ConcurrentState.java revision 9ad0984f36ff9cd133c61c4e979032988b77a995 
1/*
2 * Copyright (C) 2014 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package com.android.camera.async;
18
19import java.util.ArrayList;
20import java.util.HashSet;
21import java.util.List;
22import java.util.Set;
23import java.util.concurrent.Executor;
24
25import com.android.camera.util.Callback;
26
27/**
28 * Generic asynchronous state wrapper which supports two methods of interaction:
29 * polling for the latest value and listening for updates.
30 * <p>
31 * Note that this class only supports polling and using listeners. If
32 * synchronous consumption of state changes is required, see
33 * {@link FutureResult} or {@link BufferQueue} and its implementations.
34 * </p>
35 */
36public class ConcurrentState<T> implements Updatable<T>, Observable<T> {
37
38    private static class ExecutorListenerPair<T> {
39        private final Executor mExecutor;
40        private final Callback<T> mListener;
41
42        public ExecutorListenerPair(Executor executor, Callback<T> listener) {
43            mExecutor = executor;
44            mListener = listener;
45        }
46
47        /**
48         * Runs the callback on the executor with the given value.
49         */
50        public void run(final T t) {
51            mExecutor.execute(new Runnable() {
52                public void run() {
53                    mListener.onCallback(t);
54                }
55            });
56        }
57    }
58
59    private final Object mLock;
60    private final Set<ExecutorListenerPair<T>> mListeners;
61    private T mValue;
62
63    public ConcurrentState(T initialValue) {
64        mLock = new Object();
65        mListeners = new HashSet<>();
66        mValue = initialValue;
67    }
68
69    /**
70     * Updates the state to the latest value, notifying all listeners.
71     */
72    @Override
73    public void update(T newValue) {
74        List<ExecutorListenerPair<T>> listeners = new ArrayList<>();
75        synchronized (mLock) {
76            mValue = newValue;
77            // Copy listeners out here so we can iterate over the list outside
78            // the critical section.
79            listeners.addAll(mListeners);
80        }
81        for (ExecutorListenerPair<T> pair : listeners) {
82            pair.run(newValue);
83        }
84    }
85
86    @Override
87    public SafeCloseable addCallback(Callback callback, Executor executor) {
88        synchronized (mLock) {
89            final ExecutorListenerPair<T> pair = new ExecutorListenerPair<>(executor, callback);
90            mListeners.add(pair);
91
92            return new SafeCloseable() {
93                @Override
94                public void close() {
95                    synchronized (mLock) {
96                        mListeners.remove(pair);
97                    }
98                }
99            };
100        }
101    }
102
103    /**
104     * Polls for the latest value.
105     *
106     * @return The latest state.
107     */
108    @Override
109    public T get() {
110        synchronized (mLock) {
111            return mValue;
112        }
113    }
114}
115