bundles/org.eclipse.ui.workbench/Eclipse UI/org/eclipse/ui/actions/BaseSelectionListenerAction.java

/*******************************************************************************
 * Copyright (c) 2000, 2003 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials 
 * are made available under the terms of the Common Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/cpl-v10.html
 * 
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/

package org.eclipse.ui.actions;

import org.eclipse.jface.action.Action;
import org.eclipse.jface.viewers.ISelection;
import org.eclipse.jface.viewers.ISelectionChangedListener;
import org.eclipse.jface.viewers.IStructuredSelection;
import org.eclipse.jface.viewers.SelectionChangedEvent;
import org.eclipse.jface.viewers.StructuredSelection;

/**
 * The abstract superclass for actions that listen to selection change events.
 * This implementation tracks the current selection (see 
 * <code>getStructuredSelection</code>) and provides a convenient place to 
 * monitor selection changes that could affect the availability of the action.
 * <p>
 * Subclasses must implement the following <code>IAction</code> method:
 * <ul>
 *   <li><code>run</code> - to do the action's work</li>
 * </ul>
 * </p>
 * <p>
 * Subclasses may extend the <code>updateSelection</code> method to update
 * the action determine its availability based on the current selection.
 * </p>
 * <p>
 * The object instantiating the subclass is responsible for registering
 * the instance with a selection provider. Alternatively, the object can
 * notify the subclass instance directly of a selection change using the
 * methods:
 * <ul>
 *   <li><code>selectionChanged(IStructuredSelection)</code> - passing the selection</li>
 *   <li><code>selectionChanged(ISelectionChangedEvent)</code> - passing the selection change event</li>
 * </ul>
 * </p>
 * @since 3.0
 */
public abstract class BaseSelectionListenerAction extends Action implements ISelectionChangedListener {
	/**
	 * The current selection.
	 */
	private IStructuredSelection selection = new StructuredSelection();

	/**
	 * Creates a new action with the given text.
	 *
	 * @param text the string used as the text for the action, 
	 *   or <code>null</code> if there is no text
	 */
	protected BaseSelectionListenerAction(String text) {
		super(text);
	}
	
	/**
	 * Clears any cached state associated with the selection.
	 * Called when the selection changes.
	 * <p>
	 * The <code>BaseSelectionListenerAction</code> implementation of this method
	 * does nothing. Subclasses may override.
     * </p>
	 */
	protected void clearCache() {
		// do nothing
	}
	
	/**
	 * Returns the current structured selection in the workbench, or an empty
	 * selection if nothing is selected or if selection does not include
	 * objects (for example, raw text).
	 *
	 * @return the current structured selection in the workbench
	 */
	public IStructuredSelection getStructuredSelection() {
		return selection;
	}
	
	/**
	 * Notifies this action that the given structured selection has changed.
	 * <p>
	 * The <code>BaseSelectionListenerAction</code> implementation of this method
	 * records the given selection for future reference and calls
	 * <code>updateSelection</code>, updating the enable state of this action
	 * based on the outcome. Subclasses should override <code>updateSelection</code>
	 * to react to selection changes.
	 * </p>
	 *
	 * @param selection the new selection
	 */
	public final void selectionChanged(IStructuredSelection selection) {
		this.selection = selection;
		clearCache();
		setEnabled(updateSelection(selection));
	}
	
	/**
	 * The <code>BaseSelectionListenerAction</code> implementation of this 
	 * <code>ISelectionChangedListener</code> method calls 
	 * <code>selectionChanged(IStructuredSelection)</code> assuming the selection is
	 * a structured one. Subclasses should override the <code>updateSelection</code>
	 * method to react to selection changes.
	 */
	public final void selectionChanged(SelectionChangedEvent event) {
		ISelection selection = event.getSelection();
		if (selection instanceof IStructuredSelection)
			selectionChanged((IStructuredSelection) selection);
		else
			selectionChanged(StructuredSelection.EMPTY);
	}
	
	/**
	 * Updates this action in response to the given selection.
	 * <p>
	 * The <code>BaseSelectionListenerAction</code> implementation of this method
	 * returns <code>true</code>. Subclasses may extend to react to selection
	 * changes; however, if the super method returns <code>false</code>, the
	 * overriding method must also return <code>false</code>.
	 * </p>
	 *
	 * @param selection the new selection
	 * @return <code>true</code> if the action should be enabled for this selection,
	 *   and <code>false</code> otherwise
	 */
	protected boolean updateSelection(IStructuredSelection selection) {
		return true;
	}
}