ConcurrentState.java revision e606c4d68b74293e7d7725aecbaa9c915751cd43 
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;
26
27/**
28 * Generic asynchronous state wrapper which supports two methods of interaction:
29 * polling for the latest value and listening for updates.
30 */
31@ParametersAreNonnullByDefault
32public class ConcurrentState<T> implements Updatable<T>, Observable<T> {
33    private static class ExecutorListenerPair implements Runnable {
34        private final Executor mExecutor;
35        private final Runnable mListener;
36
37        public ExecutorListenerPair(Executor executor, Runnable listener) {
38            mExecutor = executor;
39            mListener = listener;
40        }
41
42        /**
43         * Runs the callback on the executor.
44         */
45        @Override
46        public void run() {
47            mExecutor.execute(mListener);
48        }
49    }
50
51    private final Set<ExecutorListenerPair> mListeners;
52    private volatile T mValue;
53
54    public ConcurrentState(T initialValue) {
55        // Callbacks are typically only added and removed at startup/shutdown,
56        // but {@link #update} is often called at high-frequency. So, using a
57        // read-optimized data structure is appropriate here.
58        mListeners = new CopyOnWriteArraySet<>();
59        mValue = initialValue;
60    }
61
62    /**
63     * Updates the state to the latest value, notifying all listeners.
64     */
65    @Override
66    public void update(T newValue) {
67        mValue = newValue;
68        for (ExecutorListenerPair pair : mListeners) {
69            pair.run();
70        }
71    }
72
73    @CheckReturnValue
74    @Nonnull
75    @Override
76    public SafeCloseable addCallback(Runnable callback, Executor executor) {
77        final ExecutorListenerPair pair = new ExecutorListenerPair(executor, callback);
78        mListeners.add(pair);
79        return new SafeCloseable() {
80            @Override
81            public void close() {
82                mListeners.remove(pair);
83            }
84        };
85    }
86
87    /**
88     * Polls for the latest value.
89     *
90     * @return The latest state.
91     */
92    @Nonnull
93    @Override
94    public T get() {
95        return mValue;
96    }
97}
98