/*******************************************************************************
 * Copyright (c) 2005, 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.contexts;

import org.eclipse.core.expressions.Expression;
import org.eclipse.core.expressions.IEvaluationContext;
import org.eclipse.ui.ISources;
import org.eclipse.ui.contexts.IContextActivation;
import org.eclipse.ui.contexts.IContextService;
import org.eclipse.ui.internal.services.EvaluationResultCache;

/**
 * <p>
 * A token representing the activation of a context. This token can later be
 * used to cancel that activation. Without this token, then the context will
 * only become inactive if the component in which the context was activated is
 * destroyed.
 * </p>
 * <p>
 * This caches the context id, so that they can later be identified.
 * </p>
 * <p>
 * Note: this class has a natural ordering that is inconsistent with equals.
 * </p>
 * 
 * @since 3.1
 */
final class ContextActivation extends EvaluationResultCache implements
		IContextActivation {

	/**
	 * The identifier for the context which should be active. This value is
	 * never <code>null</code>.
	 */
	private final String contextId;

	/**
	 * The context service from which this context activation was requested.
	 * This value is never <code>null</code>.
	 */
	private final IContextService contextService;

	/**
	 * Constructs a new instance of <code>ContextActivation</code>.
	 * 
	 * @param contextId
	 *            The identifier for the context which should be activated. This
	 *            value must not be <code>null</code>.
	 * @param expression
	 *            The expression that must evaluate to <code>true</code>
	 *            before this handler is active. This value may be
	 *            <code>null</code> if it is always active.
	 * @param contextService
	 *            The context service from which the handler activation was
	 *            requested; must not be <code>null</code>.
	 * @see ISources
	 */
	public ContextActivation(final String contextId,
			final Expression expression, final IContextService contextService) {
		super(expression);

		if (contextId == null) {
			throw new NullPointerException(
					"The context identifier for a context activation cannot be null"); //$NON-NLS-1$
		}

		if (contextService == null) {
			throw new NullPointerException(
					"The context service for an activation cannot be null"); //$NON-NLS-1$
		}

		this.contextId = contextId;
		this.contextService = contextService;
	}

	public final void clearActive() {
		clearResult();
	}

	public final String getContextId() {
		return contextId;
	}

	public final IContextService getContextService() {
		return contextService;
	}

	public final boolean isActive(final IEvaluationContext context) {
		return evaluate(context);
	}

	public final String toString() {
		final StringBuffer buffer = new StringBuffer();

		buffer.append("ContextActivation(contextId="); //$NON-NLS-1$
		buffer.append(contextId);
		buffer.append(",sourcePriority="); //$NON-NLS-1$
		buffer.append(getSourcePriority());
		buffer.append(')');

		return buffer.toString();
	}

}