001// Copyright 2013 The Apache Software Foundation
002//
003// Licensed under the Apache License, Version 2.0 (the "License");
004// you may not use this file except in compliance with the License.
005// You may obtain a copy of the License at
006//
007//     http:#www.apache.org/licenses/LICENSE-2.0
008//
009// Unless required by applicable law or agreed to in writing, software
010// distributed under the License is distributed on an "AS IS" BASIS,
011// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
012// See the License for the specific language governing permissions and
013// limitations under the License.
014
015package org.apache.tapestry5.internal.util;
016
017import org.apache.tapestry5.commons.services.InvalidationEventHub;
018import org.apache.tapestry5.ioc.Invokable;
019
020/**
021 * A utility class for managing a cacheable result that can be recomputed when needed.
022 */
023public class RecomputableSupport
024{
025    private volatile int masterVersion = 1;
026
027    /**
028     * Invalidates any existing {@link #create(org.apache.tapestry5.ioc.Invokable)} wrappers} such that they will
029     * re-perform the computation when next invoked.
030     */
031    public void invalidate()
032    {
033        masterVersion++;
034    }
035
036    /**
037     * Forces {@link #invalidate()} to be invoked when the hub emits an {@linkplain InvalidationEventHub#addInvalidationCallback(Runnable) invalidation callback}.
038     *
039     * @param hub
040     */
041    public void initialize(InvalidationEventHub hub)
042    {
043        hub.addInvalidationCallback(new Runnable()
044        {
045            public void run()
046            {
047                invalidate();
048            }
049        });
050    }
051
052    /**
053     * Wraps a computation with caching logic; once computed, the Invokable will return the same value, until
054     * {@link #invalidate()} is invoked.
055     *
056     * @param invokable
057     *         a computation to perform, whose results are cacheable until invalidated
058     * @param <T>
059     *         type of result
060     * @return caching-enabled wrapper around invokable
061     */
062    public <T> Invokable<T> create(final Invokable<T> invokable)
063    {
064        return new Invokable<T>()
065        {
066            private volatile int localVersion = masterVersion;
067
068            private volatile T cachedResult;
069
070            public T invoke()
071            {
072                // Has the master version changed since the computation was last executed?
073                if (localVersion != masterVersion)
074                {
075                    cachedResult = null;
076                    localVersion = masterVersion;
077                }
078
079                if (cachedResult == null)
080                {
081                    cachedResult = invokable.invoke();
082                }
083
084
085                return cachedResult;
086            }
087        };
088    }
089}