Eclipse Commands - Overview

1.1. Overview

A command in Eclipse is a declarative description of a component and is independent from the implementation details. A command can be categorized, assigned to the user interface and a key binding can be defined for the command. The behavior of a command can be defined via a handler.

Therefore to define and use a command you need

• Command - Declarative description of the component

• Handler - Defines the behavior

• UI Assignment - Where and how should the command be included in the UI

Commands are defined via the extension point org.eclipse.ui.commands.

The following example used are based on Eclipse RCP but the concept also applies to general Eclipse plugin development.

1.2. Location URI - User interface

Commands can be used in menus, toolbars and / or context menus. Where and how these commands are displayed is defined via a location URI.

Table 1.

Contribution to	Description	Uri
Application menu	Displays the command in the menu of the application	"menu:org.eclipse.ui.main.menu"
Application toolbar	displays the command in the toolbar of the application	"toolbar:org.eclipse.ui.main.toolbar"
View toolbar	displays the command in the toolbar of the view	"toolbar:viewId". For example to display a menu to view with the Id "View1" use "toolbar:Views1".
Content menu / pop-up	Command is displayed in a content menu, e.g. right mouse click on an object	n.a.

1.3. Command Handler

The behavior of a command is defined via handlers. You can define several handlers for a command but only only handler can be valid for a command at a certain point otherwise the command is not active. The handler is the class which will be executed once the command is called. A command handler must implement the interface org.eclipse.core.commands.IHandler.

IHandler (which is implemented by org.eclipse.core.commands.AbstractHandler) defines the following important methods which can be implemented:

• isEnabled: Called during instantiation, defines if this action is enabled

• isHandled: Defines if the handler can be called or not

• execute: Coding which perform the action

• fireHandlerChanged: needs to be called if isEnabled is changed.

Tip

The class HandlerUtil provides access methods to the important Eclipse services.

Handler can be defined with conditions (activeWhen) under which they are valid handlers for the command. If more then one handler is valid for a given selection the Eclipse runtime cannot decided which one is used and the command will not be enabled.