/*******************************************************************************
 * 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.common.utility.internal.model.value;

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

/**
 * A <code>CachingTransformationPropertyValueModel</code> wraps another
 * {@link PropertyValueModel} and uses a {@link Transformer}
 * to transform the wrapped value before it is returned by {@link #getValue()}.
 * 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 {@link #valueChanged(org.eclipse.jpt.common.utility.model.event.PropertyChangeEvent)}
 * can no longer be "transformed" because its state is no longer valid.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 {@link Transformer},
 * a subclass of <code>CachingTransformationPropertyValueModel</code> can
 * either override {@link #transform_(Object)} or,
 * if something other than null should be returned when the wrapped value
 * is null, override {@link #transform(Object)}.
 */
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
	 * {@link #transform_(Object)} or {@link #transform(Object)}
	 * instead of building a {@link Transformer}.
	 */
	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 engageModel() {
		super.engageModel();
		this.cachedValue = this.transform(this.valueHolder.getValue());
	}

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

	/**
	 * 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.
	 * A bit of a side-effect, but it seems reasonable.
	 */
	@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;
	}

}