View source for FAQ Why are some actions activated without a target?

Most of the time, an action is invoked on the ''current selection'' in the UI.
One such example is the '''Properties''' menu action on a resource in the 
Navigator. This action is relevant only when a particular object has been selected.
In other cases, the action is ''global'', as is the action behind
'''Project &gt; Build All''', which simply builds all projects in the 
workspace and is independent of the current selection.

The workbench does not distinguish between context-sensitive
actions and global actions, and so it calls the <tt>selectionChanged</tt>
method on all action delegates in case they want to update their enablement
based on the selection. This is somewhat of a design 
mistake as many unnecessary selection change notifications are now sent out
throughout the workbench. Furthermore, actions have to remember the current
selection and store it in a field so it can be used if the <tt>run</tt> 
method is called. This is not a good design pattern.

Luckily, the workbench added an interface called <tt>IActionDelegate2</tt>
in version 2.0 of the platform. When clients implement both 
<tt>IActionDelegate</tt> and <tt>IActionDelegate2</tt>, the workbench
will not call the <tt>run(IAction action)</tt> method but will call the 
<tt>runWithEvent</tt> method instead. 

This alternative solution allows clients to write more compact code 
with control flow localized to the execution of the action. 
A typical action would declare itself as follows:
<pre>
   class Handler extends ActionDelegate 
                                implements IActionDelegate2 {
      public void runWithEvent(IAction action, Event event) {
         MessageDialog.openInformation(new Shell(), 
            "Demo", "Handling: "+action+" on "+event);
      }
   }
</pre>

Interfaces like <tt>IActionDelegate2</tt> are an indication of how 
a platform like Eclipse struggles with the tension between adoption 
and innovation. Because too many client plug-ins already 
depended on the implementation of the old interface 
<tt>IActionDelegate</tt>, it could not be easily changed without 
breaking the existing Eclipse API. Instead, a parallel replacement was 
added, which is less elegant but comes back in multiple places in 
Eclipse. The existence of the <tt>IActionDelegateWithEvent</tt> 
interface shows how even naming mistakes have to persist for a 
while as some clients may rely on it.

<hr><font size=-2>This FAQ was originally published in [http://www.eclipsefaq.org Official Eclipse 3.0 FAQs]. Copyright 2004, Pearson Education, Inc. All rights reserved. This text is made available here under the terms of the [http://www.eclipse.org/legal/epl-v10.html Eclipse Public License v1.0].</font>