Eclipse Tips

Commands Part 1: Actions Vs Commands

As you would have seen, there are two different ways to contributing to the Workbench: Actions and Commands. Although Commands are newer and advanced, I've always preferred using Actions, simply because of my comfort level in using them. Now that I've started fixing some bugs in the Command framework, I'm forced to look into the details. The more deeper I look into the Commands, the more I'm loving it. So I decided to write a series of on the Commands and this is the first one in the series. Many of the information presented here is obtained by looking some old bugs, wiki and digging into CVS history. If I'm missing anything or wrong about something let me know.

Lets start with Actions. We are able to contribute to the menus, toolbars, pull down menu, etc. We are able to specify the UI details like label or tooltip in the plugin.xml itself, which helps in lazy loading. So whats wrong with them?

• The UI and handling are always tied. There is no way you can separate each other
• While Actions can be contributed to different parts of the workbench (popup menu/tool bar), all of them were different extension points and so you end up duplicating the XML in multiple places. The worst of it is that not all the extension points expect the same configuration.
• Specifying Actions in multiple places is a maintenance nightmare. If you have to change the icon of an action, you need to change in all the places.
• Another issue with duplicating Actions in plugin.xml is that multiple instance of the same Actions will be created in the memory

Lets see how the Commands Framework eliminates all this. Commands are defined by the org.eclipse.ui.commands extension point. A typical command would look like:

<command
         id="com.eclipse-tips.commands.someCommand"
         name="Some Command">
</command>

Yeah thats it. That defines a Command! If you want to place this in a tool bar, you have to use the extension point org.eclipse.ui.menus:

<menuContribution
      locationURI="toolbar:org.eclipse.ui.main.toolbar">
   <toolbar
         id="com.eclipse-tips.commands.toolbar1">
      <command
            commandId="com.eclipse-tips.commands.someCommand"
            id="com.eclipse-tips.commands.someCommandInToolBar">
      </command>
   </toolbar>
</menuContribution>
 
Now that takes of the placement in the UI. But what about the code that gets executed? That goes into a another extension point, org.eclipse.ui.handlers:

<handler
         class="com.eclipse_tips.commads.SomeCommandHandler"
         commandId="com.eclipse-tips.commands.someCommand">
</handler>

One last piece is to add an icon to the Command
No price for guessing that you need to use another extension point, org.eclipse.ui.commandImages:

<image
         commandId="com.eclipse-tips.commands.someCommand"
         icon="icons/sample.gif">
</image>

All set. Lets see the Command in action:

As you see, in the actions we would just use only one extension point, instead of the four which we have used now. Does it sounds like a round about way or doing things? Probably once we get to know about multiple handlers for a Command and context based association & enablement of the handlers etc, we will see the beauty of the framework. That would be the next in this series.

Update (31-Mar-2009): Thanks to Hiroki Kondo, a Japanese version of this blog entry is available here.

See also:
Part 2: Selection and Enablement of Handlers
Part 3: Parameters for Commands
Part 4: Misc items ...
Part 5: ISourceProvider & dynamically updating Commands
Part 6: 'toggle' & 'radio' style menu contribution

8 comments:

1. Channing Walton Says: January 3, 2009 3:01 AM  

   Thanks for this, I wanted to use commands and menuContributions and was just about to start digging for clues. Your blog saved me the effort.

2. mattre Says: January 3, 2009 3:55 PM  

   Hi, it would be also nice if you cover advanced topics like property testers in your series.

3. vogella Says: January 3, 2009 4:34 PM  

   Hi,
   
   I relation to this: "As you see, in the actions we would just use only one extension point, instead of the four which we have used now."
   
   You could have done this with two extensions points by using the default handler and the icons in the menucontributin.
   
   Please see here for an explaination: http://www.vogella.de/articles/RichClientPlatform/article.html#commands
   
   Best regards, Lars

4. Anonymous Says: March 31, 2009 7:04 PM  

   Hi,
   
   Can commands be used to override the Copy/Paste/Delete/Rename/... commands (retargetable actions)?
   
   If yes, how it works?
   
   Thanks

5. Prakash G.R. Says: April 2, 2009 10:29 AM  

   > Can commands be used to override the
   > Copy/Paste/Delete/Rename/... commands (retargetable actions)?
   
   Short answer is yes, the long answer will be posted as a separate tip :-)

6. Wayne Says: April 2, 2009 11:46 PM  

   What do I have to do to get you to turn these blog posts into an article for Eclipse Corner?
   
   I can get you an Eclipse-branded shirt... I can help you write it...

7. Prakash G.R. Says: April 3, 2009 8:20 AM  

   @Wayne,
   2 more posts in this series are still pending. I'll make it as an article once they are done.

8. Anonymous Says: May 27, 2009 11:34 PM  

   Hi Prakash,
   
   Did you write the 2 posts you mentionned in the last comment?
   
   What are the subjects?
   
   Is the copy/paste/undo/redo is one of this subject?
   
   Thanks,
   Vincent

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