/*******************************************************************************
 * Copyright (c) 2008, 2009 Oracle. 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:
 *     Oracle - initial API and implementation
 ******************************************************************************/
package org.eclipse.jpt.core.internal.utility;

import java.io.Serializable;

import org.eclipse.core.runtime.IProgressMonitor;
import org.eclipse.core.runtime.IStatus;
import org.eclipse.core.runtime.Status;

/**
 * Simple interface for implementing the GOF Command design pattern in an
 * Eclipse job.
 * <p>
 * Provisional API: This interface is part of an interim API that is still
 * under development and expected to change significantly before reaching
 * stability. It is available at this early stage to solicit feedback from
 * pioneering adopters on the understanding that any code that uses this API
 * will almost certainly be broken (repeatedly) as the API evolves.
 */
public interface JobCommand {

	/**
	 * Execute the command. The semantics of the command
	 * is determined by the contract between the client and server.
	 * The command should check, as appropriate, whether the specified progress
	 * monitor is {@link IProgressMonitor#isCanceled() "canceled"}; if it is,
	 * the command should throw an
	 * {@link org.eclipse.core.runtime.OperationCanceledException OperationCanceledException}.
	 * @see IProgressMonitor#isCanceled()
	 */
	IStatus execute(IProgressMonitor monitor);

	/**
	 * Singleton implementation of the job command interface that will do
	 * nothing when executed.
	 */
	final class Null implements JobCommand, Serializable {
		public static final JobCommand INSTANCE = new Null();
		public static JobCommand instance() {
			return INSTANCE;
		}
		// ensure single instance
		private Null() {
			super();
		}
		public IStatus execute(IProgressMonitor arg0) {
			return Status.OK_STATUS;
		}
		@Override
		public String toString() {
			return "JobCommand.Null"; //$NON-NLS-1$
		}
		private static final long serialVersionUID = 1L;
		private Object readResolve() {
			// replace this object with the singleton
			return INSTANCE;
		}
	}

	/**
	 * Singleton implementation of the job command interface that will throw an
	 * exception when executed.
	 */
	final class Disabled implements JobCommand, Serializable {
		public static final JobCommand INSTANCE = new Disabled();
		public static JobCommand instance() {
			return INSTANCE;
		}
		// ensure single instance
		private Disabled() {
			super();
		}
		// throw an exception
		public IStatus execute(IProgressMonitor arg0) {
			throw new UnsupportedOperationException();
		}
		@Override
		public String toString() {
			return "JobCommand.Disabled"; //$NON-NLS-1$
		}
		private static final long serialVersionUID = 1L;
		private Object readResolve() {
			// replace this object with the singleton
			return INSTANCE;
		}
	}

}