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 17 package com.android.camera.async; 18 19 import java.util.Set; 20 import java.util.concurrent.CopyOnWriteArraySet; 21 import java.util.concurrent.Executor; 22 23 import javax.annotation.CheckReturnValue; 24 import javax.annotation.Nonnull; 25 import 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 32 public 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 ExecutorListenerPair(Executor executor, Runnable listener)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 run()46 public void run() { 47 mExecutor.execute(mListener); 48 } 49 } 50 51 private final Set<ExecutorListenerPair> mListeners; 52 private volatile T mValue; 53 ConcurrentState(T initialValue)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 update(T newValue)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 addCallback(Runnable callback, Executor executor)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