### Undo and the IDE Workbench

We've seen code snippets that use workbench protocol for accessing the operations history and the workbench undo context. This is achieved using IWorkbenchOperationSupport, which can be obtained from the workbench. The notion of a workbench-wide undo context is fairly general. It is up to the workbench application to determine what specific scope is implied by the workbench undo context, and which views or editors use the workbench context when providing undo support.

In the case of the Eclipse IDE workbench, the workbench undo context should be assigned to any operation that affects the IDE workspace at large. This context is used by views that manipulate the workspace, such as the Resource Navigator. The IDE workbench installs an adapter on the workspace for IUndoContext that returns the workbench undo context. This model-based registration allows plug-ins that manipulate the workspace to obtain the appropriate undo context, even if they are headless and do not reference any workbench classes.

    // get the operation history
    IOperationHistory history = OperationHistoryFactory.getOperationHistory();

    // obtain the appropriate undo context for my model
    IUndoContext workspaceContext = (IUndoContext)ResourcesPlugin.getWorkspace().getAdapter(IUndoContext.class);
    if (workspaceContext != null) {
    	// create an operation and assign it the context
    }

Other plug-ins are encouraged to use this same technique for registering model-based undo contexts.

### Undoable Workspace Operations

Undoable operations are provided by the IDE plug-in that allow you to execute, undo, and redo common workspace manipulations. Operations are provided for creating, moving, copying, and deleting projects, folders, and files. The IDE plug-in uses these operations to provide undo support in the Resource Navigator. These classes can also be used by clients. They are available in the package org.eclipse.ui.ide.undo.

Using the workspace undoable operations is similar to the examples shown for using any IUndoableOperation. The primary difference is that some of the details, such as assigning the appropriate undo context, are handled internally by the operation. The following snippet shows how to use an undoable operation for creating a new file.

    IFile newFileHandle = IDEWorkbenchPlugin.getPluginWorkspace().getRoot().
    	getFile(myNewFilePath);
    CreateFileOperation op = new CreateFileOperation(newFileHandle, null,
    	initialContentStream, "Create New File");

The operation is configured with a file handle for the file to be created, an input stream with the initial contents, and a label describing the operation. Internally, the operation will assign the workspace undo context and handle other details such as keeping track of which resources the operation manipulates. To execute the operation, the caller uses the standard IOperationHistory protocol.

    try {
    	PlatformUI.getWorkbench().getOperationSupport().getOperationHistory()
    		.execute(op, myProgressMonitor,
    			WorkspaceUndoUtil.getUIInfoAdapter(getShell()));
    } catch (final ExecutionException e) {
    	// handle exceptions
    }

Note that the caller is responsible for providing a progress monitor if desired, and an IAdaptable that can provide the shell that should be used for any prompting. The utility class WorkspaceUndoUtil provides a utility method for creating the necessary adapter from a shell.

Callers are also responsible for determining whether to execute the operation in the UI thread, the background, or a workbench job. If the operation should be executed in the background, then the caller should create the appropriate progress monitor and run the snippet shown above in the appropriate runnable.

The undo and redo of the operation will be performed by the undo and redo action handlers when the user initiates undo or redo. The standard workbench action handlers use IAdvancedUndoableOperation2 protocol to determine how to perform the undo and redo of operations. All of the workspace operations implement protocol that instructs clients to run the undo or redo of the operation in the background if possible.