Eclipse Workbench: Using the Selection Service

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Multiple Selection Providers Within a Part

Be aware that the part's site accepts a single selection provider only, which should be registered within the createPartControl() method only:

    getSite().setSelectionProvider(provider);

Replacing the selection provider during the lifetime of the part is not properly supported by the workbench. If a part contains multiple viewers providing selections, like the "Java Hierarchy" view does, a intermediate ISelectionProvider implementation has to be provided that allows dynamically delegating to the currently active viewer within the part. As a starting point you may look into SelectionProviderIntermediate.java provided with this article.

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What to do With the Selected Objects?

This article claims that the selection service helps to decouple views reacting on each others selection. But the view handling a selection still needs to deal with the selected objects to provide any useful functionality. Check out the adapter pattern provided by the Eclipse runtime core, which allows attaching new functionality to existing model objects or the other way round providing required functionality for newly contributed objects.

Actually our example plug-in does exactly this by using the workbench label provider which in turn relies on the IWorkbenchAdapter to retrieve icons and text labels for the listed objects. The same mechanism is utilized by the "Properties" view, see article "Take control of your properties" for details.

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Example Plug-in

This article comes with a small example plug-in that demonstrates the explained techniques. Additionally the plug-in may serve for debugging your selection providers. The example contributes a new view "Workbench Selection" which simply mirrors the current selection in the workbench. It works for element selections as well as for text selections.
Getting and Running the Example

Download the plug-in com.mountainminds.eclipse.selectionsample_1.1.0.jar and import it into your workspace via the "Import..." wizard from the "File" menu. Select "Existing Project into Workspace" on the first wizard page. On the second page simply use the option "Select archive file" to import the downloaded archive. The fastest way to launch the example is right-clicking the plug-in project and select "Run As" → "Eclipse Application" in the context menu.

If don't want to play with the source code you can also drop the bundle into an Eclipse installation and use it directly.

From the menu of the launched workbench activate "Window" → "Show View" → "Other..." and select our view "Workbench Selection" in the category "Other". Now you can play around with selections in various workbench parts and see how our "Workbench Selection" view reflects these selections:
[Screen 2]

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The same works for text selections:
[Screen 3]

The example is implemented in a single class SelectionView.java applying the techniques discussed in this article. When reading through this short piece of code you may note that:

    * The ISelectionListener implementation makes sure that we do not react on our own selections.
    * The title of the source part of the current selection as well as the implementation class of ISelection is shown in the view description (the optional grey bar at the top).
    * The view uses two viewers: One for object lists and one for text blocks. Switching between the viewers is implemented done with a PageBook.
    *
      The viewer for object lists itself registeres as a selection provider. Check it out: If you select a element listed in our "Workbench Selection" view its properties are shown in turn in the "Properties" view.

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Conclusion

The mechanisms provided by the Eclipse workbench are simple to use and powerful. Not using this mechanisms result in plug-ins that poorly integrate with the rest of the workbench and will be hard to extend. To avoid such pitfalls simply follow these rules when selections come into picture:

    * Avoid direct hard-wired inter-view communication. If one view needs to react on the selection in another view use the ISelectionService.
    * Be cooperative and open for future extensions: Always register your viewers as selection providers with the part site.
    * Use existing selection specific views like the "Properties" view when appropriate instead of creating new ones.

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