ConcurrentState.java revision 63204dc989dbd0eba56f65086fde0ebe29ed6bdb 
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.Set;
20import java.util.concurrent.CopyOnWriteArraySet;
21import java.util.concurrent.Executor;
22
23import javax.annotation.CheckReturnValue;
24import javax.annotation.Nonnull;
25import javax.annotation.ParametersAreNonnullByDefault;
26import javax.annotation.concurrent.GuardedBy;
27
28/**
29 * Generic asynchronous state wrapper which supports two methods of interaction:
30 * polling for the latest value and listening for updates.
31 */
32@ParametersAreNonnullByDefault
33public class ConcurrentState<T> implements Updatable<T>, Observable<T> {
34    private static class ExecutorListenerPair implements Runnable {
35        private final Executor mExecutor;
36        private final Runnable mListener;
37
38        public ExecutorListenerPair(Executor executor, Runnable listener) {
39            mExecutor = executor;
40            mListener = listener;
41        }
42
43        /**
44         * Runs the callback on the executor.
45         */
46        @Override
47        public void run() {
48            mExecutor.execute(mListener);
49        }
50    }
51
52    private final Set<ExecutorListenerPair> mListeners;
53    private volatile T mValue;
54
55    public ConcurrentState(T initialValue) {
56        // Callbacks are typically only added and removed at startup/shutdown,
57        // but {@link #update} is often called at high-frequency. So, using a
58        // read-optimized data structure is appropriate here.
59        mListeners = new CopyOnWriteArraySet<>();
60        mValue = initialValue;
61    }
62
63    /**
64     * Updates the state to the latest value, notifying all listeners.
65     */
66    @Override
67    public void update(T newValue) {
68        mValue = newValue;
69        for (ExecutorListenerPair pair : mListeners) {
70            pair.run();
71        }
72    }
73
74    @CheckReturnValue
75    @Nonnull
76    @Override
77    public SafeCloseable addCallback(Runnable callback, Executor executor) {
78        final ExecutorListenerPair pair = new ExecutorListenerPair(executor, callback);
79        mListeners.add(pair);
80        return new SafeCloseable() {
81            @Override
82            public void close() {
83                mListeners.remove(pair);
84            }
85        };
86    }
87
88    /**
89     * Polls for the latest value.
90     *
91     * @return The latest state.
92     */
93    @Nonnull
94    @Override
95    public T get() {
96        return mValue;
97    }
98}
99