApp(event.source):
          self.last_focused = event.source
    </code>
    <p>
       In the hotkey action callback, we update the application wide node with the
       last focused accessible item, if we have recorded it:
    </p>
    <code>
      def inspectLastFocused(self):
        if self.last_focused:
          self.node.update(self.last_focused)
    </code>
  </section>
  <section id="console_plugin">
    <title>Creating a Console Plugin</title>
    <p>
      We will create a console plugin to display focus changes emitted by an accessible
      item with a "push button" role - remember that it is easy to check what is the role
      of any item with <app>Accerciser</app>; you can verify it in the <link xref="desktop_tree_view">Application Tree View</link>,
      for example. 
    </p>
    <p>
      The needed import lines are:
    </p>
    <code>
      from accerciser.plugin import ConsolePlugin
      import pyatspi
    </code>
    <p>
      Then we add a class definition, with a plugin name and description: 
    </p> 
    <code>
      class PushButtonFocus(ConsolePlugin):
        plugin_name = 'Push Button Focus'
        plugin_description = 'Print event when pushbutton get\'s focus.'
    </code>
    <p>
      We override the init method adding a register listener: 
    </p>
    <code>
       def init(self):
         pyatspi.Registry.registerEventListener(self.accEventFocusChanged, 'focus')
    </code>
    <p>
      In the callback method, all push button events are printed. 
    </p>
    <code>
      def accEventFocusChanged(self, event):
        if event.source.getRole() == pyatspi.ROLE_PUSH_BUTTON:
          self.appendText(str(event)+'\n')
    </code>
  </section>
  <section id="viewport_plugin">
    <title>Creating a Viewport Plugin</title>
    <p>
      We will create a viewport plugin that allows quick testing of the
      "click" action in accessible items that support the AT-SPI Action interface
      and have an action named "click". It will be a simple button that, once
      clicked, does the "click" action in the accessible. 
    </p>
    <p>
      First off, some mandatory import lines:
    </p>
    <code>
      import gtk
      from accerciser.plugin import ViewportPlugin
    </code>
    <p>
      Next, a class definition, with a name and description: 
    </p>
    <code>
      class Clicker(ViewportPlugin):
        plugin_name = 'Clicker'
        plugin_description = 'Test the "click" action in relevant accessibles.'
    </code>
    <p>
      We override the init method with some UI building, and connecting a callback
      to a signal for the button. We set the alignment to allow the button
      to be centered in the plugin tab, and not monstrously take up the entire plugin
      space. Notice that the <cmd>plugin_area</cmd> instance variable contains a gtk.Frame
      that could be populated with all the plugin's widgets. 
    </p>
    <code>
       def init(self):
         self.click_button = gtk.Button('Click me!')
         self.click_button.set_halign(gtk.Align.CENTER)
         self.click_button.set_valign(gtk.Align.CENTER)
         self.plugin_area.add(self.click_button)

         self.click_button.connect('clicked', self.onClick)

         self.show_all()
    </code>
    <p>
       We also created a convenience method that returns a list of supported actions
       of the currently selected accessible item - if it does not support the Action
       interface, it returns an empty list: 
    </p> 
    <code>
       def accSupportedActions(self):
       try:
         ai = self.node.acc.queryAction()
       except NotImplementedError:
         action_names = []
       else:
         action_names = [ai.getName(i) for i in xrange(ai.nActions)]
       return action_names
    </code>
    <p>
      The base plugin class has a method call onAccChanged that is called everytime the
      target application's selected accessible item changes. We will override it setting
      the button to be sensitive only when the current accessible item has the "click" action: 
    </p>
    <code>
       def onAccChanged(self, acc):
         has_click = 'click' in self.accSupportedActions()
         self.click_button.set_sensitive(has_click)
    </code>
    <p>
      The callback method for button "clicked" performs the "click" action on the accessible item. 
      Since this callback could only be called when the button is sensitive, we don't need to worry 
      about checking if the current accessible has the "click" action?
    </p>
    <code>
      def onClick(self, button):
        ai = self.node.acc.queryAction()
        action_names = [ai.getName(i) for i in xrange(ai.nActions)]
        ai.doAction(action_names.index('click'))
    </code>
   </section>
</page>
