bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/extensions/NavigatorContentDescriptorManager.java - platform/eclipse.platform.ui - Git at Google

 /*******************************************************************************
  * Copyright (c) 2003, 2006 IBM Corporation and others.
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License v1.0
  * which accompanies this distribution, and is available at
  * http://www.eclipse.org/legal/epl-v10.html
  *
  * Contributors:
  * IBM Corporation - initial API and implementation
  *******************************************************************************/
 package org.eclipse.ui.internal.navigator.extensions;

 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
 import java.util.Map;
 import java.util.Set;
 import java.util.TreeSet;
 import java.util.WeakHashMap;

 import org.eclipse.core.runtime.IConfigurationElement;
 import org.eclipse.jface.resource.ImageDescriptor;
 import org.eclipse.jface.resource.ImageRegistry;
 import org.eclipse.swt.graphics.Image;
 import org.eclipse.ui.WorkbenchException;
 import org.eclipse.ui.internal.navigator.NavigatorPlugin;
 import org.eclipse.ui.internal.navigator.VisibilityAssistant;
 import org.eclipse.ui.internal.navigator.VisibilityAssistant.VisibilityListener;
 import org.eclipse.ui.navigator.INavigatorContentDescriptor;
 import org.eclipse.ui.plugin.AbstractUIPlugin;

 /**
  * <p>
  * <strong>EXPERIMENTAL</strong>. This class or interface has been added as
  * part of a work in progress. There is a guarantee neither that this API will
  * work nor that it will remain the same. Please do not use this API without
  * consulting with the Platform/UI team.
  * </p>
  *
  * @since 3.2
  */
 public class NavigatorContentDescriptorManager {

 	private static final NavigatorContentDescriptorManager INSTANCE = new NavigatorContentDescriptorManager();

 	private final Map firstClassDescriptorsMap = new HashMap();

 	private final Map allDescriptors = new HashMap();

 	private class EvaluationCache implements VisibilityListener {

 		private final Map evaluations = new WeakHashMap();
 		private final Map evaluationsWithOverrides = new WeakHashMap();

 		EvaluationCache(VisibilityAssistant anAssistant) {
 			anAssistant.addListener(this);
 		}

 		protected final Set getDescriptors(Object anElement) {
 			return getDescriptors(anElement, true);
 		}

 		protected final void setDescriptors(Object anElement, Set theDescriptors) {
 			setDescriptors(anElement, theDescriptors, true);
 		}

 		protected final Set getDescriptors(Object anElement, boolean toComputeOverrides) {
 			if(toComputeOverrides)
 				return (Set) evaluations.get(anElement);
 			return (Set) evaluationsWithOverrides.get(anElement);
 		}

 		protected final void setDescriptors(Object anElement, Set theDescriptors, boolean toComputeOverrides) {
 			if(toComputeOverrides)
 				evaluations.put(anElement, theDescriptors);
 			else
 				evaluationsWithOverrides.put(anElement, theDescriptors);
 		}

 		/*
 		 * (non-Javadoc)
 		 *
 		 * @see org.eclipse.ui.internal.navigator.VisibilityAssistant.VisibilityListener#onVisibilityOrActivationChange()
 		 */
 		public void onVisibilityOrActivationChange() {
 			evaluations.clear();
 			evaluationsWithOverrides.clear();
 		}
 	}

 	/* Map of (VisibilityAssistant, EvaluationCache)-pairs */
 	private final Map cachedTriggerPointEvaluations = new WeakHashMap();

 	/* Map of (VisibilityAssistant, EvaluationCache)-pairs */
 	private final Map cachedPossibleChildrenEvaluations = new WeakHashMap();

 	private ImageRegistry imageRegistry;

 	private final Set overridingDescriptors = new HashSet();

 	private final Set saveablesProviderDescriptors = new HashSet();

 	private final Set firstClassDescriptorsSet = new HashSet();

 	/**
 	 * @return the singleton instance of the manager
 	 */
 	public static NavigatorContentDescriptorManager getInstance() {
 		return INSTANCE;
 	}

 	private NavigatorContentDescriptorManager() {
 		new NavigatorContentDescriptorRegistry().readRegistry();
 	}

 	/**
 	 *
 	 * @return Returns all content descriptor(s).
 	 */
 	public NavigatorContentDescriptor[] getAllContentDescriptors() {
 		NavigatorContentDescriptor[] finalDescriptors = new NavigatorContentDescriptor[allDescriptors
 				.size()];
 		finalDescriptors = (NavigatorContentDescriptor[]) allDescriptors.values().toArray(finalDescriptors);
 		Arrays.sort(finalDescriptors, ExtensionPriorityComparator.INSTANCE);
 		return finalDescriptors;
 	}

 	/**
 	 *
 	 * @return Returns all content descriptors that provide saveables.
 	 */
 	public NavigatorContentDescriptor[] getContentDescriptorsWithSaveables() {
 		NavigatorContentDescriptor[] finalDescriptors = new NavigatorContentDescriptor[saveablesProviderDescriptors
 		                                                                               .size()];
 		saveablesProviderDescriptors.toArray(finalDescriptors);
 		Arrays.sort(finalDescriptors, ExtensionPriorityComparator.INSTANCE);
 		return finalDescriptors;
 	}

 	/**
 	 *
 	 * Returns all content descriptor(s) which enable for the given element.
 	 *
 	 * @param anElement
 	 *            the element to return the best content descriptor for
 	 *
 	 * @param aVisibilityAssistant
 	 *            The relevant viewer assistant; used to filter out unbound
 	 *            content descriptors.
 	 * @return the best content descriptor for the given element.
 	 */
 	public Set findDescriptorsForTriggerPoint(Object anElement,
 			VisibilityAssistant aVisibilityAssistant) {
 		EvaluationCache cache = getEvaluationCache(
 				cachedTriggerPointEvaluations, aVisibilityAssistant);
 		if (cache.getDescriptors(anElement) != null) {
 			return cache.getDescriptors(anElement);
 		}

 		Set descriptors = new TreeSet(ExtensionPriorityComparator.INSTANCE);

 	 	/* Find other ContentProviders which enable for this object */
 		for (Iterator contentDescriptorsItr = firstClassDescriptorsMap.values()
 				.iterator(); contentDescriptorsItr.hasNext();) {
 			NavigatorContentDescriptor descriptor = (NavigatorContentDescriptor) contentDescriptorsItr
 					.next();

 			if (aVisibilityAssistant.isActive(descriptor)
 					&& aVisibilityAssistant.isVisible(descriptor)
 					&& descriptor.isTriggerPoint(anElement)) {
 				descriptors.add(descriptor);
 			}
 		}

 		cache.setDescriptors(anElement, descriptors);

 		return descriptors;
 	}

 	private EvaluationCache getEvaluationCache(Map anEvaluationMap,
 			VisibilityAssistant aVisibilityAssistant) {
 		EvaluationCache c = (EvaluationCache) anEvaluationMap
 				.get(aVisibilityAssistant);
 		if (c == null) {
 			anEvaluationMap.put(aVisibilityAssistant, c = new EvaluationCache(
 					aVisibilityAssistant));
 		}
 		return c;

 	}

 	/**
 	 *
 	 * Returns all content descriptor(s) which enable for the given element.
 	 *
 	 * @param anElement
 	 *            the element to return the best content descriptor for
 	 *
 	 * @param aVisibilityAssistant
 	 *            The relevant viewer assistant; used to filter out unbound
 	 *            content descriptors.
 	 * @return the best content descriptor for the given element.
 	 */
 	public Set findDescriptorsForPossibleChild(Object anElement,
 			VisibilityAssistant aVisibilityAssistant) {
 		return findDescriptorsForPossibleChild(anElement, aVisibilityAssistant, true);
 	}

 	/**
 	 *
 	 * Returns all content descriptor(s) which enable for the given element.
 	 *
 	 * @param anElement
 	 *            the element to return the best content descriptor for
 	 *
 	 * @param aVisibilityAssistant
 	 *            The relevant viewer assistant; used to filter out unbound
 	 *            content descriptors.
 	 * @return the best content descriptor for the given element.
 	 */
 	public Set findDescriptorsForPossibleChild(Object anElement,
 			VisibilityAssistant aVisibilityAssistant, boolean toComputeOverrides) {

 		EvaluationCache cache = getEvaluationCache(
 				cachedPossibleChildrenEvaluations, aVisibilityAssistant);
 		if (cache.getDescriptors(anElement, toComputeOverrides) != null) {
 			return cache.getDescriptors(anElement, toComputeOverrides);
 		}

 		Set descriptors = new TreeSet(ExtensionPriorityComparator.INSTANCE);
 		if(toComputeOverrides) {
 			addDescriptorsForPossibleChild(anElement, firstClassDescriptorsSet,
 				aVisibilityAssistant, descriptors);
 		} else {

 			NavigatorContentDescriptor descriptor;
 			/* Find other ContentProviders which enable for this object */
 			for (Iterator contentDescriptorsItr = allDescriptors.values().iterator(); contentDescriptorsItr
 					.hasNext();) {
 				descriptor = (NavigatorContentDescriptor) contentDescriptorsItr
 						.next();

 				boolean isApplicable = aVisibilityAssistant.isActive(descriptor)
 						&& aVisibilityAssistant.isVisible(descriptor)
 						&& descriptor.isPossibleChild(anElement);

 				 if (isApplicable) {
 					descriptors.add(descriptor);
 				}

 			}
 		}
 		cache.setDescriptors(anElement, descriptors, toComputeOverrides);

 		return descriptors;
 	}

 	private boolean addDescriptorsForPossibleChild(Object anElement,
 			Set theChildDescriptors, VisibilityAssistant aVisibilityAssistant,
 			Set theFoundDescriptors) {
 		int initialSize = theFoundDescriptors.size();

 		NavigatorContentDescriptor descriptor;
 		/* Find other ContentProviders which enable for this object */
 		for (Iterator contentDescriptorsItr = theChildDescriptors.iterator(); contentDescriptorsItr
 				.hasNext();) {
 			descriptor = (NavigatorContentDescriptor) contentDescriptorsItr
 					.next();

 			boolean isApplicable = aVisibilityAssistant.isActive(descriptor)
 					&& aVisibilityAssistant.isVisible(descriptor)
 					&& descriptor.isPossibleChild(anElement);

 			if (descriptor.hasOverridingExtensions()) {

 				boolean isOverridden = addDescriptorsForPossibleChild(
 						anElement, descriptor.getOverriddingExtensions(),
 						aVisibilityAssistant, theFoundDescriptors);

 				if (!isOverridden && isApplicable) {
 					theFoundDescriptors.add(descriptor);
 				}

 			} else if (isApplicable) {
 				theFoundDescriptors.add(descriptor);
 			}

 		}
 		return initialSize < theFoundDescriptors.size();

 	}

 	/**
 	 * Returns the navigator content descriptor with the given id.
 	 *
 	 * @param id
 	 *            The id of the content descriptor that should be returned
 	 * @return The content descriptor of the given id
 	 */
 	public NavigatorContentDescriptor getContentDescriptor(String id) {
 		return (NavigatorContentDescriptor) allDescriptors.get(id);
 	}

 	/**
 	 *
 	 * @param descriptorId
 	 *            The unique id of a particular descriptor
 	 * @return The name (value of the 'name' attribute) of the given descriptor
 	 */
 	public String getText(String descriptorId) {
 		INavigatorContentDescriptor descriptor = getContentDescriptor(descriptorId);
 		if (descriptor != null) {
 			return descriptor.getName();
 		}
 		return descriptorId;
 	}

 	/**
 	 *
 	 * @param descriptorId
 	 *            The unique id of a particular descriptor
 	 * @return The image (corresponding to the value of the 'icon' attribute) of
 	 *         the given descriptor
 	 */
 	public Image getImage(String descriptorId) {
 		return retrieveAndStoreImage(descriptorId);
 	}

 	protected Image retrieveAndStoreImage(String descriptorId) {
 		NavigatorContentDescriptor contentDescriptor = getContentDescriptor(descriptorId);

 		Image image = null;
 		if (contentDescriptor != null) {
 			String icon = contentDescriptor.getIcon();
 			if (icon != null) {
 				image = getImageRegistry().get(icon);
 				if (image == null || image.isDisposed()) {
 					ImageDescriptor imageDescriptor = AbstractUIPlugin
 							.imageDescriptorFromPlugin(contentDescriptor
 									.getContribution().getPluginId(), icon);
 					if (imageDescriptor != null) {
 						image = imageDescriptor.createImage();
 						if (image != null) {
 							getImageRegistry().put(icon, image);
 						}
 					}
 				}
 			}
 		}
 		return image;
 	}

 	/**
 	 * @param desc
 	 */
 	private void addNavigatorContentDescriptor(NavigatorContentDescriptor desc) {
 		if (desc == null) {
 			return;
 		}
 		synchronized (firstClassDescriptorsMap) {
 			if (firstClassDescriptorsMap.containsKey(desc.getId())) {
 				NavigatorPlugin
 						.logError(
 								0,
 								"An extension already exists with id \"" + desc.getId() + "\".", null); //$NON-NLS-1$ //$NON-NLS-2$
 			} else {
 				if (desc.getSuppressedExtensionId() == null) {
 					firstClassDescriptorsMap.put(desc.getId(), desc);
 					firstClassDescriptorsSet.add(desc);
 				} else {
 					overridingDescriptors.add(desc);
 				}
 				allDescriptors.put(desc.getId(), desc);
 				if (desc.hasSaveablesProvider()) {
 					saveablesProviderDescriptors.add(desc);
 				}
 			}
 		}
 	}

 	/**
 	 *
 	 */
 	private void computeOverrides() {
 		if (overridingDescriptors.size() > 0) {
 			NavigatorContentDescriptor descriptor;
 			NavigatorContentDescriptor overriddenDescriptor;
 			for (Iterator overridingIterator = overridingDescriptors.iterator(); overridingIterator
 					.hasNext();) {
 				descriptor = (NavigatorContentDescriptor) overridingIterator
 						.next();
 				overriddenDescriptor = (NavigatorContentDescriptor) allDescriptors
 						.get(descriptor.getSuppressedExtensionId());
 				if (overriddenDescriptor != null) {

 					/*
 					 * add the descriptor as an overriding extension for its
 					 * suppressed extension
 					 */
 					overriddenDescriptor.getOverriddingExtensions().add(
 							descriptor);
 					descriptor.setOverriddenDescriptor(overriddenDescriptor);
 					/*
 					 * the always policy implies this is also a top-level
 					 * extension
 					 */
 					if (descriptor.getOverridePolicy() == OverridePolicy.InvokeAlwaysRegardlessOfSuppressedExt) {
 						firstClassDescriptorsMap.put(descriptor.getId(),
 								descriptor);
 						firstClassDescriptorsSet.add(descriptor);
 					}

 				} else {
 					NavigatorPlugin.logError(0,
 							"Invalid suppressedExtensionId (\"" //$NON-NLS-1$
 									+ descriptor.getSuppressedExtensionId()
 									+ "\" specified from " //$NON-NLS-1$
 									+ descriptor.getContribution()
 											.getPluginId()
 									+ ". No extension with matching id found.", //$NON-NLS-1$
 							null);
 				}
 			}
 		}
 	}

 	private ImageRegistry getImageRegistry() {
 		if (imageRegistry == null) {
 			imageRegistry = new ImageRegistry();
 		}
 		return imageRegistry;
 	}

 	private class NavigatorContentDescriptorRegistry extends
 			NavigatorContentRegistryReader {

 		/*
 		 * (non-Javadoc)
 		 *
 		 * @see org.eclipse.ui.internal.navigator.extensions.RegistryReader#readRegistry()
 		 */
 		public void readRegistry() {
 			super.readRegistry();
 			computeOverrides();
 		}

 		protected boolean readElement(IConfigurationElement anElement) {
 			if (TAG_NAVIGATOR_CONTENT.equals(anElement.getName())) {
 				try {
 					addNavigatorContentDescriptor(new NavigatorContentDescriptor(
 							anElement));

 				} catch (WorkbenchException e) {
 					// log an error since its not safe to open a dialog here
 					NavigatorPlugin.log(e.getStatus());
 				}
 			}
 			return super.readElement(anElement);
 		}
 	}

 }
	/*******************************************************************************
	* Copyright (c) 2003, 2006 IBM Corporation and others.
	* All rights reserved. This program and the accompanying materials
	* are made available under the terms of the Eclipse Public License v1.0
	* which accompanies this distribution, and is available at
	* http://www.eclipse.org/legal/epl-v10.html
	*
	* Contributors:
	* IBM Corporation - initial API and implementation
	*******************************************************************************/
	package org.eclipse.ui.internal.navigator.extensions;

	import java.util.Arrays;
	import java.util.HashMap;
	import java.util.HashSet;
	import java.util.Iterator;
	import java.util.Map;
	import java.util.Set;
	import java.util.TreeSet;
	import java.util.WeakHashMap;

	import org.eclipse.core.runtime.IConfigurationElement;
	import org.eclipse.jface.resource.ImageDescriptor;
	import org.eclipse.jface.resource.ImageRegistry;
	import org.eclipse.swt.graphics.Image;
	import org.eclipse.ui.WorkbenchException;
	import org.eclipse.ui.internal.navigator.NavigatorPlugin;
	import org.eclipse.ui.internal.navigator.VisibilityAssistant;
	import org.eclipse.ui.internal.navigator.VisibilityAssistant.VisibilityListener;
	import org.eclipse.ui.navigator.INavigatorContentDescriptor;
	import org.eclipse.ui.plugin.AbstractUIPlugin;

	/**
	* <p>
	* <strong>EXPERIMENTAL</strong>. This class or interface has been added as
	* part of a work in progress. There is a guarantee neither that this API will
	* work nor that it will remain the same. Please do not use this API without
	* consulting with the Platform/UI team.
	* </p>
	*
	* @since 3.2
	*/
	public class NavigatorContentDescriptorManager {

	private static final NavigatorContentDescriptorManager INSTANCE = new NavigatorContentDescriptorManager();

	private final Map firstClassDescriptorsMap = new HashMap();

	private final Map allDescriptors = new HashMap();

	private class EvaluationCache implements VisibilityListener {

	private final Map evaluations = new WeakHashMap();
	private final Map evaluationsWithOverrides = new WeakHashMap();

	EvaluationCache(VisibilityAssistant anAssistant) {
	anAssistant.addListener(this);
	}

	protected final Set getDescriptors(Object anElement) {
	return getDescriptors(anElement, true);
	}

	protected final void setDescriptors(Object anElement, Set theDescriptors) {
	setDescriptors(anElement, theDescriptors, true);
	}

	protected final Set getDescriptors(Object anElement, boolean toComputeOverrides) {
	if(toComputeOverrides)
	return (Set) evaluations.get(anElement);
	return (Set) evaluationsWithOverrides.get(anElement);
	}

	protected final void setDescriptors(Object anElement, Set theDescriptors, boolean toComputeOverrides) {
	if(toComputeOverrides)
	evaluations.put(anElement, theDescriptors);
	else
	evaluationsWithOverrides.put(anElement, theDescriptors);
	}

	/*
	* (non-Javadoc)
	*
	* @see org.eclipse.ui.internal.navigator.VisibilityAssistant.VisibilityListener#onVisibilityOrActivationChange()
	*/
	public void onVisibilityOrActivationChange() {
	evaluations.clear();
	evaluationsWithOverrides.clear();
	}
	}

	/* Map of (VisibilityAssistant, EvaluationCache)-pairs */
	private final Map cachedTriggerPointEvaluations = new WeakHashMap();

	/* Map of (VisibilityAssistant, EvaluationCache)-pairs */
	private final Map cachedPossibleChildrenEvaluations = new WeakHashMap();

	private ImageRegistry imageRegistry;

	private final Set overridingDescriptors = new HashSet();

	private final Set saveablesProviderDescriptors = new HashSet();

	private final Set firstClassDescriptorsSet = new HashSet();

	/**
	* @return the singleton instance of the manager
	*/
	public static NavigatorContentDescriptorManager getInstance() {
	return INSTANCE;
	}

	private NavigatorContentDescriptorManager() {
	new NavigatorContentDescriptorRegistry().readRegistry();
	}

	/**
	*
	* @return Returns all content descriptor(s).
	*/
	public NavigatorContentDescriptor[] getAllContentDescriptors() {
	NavigatorContentDescriptor[] finalDescriptors = new NavigatorContentDescriptor[allDescriptors
	.size()];
	finalDescriptors = (NavigatorContentDescriptor[]) allDescriptors.values().toArray(finalDescriptors);
	Arrays.sort(finalDescriptors, ExtensionPriorityComparator.INSTANCE);
	return finalDescriptors;
	}

	/**
	*
	* @return Returns all content descriptors that provide saveables.
	*/
	public NavigatorContentDescriptor[] getContentDescriptorsWithSaveables() {
	NavigatorContentDescriptor[] finalDescriptors = new NavigatorContentDescriptor[saveablesProviderDescriptors
	.size()];
	saveablesProviderDescriptors.toArray(finalDescriptors);
	Arrays.sort(finalDescriptors, ExtensionPriorityComparator.INSTANCE);
	return finalDescriptors;
	}

	/**
	*
	* Returns all content descriptor(s) which enable for the given element.
	*
	* @param anElement
	* the element to return the best content descriptor for
	*
	* @param aVisibilityAssistant
	* The relevant viewer assistant; used to filter out unbound
	* content descriptors.
	* @return the best content descriptor for the given element.
	*/
	public Set findDescriptorsForTriggerPoint(Object anElement,
	VisibilityAssistant aVisibilityAssistant) {
	EvaluationCache cache = getEvaluationCache(
	cachedTriggerPointEvaluations, aVisibilityAssistant);
	if (cache.getDescriptors(anElement) != null) {
	return cache.getDescriptors(anElement);
	}

	Set descriptors = new TreeSet(ExtensionPriorityComparator.INSTANCE);

	/* Find other ContentProviders which enable for this object */
	for (Iterator contentDescriptorsItr = firstClassDescriptorsMap.values()
	.iterator(); contentDescriptorsItr.hasNext();) {
	NavigatorContentDescriptor descriptor = (NavigatorContentDescriptor) contentDescriptorsItr
	.next();

	if (aVisibilityAssistant.isActive(descriptor)
	&& aVisibilityAssistant.isVisible(descriptor)
	&& descriptor.isTriggerPoint(anElement)) {
	descriptors.add(descriptor);
	}
	}

	cache.setDescriptors(anElement, descriptors);

	return descriptors;
	}

	private EvaluationCache getEvaluationCache(Map anEvaluationMap,
	VisibilityAssistant aVisibilityAssistant) {
	EvaluationCache c = (EvaluationCache) anEvaluationMap
	.get(aVisibilityAssistant);
	if (c == null) {
	anEvaluationMap.put(aVisibilityAssistant, c = new EvaluationCache(
	aVisibilityAssistant));
	}
	return c;

	}

	/**
	*
	* Returns all content descriptor(s) which enable for the given element.
	*
	* @param anElement
	* the element to return the best content descriptor for
	*
	* @param aVisibilityAssistant
	* The relevant viewer assistant; used to filter out unbound
	* content descriptors.
	* @return the best content descriptor for the given element.
	*/
	public Set findDescriptorsForPossibleChild(Object anElement,
	VisibilityAssistant aVisibilityAssistant) {
	return findDescriptorsForPossibleChild(anElement, aVisibilityAssistant, true);
	}

	/**
	*
	* Returns all content descriptor(s) which enable for the given element.
	*
	* @param anElement
	* the element to return the best content descriptor for
	*
	* @param aVisibilityAssistant
	* The relevant viewer assistant; used to filter out unbound
	* content descriptors.
	* @return the best content descriptor for the given element.
	*/
	public Set findDescriptorsForPossibleChild(Object anElement,
	VisibilityAssistant aVisibilityAssistant, boolean toComputeOverrides) {

	EvaluationCache cache = getEvaluationCache(
	cachedPossibleChildrenEvaluations, aVisibilityAssistant);
	if (cache.getDescriptors(anElement, toComputeOverrides) != null) {
	return cache.getDescriptors(anElement, toComputeOverrides);
	}

	Set descriptors = new TreeSet(ExtensionPriorityComparator.INSTANCE);
	if(toComputeOverrides) {
	addDescriptorsForPossibleChild(anElement, firstClassDescriptorsSet,
	aVisibilityAssistant, descriptors);
	} else {

	NavigatorContentDescriptor descriptor;
	/* Find other ContentProviders which enable for this object */
	for (Iterator contentDescriptorsItr = allDescriptors.values().iterator(); contentDescriptorsItr
	.hasNext();) {
	descriptor = (NavigatorContentDescriptor) contentDescriptorsItr
	.next();

	boolean isApplicable = aVisibilityAssistant.isActive(descriptor)
	&& aVisibilityAssistant.isVisible(descriptor)
	&& descriptor.isPossibleChild(anElement);

	if (isApplicable) {
	descriptors.add(descriptor);
	}

	}
	}
	cache.setDescriptors(anElement, descriptors, toComputeOverrides);

	return descriptors;
	}

	private boolean addDescriptorsForPossibleChild(Object anElement,
	Set theChildDescriptors, VisibilityAssistant aVisibilityAssistant,
	Set theFoundDescriptors) {
	int initialSize = theFoundDescriptors.size();

	NavigatorContentDescriptor descriptor;
	/* Find other ContentProviders which enable for this object */
	for (Iterator contentDescriptorsItr = theChildDescriptors.iterator(); contentDescriptorsItr
	.hasNext();) {
	descriptor = (NavigatorContentDescriptor) contentDescriptorsItr
	.next();

	boolean isApplicable = aVisibilityAssistant.isActive(descriptor)
	&& aVisibilityAssistant.isVisible(descriptor)
	&& descriptor.isPossibleChild(anElement);

	if (descriptor.hasOverridingExtensions()) {

	boolean isOverridden = addDescriptorsForPossibleChild(
	anElement, descriptor.getOverriddingExtensions(),
	aVisibilityAssistant, theFoundDescriptors);

	if (!isOverridden && isApplicable) {
	theFoundDescriptors.add(descriptor);
	}

	} else if (isApplicable) {
	theFoundDescriptors.add(descriptor);
	}

	}
	return initialSize < theFoundDescriptors.size();

	}

	/**
	* Returns the navigator content descriptor with the given id.
	*
	* @param id
	* The id of the content descriptor that should be returned
	* @return The content descriptor of the given id
	*/
	public NavigatorContentDescriptor getContentDescriptor(String id) {
	return (NavigatorContentDescriptor) allDescriptors.get(id);
	}

	/**
	*
	* @param descriptorId
	* The unique id of a particular descriptor
	* @return The name (value of the 'name' attribute) of the given descriptor
	*/
	public String getText(String descriptorId) {
	INavigatorContentDescriptor descriptor = getContentDescriptor(descriptorId);
	if (descriptor != null) {
	return descriptor.getName();
	}
	return descriptorId;
	}

	/**
	*
	* @param descriptorId
	* The unique id of a particular descriptor
	* @return The image (corresponding to the value of the 'icon' attribute) of
	* the given descriptor
	*/
	public Image getImage(String descriptorId) {
	return retrieveAndStoreImage(descriptorId);
	}

	protected Image retrieveAndStoreImage(String descriptorId) {
	NavigatorContentDescriptor contentDescriptor = getContentDescriptor(descriptorId);

	Image image = null;
	if (contentDescriptor != null) {
	String icon = contentDescriptor.getIcon();
	if (icon != null) {
	image = getImageRegistry().get(icon);
	if (image == null \|\| image.isDisposed()) {
	ImageDescriptor imageDescriptor = AbstractUIPlugin
	.imageDescriptorFromPlugin(contentDescriptor
	.getContribution().getPluginId(), icon);
	if (imageDescriptor != null) {
	image = imageDescriptor.createImage();
	if (image != null) {
	getImageRegistry().put(icon, image);
	}
	}
	}
	}
	}
	return image;
	}

	/**
	* @param desc
	*/
	private void addNavigatorContentDescriptor(NavigatorContentDescriptor desc) {
	if (desc == null) {
	return;
	}
	synchronized (firstClassDescriptorsMap) {
	if (firstClassDescriptorsMap.containsKey(desc.getId())) {
	NavigatorPlugin
	.logError(
	0,
	"An extension already exists with id \"" + desc.getId() + "\".", null); //$NON-NLS-1$ //$NON-NLS-2$
	} else {
	if (desc.getSuppressedExtensionId() == null) {
	firstClassDescriptorsMap.put(desc.getId(), desc);
	firstClassDescriptorsSet.add(desc);
	} else {
	overridingDescriptors.add(desc);
	}
	allDescriptors.put(desc.getId(), desc);
	if (desc.hasSaveablesProvider()) {
	saveablesProviderDescriptors.add(desc);
	}
	}
	}
	}

	/**
	*
	*/
	private void computeOverrides() {
	if (overridingDescriptors.size() > 0) {
	NavigatorContentDescriptor descriptor;
	NavigatorContentDescriptor overriddenDescriptor;
	for (Iterator overridingIterator = overridingDescriptors.iterator(); overridingIterator
	.hasNext();) {
	descriptor = (NavigatorContentDescriptor) overridingIterator
	.next();
	overriddenDescriptor = (NavigatorContentDescriptor) allDescriptors
	.get(descriptor.getSuppressedExtensionId());
	if (overriddenDescriptor != null) {

	/*
	* add the descriptor as an overriding extension for its
	* suppressed extension
	*/
	overriddenDescriptor.getOverriddingExtensions().add(
	descriptor);
	descriptor.setOverriddenDescriptor(overriddenDescriptor);
	/*
	* the always policy implies this is also a top-level
	* extension
	*/
	if (descriptor.getOverridePolicy() == OverridePolicy.InvokeAlwaysRegardlessOfSuppressedExt) {
	firstClassDescriptorsMap.put(descriptor.getId(),
	descriptor);
	firstClassDescriptorsSet.add(descriptor);
	}

	} else {
	NavigatorPlugin.logError(0,
	"Invalid suppressedExtensionId (\"" //$NON-NLS-1$
	+ descriptor.getSuppressedExtensionId()
	+ "\" specified from " //$NON-NLS-1$
	+ descriptor.getContribution()
	.getPluginId()
	+ ". No extension with matching id found.", //$NON-NLS-1$
	null);
	}
	}
	}
	}

	private ImageRegistry getImageRegistry() {
	if (imageRegistry == null) {
	imageRegistry = new ImageRegistry();
	}
	return imageRegistry;
	}

	private class NavigatorContentDescriptorRegistry extends
	NavigatorContentRegistryReader {

	/*
	* (non-Javadoc)
	*
	* @see org.eclipse.ui.internal.navigator.extensions.RegistryReader#readRegistry()
	*/
	public void readRegistry() {
	super.readRegistry();
	computeOverrides();
	}

	protected boolean readElement(IConfigurationElement anElement) {
	if (TAG_NAVIGATOR_CONTENT.equals(anElement.getName())) {
	try {
	addNavigatorContentDescriptor(new NavigatorContentDescriptor(
	anElement));

	} catch (WorkbenchException e) {
	// log an error since its not safe to open a dialog here
	NavigatorPlugin.log(e.getStatus());
	}
	}
	return super.readElement(anElement);
	}
	}

	}