TreeViewerExtension.java

/*******************************************************************************
 * Copyright (c) 2011, 2015 Wind River Systems, Inc. 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:
 * Wind River Systems - initial API and implementation
 *******************************************************************************/
package org.eclipse.tcf.te.ui.trees;

import java.lang.reflect.Constructor;
import java.net.URL;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.concurrent.atomic.AtomicReference;

import org.eclipse.core.expressions.EvaluationContext;
import org.eclipse.core.expressions.EvaluationResult;
import org.eclipse.core.expressions.Expression;
import org.eclipse.core.expressions.ExpressionConverter;
import org.eclipse.core.expressions.IEvaluationContext;
import org.eclipse.core.runtime.Assert;
import org.eclipse.core.runtime.CoreException;
import org.eclipse.core.runtime.IConfigurationElement;
import org.eclipse.core.runtime.IExtensionPoint;
import org.eclipse.core.runtime.IExtensionRegistry;
import org.eclipse.core.runtime.Platform;
import org.eclipse.core.runtime.SafeRunner;
import org.eclipse.jface.resource.ImageDescriptor;
import org.eclipse.jface.util.LocalSelectionTransfer;
import org.eclipse.jface.util.SafeRunnable;
import org.eclipse.jface.viewers.ILabelProvider;
import org.eclipse.jface.viewers.ITreeContentProvider;
import org.eclipse.jface.viewers.TreeViewer;
import org.eclipse.jface.viewers.ViewerFilter;
import org.eclipse.swt.SWT;
import org.eclipse.swt.dnd.DND;
import org.eclipse.swt.dnd.DragSourceListener;
import org.eclipse.swt.dnd.DropTargetListener;
import org.eclipse.swt.dnd.FileTransfer;
import org.eclipse.swt.dnd.ImageTransfer;
import org.eclipse.swt.dnd.TextTransfer;
import org.eclipse.swt.dnd.Transfer;
import org.eclipse.swt.graphics.Image;
import org.eclipse.ui.PlatformUI;
import org.eclipse.ui.handlers.IHandlerService;
import org.osgi.framework.Bundle;

/**
 * The implementation of the tree viewer extension which is used to parse
 * the column, filter and content contributions declared in this extension
 * for a specified tree viewer.
 */
public class TreeViewerExtension {
	// The extension point id constant.
	private static final String EXTENSION_POINT_ID = "org.eclipse.tcf.te.ui.viewers"; //$NON-NLS-1$
	// The id of the tree viewer for which the extension is parsed.
	private String viewerId;

	/**
	 * Create an instance and parse all tree viewer extensions to get the
	 * column descriptors and filter descriptors for the specified viewer.
	 *
	 * @param viewerId The tree viewer's id.
	 */
	public TreeViewerExtension(String viewerId) {
		this.viewerId = viewerId;
	}

	/**
	 * Parse the viewer extensions and return the descriptor of the tree viewer.
	 *
	 * @return The viewer descriptor.
	 */
	public ViewerDescriptor parseViewer() {
		Assert.isNotNull(viewerId);
		IExtensionRegistry registry = Platform.getExtensionRegistry();
		IExtensionPoint extensionPoint = registry.getExtensionPoint(EXTENSION_POINT_ID);
		IConfigurationElement[] configurations = extensionPoint.getConfigurationElements();
		for (IConfigurationElement configuration : configurations) {
			String name = configuration.getName();
			if ("viewer".equals(name)) { //$NON-NLS-1$
				String id = configuration.getAttribute("id"); //$NON-NLS-1$
				if (viewerId.equals(id)) {
					return createViewerDescriptor(configuration);
				}
			}
		}
		return null;
	}

	/**
	 * Create a viewer descriptor from the given configuration element.
	 *
	 * @param configuration The configuration element that defines the viewer.
	 * @return The viewer descriptor.
	 */
	private ViewerDescriptor createViewerDescriptor(final IConfigurationElement configuration) {
		final ViewerDescriptor descriptor = new ViewerDescriptor();
		IConfigurationElement[] children = configuration.getChildren("creation"); //$NON-NLS-1$
		if (children != null && children.length > 0) {
			Assert.isTrue(children.length == 1);
			descriptor.setStyleConfig(children[0]);
		}
		children = configuration.getChildren("dragSupport"); //$NON-NLS-1$
		if (children != null && children.length > 0) {
			Assert.isTrue(children.length == 1);
			descriptor.setDragConfig(children[0]);
		}
		children = configuration.getChildren("dropSupport"); //$NON-NLS-1$
		if (children != null && children.length > 0) {
			Assert.isTrue(children.length == 1);
			descriptor.setDropConfig(children[0]);
		}
		SafeRunner.run(new SafeRunnable() {
			@Override
            public void handleException(Throwable e) {
				// Ignore exception
            }
			@Override
			public void run() throws Exception {
				ITreeContentProvider contentProvider = (ITreeContentProvider) configuration.createExecutableExtension("contentProvider"); //$NON-NLS-1$
				descriptor.setContentProvider(contentProvider);
			}
		});
		String value = configuration.getAttribute("persistent"); //$NON-NLS-1$
		if (value != null) {
			descriptor.setPersistent(Boolean.valueOf(value).booleanValue());
		}
		value = configuration.getAttribute("useLabelDecorator"); //$NON-NLS-1$
		if (value != null) {
			descriptor.setUseLabelDecorator(Boolean.valueOf(value).booleanValue());
		}
		value = configuration.getAttribute("autoExpandLevel"); //$NON-NLS-1$
		if (value != null) {
			try {
				int level = Integer.parseInt(value);
				descriptor.setAutoExpandLevel(level);
			}
			catch (NumberFormatException nfe) {
			}
		}
		return descriptor;
	}

	/**
	 * Parse the column declarations of this extension point and return the
	 * column descriptors.
	 *
	 * @param input The input used to initialize the columns.
	 * @return The column descriptors from this extension point.
	 */
	public ColumnDescriptor[] parseColumns(Object input) {
		Assert.isNotNull(viewerId);
		List<ColumnDescriptor> columns = Collections.synchronizedList(new ArrayList<ColumnDescriptor>());
		IExtensionRegistry registry = Platform.getExtensionRegistry();
		IExtensionPoint extensionPoint = registry.getExtensionPoint(EXTENSION_POINT_ID);
		IConfigurationElement[] configurations = extensionPoint.getConfigurationElements();
		for (IConfigurationElement configuration : configurations) {
			String name = configuration.getName();
			if ("columnContribution".equals(name)) { //$NON-NLS-1$
				String aViewerId = configuration.getAttribute("viewerId"); //$NON-NLS-1$
				if (viewerId.equals(aViewerId)) {
					IConfigurationElement[] children = configuration.getChildren("column"); //$NON-NLS-1$
					if (children != null && children.length > 0) {
						for (IConfigurationElement child : children) {
							createColumnDescriptor(input, columns, child);
						}
					}
				}
			}
		}
		return columns.toArray(new ColumnDescriptor[columns.size()]);
	}

	/**
	 * Parse the viewer filter declarations of this extension point and return the
	 * filter descriptors.
	 *
	 * @param input the new input
	 * @return The filter descriptors from this extension point.
	 */
	public FilterDescriptor[] parseFilters(Object input) {
		Assert.isNotNull(viewerId);
		List<FilterDescriptor> descriptors = Collections.synchronizedList(new ArrayList<FilterDescriptor>());
		IExtensionRegistry registry = Platform.getExtensionRegistry();
		IExtensionPoint extensionPoint = registry.getExtensionPoint(EXTENSION_POINT_ID);
		IConfigurationElement[] configurations = extensionPoint.getConfigurationElements();
		for (IConfigurationElement configuration : configurations) {
			String name = configuration.getName();
			if ("filterContribution".equals(name)) { //$NON-NLS-1$
				String aViewerId = configuration.getAttribute("viewerId"); //$NON-NLS-1$
				if (viewerId.equals(aViewerId)) {
					IConfigurationElement[] children = configuration.getChildren("filter"); //$NON-NLS-1$
					if (children != null && children.length > 0) {
						for (IConfigurationElement child : children) {
							createFilterDescriptor(input, descriptors, child);
						}
					}
				}
			}
		}
		return descriptors.toArray(new FilterDescriptor[descriptors.size()]);
	}

	/**
	 * Create an filter descriptor from the specified configuration element and
	 * add it to the filter list.
	 *
	 * @param input the input of the viewer to initialize the descriptors.
	 * @param descriptors The filter list to add the created descriptor to.
	 * @param configuration The extension configuration element to create the descriptor from.
	 */
	private void createFilterDescriptor(Object input, List<FilterDescriptor> descriptors, final IConfigurationElement configuration) {
		if (isElementActivated(input, configuration)) {
			String id = configuration.getAttribute("id"); //$NON-NLS-1$
			Assert.isNotNull(id);
			final FilterDescriptor descriptor = new FilterDescriptor();
			descriptor.setId(id);
			descriptors.add(descriptor);
			SafeRunner.run(new SafeRunnable() {
				@Override
                public void handleException(Throwable e) {
					// Ignore exception
                }
				@Override
				public void run() throws Exception {
					initFilter(descriptor, configuration);
				}
			});
		}
	}

	/**
	 * Initialize the filter descriptor from the specified configuration element.
	 *
	 * @param descriptor The new descriptor to be initialized.
	 * @param configuration The configuration element to initialize the filter.
	 * @throws CoreException Thrown during parsing.
	 */
	void initFilter(FilterDescriptor descriptor, IConfigurationElement configuration) throws CoreException {
		String attribute = configuration.getAttribute("name"); //$NON-NLS-1$
		Assert.isNotNull(attribute);
		descriptor.setName(attribute);
		attribute = configuration.getAttribute("description"); //$NON-NLS-1$
		if (attribute != null) {
			descriptor.setDescription(attribute);
		}
		attribute = configuration.getAttribute("image"); //$NON-NLS-1$
		if (attribute != null) {
			String symbolicName = configuration.getContributor().getName();
			URL resource = Platform.getBundle(symbolicName).getResource(attribute);
			Image image = ImageDescriptor.createFromURL(resource).createImage();
			descriptor.setImage(image);
		}
		attribute = configuration.getAttribute("enabled"); //$NON-NLS-1$
		if (attribute != null) {
			descriptor.setEnabled(Boolean.valueOf(attribute).booleanValue());
		}
		attribute = configuration.getAttribute("class"); //$NON-NLS-1$
		Assert.isNotNull(attribute);
		ViewerFilter filter = (ViewerFilter) configuration.createExecutableExtension("class"); //$NON-NLS-1$
		Assert.isNotNull(filter);
		descriptor.setFilter(filter);
		attribute = configuration.getAttribute("visibleInUI");  //$NON-NLS-1$
		if(attribute != null) {
			descriptor.setVisible(Boolean.valueOf(attribute).booleanValue());
		}
	}

	/**
	 * Create a column descriptor from the specified configuration element and add it to
	 * the column descriptor list.
	 *
	 * @param input the new input.
	 * @param columns The column descriptor.
	 * @param configuration The configuration element to read the descriptor from.
	 */
	private void createColumnDescriptor(Object input, final List<ColumnDescriptor> columns, final IConfigurationElement configuration) {
		if (isElementActivated(input, configuration)) {
			String id = configuration.getAttribute("id"); //$NON-NLS-1$
			Assert.isNotNull(id);
			final ColumnDescriptor column = new ColumnDescriptor(id);
			columns.add(column);
			SafeRunner.run(new SafeRunnable() {
				@Override
                public void handleException(Throwable e) {
					// Ignore exception
                }
				@Override
				public void run() throws Exception {
					initColumn(column, configuration);
					column.setOrder(columns.size());
				}
			});
		}
	}

	/**
	 * Parse the content contribution declarations of this extension point and return the
	 * content descriptors.
	 *
	 * @param input the new input
	 * @return The content descriptors from this extension point.
	 */
	public ContentDescriptor[] parseContents(Object input) {
		Assert.isNotNull(viewerId);
		List<ContentDescriptor> descriptors = Collections.synchronizedList(new ArrayList<ContentDescriptor>());
		IExtensionRegistry registry = Platform.getExtensionRegistry();
		IExtensionPoint extensionPoint = registry.getExtensionPoint(EXTENSION_POINT_ID);
		IConfigurationElement[] configurations = extensionPoint.getConfigurationElements();
		for (IConfigurationElement configuration : configurations) {
			String name = configuration.getName();
			if ("contentContribution".equals(name)) { //$NON-NLS-1$
				String aViewerId = configuration.getAttribute("viewerId"); //$NON-NLS-1$
				if (viewerId.equals(aViewerId)) {
					IConfigurationElement[] children = configuration.getChildren("content"); //$NON-NLS-1$
					if (children != null && children.length > 0) {
						for (IConfigurationElement child : children) {
							createContentDescriptor(input, descriptors, child);
						}
					}
				}
			}
		}
		return descriptors.toArray(new ContentDescriptor[descriptors.size()]);
	}

	/**
	 * Create an content descriptor from the specified configuration element and
	 * add it to the content descriptor list.
	 *
	 * @param input the input of the viewer to initialize the descriptors.
	 * @param descriptors The content descriptor list to add the created descriptor to.
	 * @param configuration The extension configuration element to create the descriptor from.
	 */
	private void createContentDescriptor(Object input, List<ContentDescriptor> descriptors, final IConfigurationElement configuration) {
		Assert.isNotNull(descriptors);
		Assert.isNotNull(configuration);

		if (isElementActivated(input, configuration)) {
			String id = configuration.getAttribute("id"); //$NON-NLS-1$
			Assert.isNotNull(id);
			final ContentDescriptor descriptor = new ContentDescriptor(id, configuration);
			descriptors.add(descriptor);
		}
	}

	/**
	 * If the specified configuration element is activated under the current input.
	 *
	 * @param input The input object.
	 * @param configuration The configuration element that defines the activation element.
	 * @return true if it is activated.
	 */
	private boolean isElementActivated(Object input, final IConfigurationElement configuration) {
		IConfigurationElement[] children = configuration.getChildren("activation"); //$NON-NLS-1$
		if(children == null || children.length == 0)
			return true;
		children = children[0].getChildren();
		if(children == null || children.length == 0)
			return true;
		final IConfigurationElement config = children[0];
		IEvaluationContext currentState = ((IHandlerService)PlatformUI.getWorkbench().getService(IHandlerService.class)).getCurrentState();
		final EvaluationContext context = new EvaluationContext(currentState, input);
		context.addVariable("input", input); //$NON-NLS-1$
		context.setAllowPluginActivation(true);
		final boolean[] result = new boolean[1];
		SafeRunner.run(new SafeRunnable() {
			@Override
            public void handleException(Throwable e) {
				// Ignore exception
            }
			@Override
			public void run() throws Exception {
				Expression expression = ExpressionConverter.getDefault().perform(config);
				EvaluationResult evaluate = expression.evaluate(context);
				if (evaluate == EvaluationResult.TRUE) {
					result[0] = true;
				}
			}
		});
		return result[0];
	}

	/**
	 * Initialize the column descriptor by reading the attributes from the configuration element.
	 *
	 * @param column The column descriptor to be initialized.
	 * @param configuration The configuration element.
	 * @throws CoreException Thrown during parsing.
	 */
	void initColumn(ColumnDescriptor column, IConfigurationElement configuration) throws CoreException {
		String name = configuration.getAttribute("name"); //$NON-NLS-1$
		Assert.isNotNull(name);
		column.setName(name);
		column.setDescription(configuration.getAttribute("description")); //$NON-NLS-1$
		String attribute = configuration.getAttribute("moveable"); //$NON-NLS-1$
		if (attribute != null) {
			column.setMoveable(Boolean.valueOf(attribute).booleanValue());
		}
		attribute = configuration.getAttribute("resizable"); //$NON-NLS-1$
		if (attribute != null) {
			column.setResizable(Boolean.valueOf(attribute).booleanValue());
		}
		attribute = configuration.getAttribute("visible"); //$NON-NLS-1$
		if (attribute != null) {
			column.setVisible(Boolean.valueOf(attribute).booleanValue());
		}
		attribute = configuration.getAttribute("style"); //$NON-NLS-1$
		if (attribute != null) {
			column.setStyle(parseAlignment(attribute));
		}
		attribute = configuration.getAttribute("alignment"); //$NON-NLS-1$
		if (attribute != null) {
			column.setAlignment(parseAlignment(attribute));
		}
		attribute = configuration.getAttribute("width"); //$NON-NLS-1$
		if (attribute != null) {
			try {
				column.setWidth(Integer.parseInt(attribute));
			}
			catch (NumberFormatException e) {
			}
		}
		attribute = configuration.getAttribute("image"); //$NON-NLS-1$
		if (attribute != null) {
			String symbolicName = configuration.getContributor().getName();
			URL resource = Platform.getBundle(symbolicName).getResource(attribute);
			Image image = ImageDescriptor.createFromURL(resource).createImage();
			column.setImage(image);
		}
		attribute = configuration.getAttribute("labelProvider"); //$NON-NLS-1$
		if (attribute != null) {
			ILabelProvider labelProvider = (ILabelProvider) configuration.createExecutableExtension("labelProvider"); //$NON-NLS-1$
			if (labelProvider != null) {
				column.setLabelProvider(labelProvider);
			}
		}
		attribute = configuration.getAttribute("comparator"); //$NON-NLS-1$
		if (attribute != null) {
			@SuppressWarnings("unchecked")
            Comparator<Object> comparator = (Comparator<Object>) configuration.createExecutableExtension("comparator"); //$NON-NLS-1$
			if (comparator != null) {
				column.setComparator(comparator);
			}
		}
	}

	/**
	 * Parse the alignment attribute from a string to integer value.
	 *
	 * @param attribute The attribute value.
	 * @return The alignment/style value.
	 */
	private int parseAlignment(String attribute) {
		if ("SWT.LEFT".equals(attribute)) { //$NON-NLS-1$
			return SWT.LEFT;
		}
		else if ("SWT.RIGHT".equals(attribute)) { //$NON-NLS-1$
			return SWT.RIGHT;
		}
		else if ("SWT.CENTER".equals(attribute)) { //$NON-NLS-1$
			return SWT.CENTER;
		}
		return SWT.NONE;
	}

	/**
	 * Parse the style name from a string to integer value.
	 *
	 * @param attribute The attribute value.
	 * @return The alignment/style value.
	 */
	private int parseStyleName(String name) {
		if ("SWT.NONE".equals(name)) { //$NON-NLS-1$
			return SWT.NONE;
		}
		else if ("SWT.SINGLE".equals(name)) { //$NON-NLS-1$
			return SWT.SINGLE;
		}
		else if ("SWT.MULTI".equals(name)) { //$NON-NLS-1$
			return SWT.MULTI;
		}
		else if ("SWT.CHECK".equals(name)) { //$NON-NLS-1$
			return SWT.CHECK;
		}
		else if ("SWT.FULL_SELECTION".equals(name)) { //$NON-NLS-1$
			return SWT.FULL_SELECTION;
		}
		else if ("SWT.VIRTUAL".equals(name)) { //$NON-NLS-1$
			return SWT.VIRTUAL;
		}
		else if ("SWT.NO_SCROLL".equals(name)) { //$NON-NLS-1$
			return SWT.NO_SCROLL;
		}
		return SWT.NONE;
	}

	/**
	 * Parse the DND operation style and return a operation.
	 *
	 * @param name The name of the DND operation.
	 * @return an integer that represents the operation.
	 */
	private int parseDndOp(String name) {
		if("DND.DROP_COPY".equals(name)) { //$NON-NLS-1$
			return DND.DROP_COPY;
		}
		else if("DND.DROP_MOVE".equals(name)) { //$NON-NLS-1$
			return DND.DROP_MOVE;
		}
		else if("DND.DROP_LINK".equals(name)) { //$NON-NLS-1$
			return DND.DROP_LINK;
		}
		return 0;
	}

	/**
	 * Parse and calculate the style from the give configuration element.
	 *
	 * @param configuration The configuration element that defines the styles.
	 * @return An integer that represents the defined styles.
	 */
	public int parseStyle(IConfigurationElement configuration) {
		IConfigurationElement[] children = configuration.getChildren();
		int style = SWT.NONE;
		for(IConfigurationElement child : children) {
			String name = child.getAttribute("name"); //$NON-NLS-1$
			style |= parseStyleName(name);
		}
	    return style;
    }

	/**
	 * Parse the DND operation including DROP_COPY,
	 * DROP_MOVE, DROP_LINK
	 *
	 * @param configuration The configuration element in which the DND operation is defined.
	 * @return The operations.
	 */
	public int parseDnd(IConfigurationElement configuration) {
		IConfigurationElement[] children = configuration.getChildren("operations"); //$NON-NLS-1$
		Assert.isTrue(children != null && children.length == 1);
		children = children[0].getChildren();
		int operations = 0;
		for(IConfigurationElement child : children) {
			String name = child.getAttribute("name"); //$NON-NLS-1$
			operations |= parseDndOp(name);
		}
	    return operations;
    }

	/**
	 * Parse the transfer type from the give configuration element.
	 *
	 * @param configuration The configuration element.
	 * @return An array of transfer object that represents the transfer types.
	 */
	public Transfer[] parseTransferTypes(IConfigurationElement configuration) {
		List<Transfer> transferTypes = new ArrayList<Transfer>();
		IConfigurationElement[] children = configuration.getChildren("transferTypes"); //$NON-NLS-1$
		Assert.isTrue(children != null && children.length == 1);
		children = children[0].getChildren();
		for(IConfigurationElement child : children) {
			String name = child.getAttribute("name"); //$NON-NLS-1$
			Transfer transfer = parseTransferType(name);
			if (transfer != null) transferTypes.add(transfer);
		}
	    return transferTypes.toArray(new Transfer[transferTypes.size()]);
    }

	/**
	 * Translate the transfer type from this element.
	 *
	 * @param name The attribute name.
	 * @return The transfer instance.
	 */
	private Transfer parseTransferType(String name) {
		if("TextTransfer".equals(name)) { //$NON-NLS-1$
			return TextTransfer.getInstance();
		}
		if("ImageTransfer".equals(name)) { //$NON-NLS-1$
			return ImageTransfer.getInstance();
		}
		if("FileTransfer".equals(name)) { //$NON-NLS-1$
			return FileTransfer.getInstance();
		}
		if("LocalSelectionTransfer".equals(name)) { //$NON-NLS-1$
			return LocalSelectionTransfer.getTransfer();
		}
	    return null;
    }

	/**
	 * Parse a DragSourceListener and return its instance.
	 *
	 * @param viewer The tree viewer to create an element.
	 * @param configuration The configuration that wraps the instance.
	 * @return The drag source listener created.
	 */
	public DragSourceListener parseDragSourceListener(final TreeViewer viewer, final IConfigurationElement configuration) {
		final AtomicReference<DragSourceListener> reference = new AtomicReference<DragSourceListener>();
		SafeRunner.run(new SafeRunnable(){
			@Override
            public void handleException(Throwable e) {
				// Ignore exception
            }
			@Override
            public void run() throws Exception {
				reference.set((DragSourceListener) createExecutableExtension(DragSourceListener.class, viewer, configuration));
            }});
	    return reference.get();
    }

	/**
	 * Create an executable instance from the given configuration element, with the tree viewer
	 * as the constructor parameter.
	 *
	 * @param aInterface The interface of the element should implement.
	 * @param viewer The tree viewer to be passed
	 * @param configuration The configuration element.
	 * @return The object created.
	 * @throws Exception
	 */
	Object createExecutableExtension(Class<?> aInterface, TreeViewer viewer, IConfigurationElement configuration) throws Exception{
		String classname = configuration.getAttribute("class"); //$NON-NLS-1$
		Assert.isNotNull(classname);
		String contributorId = configuration.getContributor().getName();
		Bundle bundle = Platform.getBundle(contributorId);
		Assert.isNotNull(bundle);
		Class<?> clazz = bundle.loadClass(classname);
		Assert.isTrue(aInterface.isAssignableFrom(clazz));
		try {
			Constructor<?> constructor = clazz.getConstructor(TreeViewer.class);
			return constructor.newInstance(viewer);
		}
		catch (NoSuchMethodException e) {
			Constructor<?> constructor = clazz.getConstructor();
			return constructor.newInstance();
		}
	}

	/**
	 * Parse a DropTargetListener and return its instance.
	 *
	 * @param viewer The tree viewer to create an element.
	 * @param configuration The configuration that wraps the instance.
	 * @return The drop target listener created.
	 */
	public DropTargetListener parseDropTargetListener(final TreeViewer viewer, final IConfigurationElement configuration) {
		final AtomicReference<DropTargetListener> reference = new AtomicReference<DropTargetListener>();
		SafeRunner.run(new SafeRunnable(){
			@Override
            public void handleException(Throwable e) {
				// Ignore exception
            }
			@Override
            public void run() throws Exception {
				reference.set((DropTargetListener) createExecutableExtension(DropTargetListener.class, viewer, configuration));
            }});
	    return reference.get();
   }
}