bundles/org.eclipse.ui.workbench/Eclipse UI/org/eclipse/ui/internal/services/EvaluationResultCache.java

/*******************************************************************************
 * Copyright (c) 2005, 2008 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.services;

import org.eclipse.core.expressions.EvaluationResult;
import org.eclipse.core.expressions.Expression;
import org.eclipse.core.expressions.IEvaluationContext;
import org.eclipse.core.runtime.CoreException;
import org.eclipse.ui.ISources;

/**
 * <p>
 * A token representing the activation or contribution of some expression-based
 * element. This caches the evaluation result so that it is only re-computed as
 * necessary.
 * </p>
 * 
 * @since 3.2
 */
public abstract class EvaluationResultCache implements IEvaluationResultCache {

	/**
	 * The previous computed evaluation result. If no evaluation result is
	 * available, then this value is <code>null</code>.
	 */
	private EvaluationResult evaluationResult = null;

	/**
	 * The expression to evaluate. This value may be <code>null</code>, in
	 * which case the evaluation result is always <code>true</code>.
	 */
	private final Expression expression;

	/**
	 * The priority that has been given to this expression.
	 */
	private final int sourcePriority;

	/**
	 * Constructs a new instance of <code>EvaluationResultCache</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.
	 * @see ISources
	 */
	protected EvaluationResultCache(final Expression expression) {
		this.expression = expression;
		this.sourcePriority = SourcePriorityNameMapping
				.computeSourcePriority(expression);
	}

	public final void clearResult() {
		evaluationResult = null;
	}

	public final boolean evaluate(final IEvaluationContext context) {
		if (expression == null) {
			return true;
		}

		if (evaluationResult == null) {
			try {
				evaluationResult = expression.evaluate(context);
			} catch (final CoreException e) {
				/*
				 * Swallow the exception. It simply means the variable is not
				 * valid it some (most frequently, that the value is null). This
				 * kind of information is not really useful to us, so we can
				 * just treat it as null.
				 */
				evaluationResult = EvaluationResult.FALSE;
				return false;
			}
		}

		// return true if  the result is FALSE or NOT_LOADED
		return evaluationResult != EvaluationResult.FALSE;
	}

	public final Expression getExpression() {
		return expression;
	}

	public final int getSourcePriority() {
		return sourcePriority;
	}

	public final void setResult(final boolean result) {
		if (result) {
			evaluationResult = EvaluationResult.TRUE;
		} else {
			evaluationResult = EvaluationResult.FALSE;
		}
	}
}