Eclipse command framework core expression : Multiple handlers

Eclipse platform command framework supports core expression to control commands behavior runtime. In this series of posts I have explained different ways of using core expressions . This post explains having multiple handler for same command but activate only one of it at runtime based on some of the condition.

Eclipse command framework is based on MVC framework. Command declaration, handler declaration  and contributing command to workbench helps to keep declaration separate from presentation. You can have multiple handlers for same command, but only one will be active at one moment.

Core expression provides “activeWhen” declarative clause to activate specific handler for given command based on specific condition. Here is how we use it . I have created a new eclipse plug-in project. I have extended “org.eclipse.ui.editors” extension point to define two simple text editors. Here I am going to define two different handlers for same commands but only one will be active based on editor instance.

1. Extend “org.eclipse.ui.commands” extension point to declare command

2. Extends “org.eclipse.ui.handlers” extension point to define handles for command

I have define two handlers for same command.

3. Use “activeWhen” clause, here I have used “instance of” expression to decide which handler will be active based on editor instance.

 

 

4. Define “org.eclipse.ui.menu” extension point to contribute command to text editor context menu.

 

The final plugin.xml will be

<?xml version="1.0" encoding="UTF-8"?>

<?eclipse version="3.4"?>

<plugin>

   <extension

         point="org.eclipse.ui.commands">

      <command

            id="com.example.advance.cmd.command1"

            name="Command 1">

      </command>

   </extension>

   <extension

         point="org.eclipse.ui.editors">

      <editor

            class="com.example.advance.cmd.editor.Editor1"

            default="true"

            extensions=".txt"

            id="com.example.advance.cmd.editor1"

            name="Editor 1">

      </editor>

      <editor

            class="com.example.advance.cmd.editor.Editor2"

            default="false"

            id="com.example.advance.cmd.editor2"

            name="Editor 2">

      </editor>

   </extension>

   <extension

         point="org.eclipse.ui.menus">

      <menuContribution

            allPopups="true"

            locationURI="popup:#TextEditorContext?after=additions">

         <command

               commandId="com.example.advance.cmd.command1"

               label="Command One"

               style="push"

               tooltip="Command 1">

         </command>

      </menuContribution>

   </extension>

   <extension

         point="org.eclipse.ui.handlers">

      <handler

            class="com.example.advance.cmd.Cmd1Handler1"

            commandId="com.example.advance.cmd.command1">

         <activeWhen>

            <with

                  variable="activeEditor">

               <instanceof

                     value="com.example.advance.cmd.editor.Editor1">

               </instanceof>

            </with>

         </activeWhen>

      </handler>

      <handler

            class="com.example.advance.cmd.Cmd1Handler2"

            commandId="com.example.advance.cmd.command1">

         <activeWhen>

            <with

                  variable="activeEditor">

               <instanceof

                     value="com.example.advance.cmd.editor.Editor2">

               </instanceof>

            </with>

         </activeWhen>

      </handler>

   </extension>

</plugin>

Things to be noticed here is

      <handler
            class="com.example.advance.cmd.Cmd1Handler2"
            commandId="com.example.advance.cmd.command1">
        <activeWhen>
            <with
                  variable="activeEditor">
               <instanceof
                     value="com.example.advance.cmd.editor.Editor2">
               </instanceof>
            </with>
         </activeWhen>      </handler>


This handler will be active only when Editor2 instance’s context menu. I have opened file.txt in two different editor “Editor1” & “Editor2”






As shown in above image when clicking on “Command One” option, you will see “Inside Cm1Handler1.execute()” message. It means Handler1 is active for Editor1 context menu (notice Editor name in file title “Editor1:file1.txt”)



Now I have opened same file in different editor “Editor2” and selected same command “Cammand One” from context menu. As shown in above image you will notice Handler2 has been invoked


Here is the complete source code.