/*******************************************************************************
 * Copyright (c) 2007, 2013 IBM Corporation and others.
 *
 * This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License 2.0
 * which accompanies this distribution, and is available at
 * https://www.eclipse.org/legal/epl-2.0/
 *
 * SPDX-License-Identifier: EPL-2.0
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *     Wind River - Pawel Piech - NPE when closing the Variables view (Bug 213719)
 *******************************************************************************/
package org.eclipse.debug.internal.ui.viewers.model.provisional;

import org.eclipse.debug.internal.ui.viewers.model.ViewerAdapterService;
import org.eclipse.debug.internal.ui.viewers.model.ViewerInputUpdate;

/**
 * Service to compute a viewer input from a source object
 * for a given presentation context.
 * <p>
 * This class may be instantiated, but it not intended to be sub-classed.
 * </p>
 * @since 3.4
 * @noextend This class is not intended to be subclassed by clients.
 */
public class ViewerInputService {

	/**
	 * An input object which will yield a null input element.
	 *
	 * @since 3.6
	 */
	public final static Object NULL_INPUT = (IViewerInputProvider) update -> {
		update.setInputElement(null);
		update.done();
	};

	// previous update request, cancelled when a new request comes in
	private IViewerInputUpdate fPendingUpdate = null;

	private IViewerInputRequestor fRequestor = null;

	private final ITreeModelViewer fViewer;

	private final IViewerInputRequestor fProxyRequest = update -> {
		synchronized (ViewerInputService.this) {
			if (fPendingUpdate == update) {
				fPendingUpdate = null;
			}
		}
		fRequestor.viewerInputComplete(update);
	};

	/**
	 * Constructs a viewer input service for the given requester and presentation context.
	 *
	 * @param viewer for which inputs are required
	 * @param requestor client requesting viewer inputs
	 */
	public ViewerInputService(ITreeModelViewer viewer, IViewerInputRequestor requestor) {
		fRequestor = requestor;
		fViewer = viewer;
	}

	/**
	 * Resolves a viewer input derived from the given source object.
	 * Reports the result to the given this service's requester. A requester may be called back
	 * in the same or thread, or asynchronously in a different thread. Cancels any previous
	 * incomplete request from this service's requester.
	 *
	 * @param source source from which to derive a viewer input
	 */
	public void resolveViewerInput(Object source) {
		IViewerInputProvider provdier = ViewerAdapterService.getInputProvider(source);
		synchronized (this) {
			// cancel any pending update
			if (fPendingUpdate != null) {
				fPendingUpdate.cancel();
			}
			fPendingUpdate = new ViewerInputUpdate(fViewer.getPresentationContext(), fViewer.getInput(), fProxyRequest, source);
		}
		if (provdier == null) {
			fPendingUpdate.setInputElement(source);
			fPendingUpdate.done();
		} else {
			provdier.update(fPendingUpdate);
		}
	}

	/**
	 * Disposes this viewer input service, canceling any pending jobs.
	 */
	public synchronized void dispose() {
		if (fPendingUpdate != null) {
			fPendingUpdate.cancel();
			fPendingUpdate = null;
		}
	}
}
