jpa/plugins/org.eclipse.jpt.utility/src/org/eclipse/jpt/utility/internal/model/value/CachingTransformationPropertyValueModel.java

/*******************************************************************************
 * Copyright (c) 2008 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.utility.internal.model.value;

import org.eclipse.jpt.utility.internal.Transformer;
import org.eclipse.jpt.utility.model.value.PropertyValueModel;

/**
 * A <code>CachingTransformationPropertyValueModel</code> wraps another
 * <code>PropertyValueModel</code> and uses a <code>Transformer</code>
 * to transform the wrapped value before it is returned by <code>getValue()</code>.
 * The transformed value is calculated and cached during initialization and every
 * time the wrapped value changes. This can be useful when the old value
 * passed in to <code>valueChanged(PropertyChangeEvent)</code> can no longer
 * be "transformed" because its state is no longer valid.
 * This caching can also improve time performance in some situations.
 * <p>
 * As an alternative to building a <code>Transformer</code>,
 * a subclass of <code>CachingTransformationPropertyValueModel</code> can
 * either override the <code>transform_(Object)</code> method or,
 * if something other than null should be returned when the wrapped value
 * is null, override the <code>transform(Object)</code> method.
 */
public class CachingTransformationPropertyValueModel<T1, T2>
	extends TransformationPropertyValueModel<T1, T2>
{

	/**
	 * Cache the transformed value so that during property change event notification
	 * we do not have to transform the old value. The old value could no longer be valid in
	 * the model; as a result, transforming it would not be valid.
	 */
	protected T2 cachedValue;


	// ********** constructors/initialization **********

	/**
	 * Construct a property value model with the specified nested
	 * property value model and the default transformer.
	 * Use this constructor if you want to override the
	 * <code>transform_(Object)</code> or <code>transform(Object)</code>
	 * method instead of building a <code>Transformer</code>.
	 */
	public CachingTransformationPropertyValueModel(PropertyValueModel<? extends T1> valueHolder) {
		super(valueHolder);
	}

	/**
	 * Construct an property value model with the specified nested
	 * property value model and transformer.
	 */
	public CachingTransformationPropertyValueModel(PropertyValueModel<? extends T1> valueHolder, Transformer<T1, T2> transformer) {
		super(valueHolder, transformer);
	}


	// ********** behavior **********

	/**
	 * We have listeners, transform the nested value and cache the result.
	 */
	@Override
	protected void engageValueHolder() {
		super.engageValueHolder();
		this.cachedValue = this.transform(this.valueHolder.getValue());
	}

	/**
	 * We have no more listeners, clear the cached value.
	 */
	@Override
	protected void disengageValueHolder() {
		this.cachedValue = null;
		super.disengageValueHolder();
	}

	/**
	 * No need to transform the nested value, simply return the cached value,
	 * which is already transformed.
	 */
	@Override
	public T2 getValue() {
		return this.cachedValue;
	}

	/**
	 * Transform the specified new value, caching it before returning it.
	 */
	@Override
	protected T2 transformNew(T1 value) {
		this.cachedValue = super.transformNew(value);
		return this.cachedValue;
	}

	/**
	 * No need to transform the old value, simply return the cached value,
	 * which is already transformed.
	 */
	@Override
	protected T2 transformOld(T1 value) {
		return this.cachedValue;
	}

}