1 /*
  2  * Copyright (C) 2006 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 android.os;
 18 
 19 /**
 20  * Class that implements the condition variable locking paradigm.
 21  *
 22  * <p>
 23  * This differs from the built-in java.lang.Object wait() and notify()
 24  * in that this class contains the condition to wait on itself.  That means
 25  * open(), close() and block() are sticky.  If open() is called before block(),
 26  * block() will not block, and instead return immediately.
 27  *
 28  * <p>
 29  * This class uses itself as the object to wait on, so if you wait()
 30  * or notify() on a ConditionVariable, the results are undefined.
 31  */
 32 public class ConditionVariable
 33 {
 34     private volatile boolean mCondition;
 35 
 36     /**
 37      * Create the ConditionVariable in the default closed state.
 38      */
 39     public ConditionVariable()
 40     {
 41         mCondition = false;
 42     }
 43 
 44     /**
 45      * Create the ConditionVariable with the given state.
 46      * 
 47      * <p>
 48      * Pass true for opened and false for closed.
 49      */
 50     public ConditionVariable(boolean state)
 51     {
 52         mCondition = state;
 53     }
 54 
 55     /**
 56      * Open the condition, and release all threads that are blocked.
 57      *
 58      * <p>
 59      * Any threads that later approach block() will not block unless close()
 60      * is called.
 61      */
 62     public void open()
 63     {
 64         synchronized (this) {
 65             boolean old = mCondition;
 66             mCondition = true;
 67             if (!old) {
 68                 this.notifyAll();
 69             }
 70         }
 71     }
 72 
 73     /**
 74      * Reset the condition to the closed state.
 75      *
 76      * <p>
 77      * Any threads that call block() will block until someone calls open.
 78      */
 79     public void close()
 80     {
 81         synchronized (this) {
 82             mCondition = false;
 83         }
 84     }
 85 
 86     /**
 87      * Block the current thread until the condition is opened.
 88      *
 89      * <p>
 90      * If the condition is already opened, return immediately.
 91      */
 92     public void block()
 93     {
 94         synchronized (this) {
 95             while (!mCondition) {
 96                 try {
 97                     this.wait();
 98                 }
 99                 catch (InterruptedException e) {
100                 }
101             }
102         }
103     }
104 
105     /**
106      * Block the current thread until the condition is opened or until
107      * timeoutMs milliseconds have passed.
108      *
109      * <p>
110      * If the condition is already opened, return immediately.
111      *
112      * @param timeoutMs the maximum time to wait in milliseconds.
113      *
114      * @return true if the condition was opened, false if the call returns
115      * because of the timeout.
116      */
117     public boolean block(long timeoutMs)
118     {
119         // Object.wait(0) means wait forever, to mimic this, we just
120         // call the other block() method in that case.  It simplifies
121         // this code for the common case.
122         if (timeoutMs != 0) {
123             synchronized (this) {
124                 long now = SystemClock.elapsedRealtime();
125                 long end = now + timeoutMs;
126                 while (!mCondition && now < end) {
127                     try {
128                         this.wait(end-now);
129                     }
130                     catch (InterruptedException e) {
131                     }
132                     now = SystemClock.elapsedRealtime();
133                 }
134                 return mCondition;
135             }
136         } else {
137             this.block();
138             return true;
139         }
140     }
141 }