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      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 
     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