Home | History | Annotate | Download | only in media 
      1 /*
      2  * Copyright (C) 2009 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.cooliris.media;
     18 
     19 import java.util.concurrent.CancellationException;
     20 import java.util.concurrent.ExecutionException;
     21 
     22 /**
     23  * An abstract class for the interface <code>Cancelable</code>. Subclass can
     24  * simply override the <code>execute()</code> function to provide an
     25  * implementation of <code>Cancelable</code>.
     26  */
     27 public abstract class BaseCancelable<T> implements Cancelable<T> {
     28 
     29     /**
     30      * The state of the task, possible transitions are:
     31      *
     32      * <pre>
     33      *     INITIAL -&gt; CANCELED
     34      *     EXECUTING -&gt; COMPLETE, CANCELING, ERROR, CANCELED
     35      *     CANCELING -&gt; CANCELED
     36      * </pre>
     37      *
     38      * When the task stop, it must be end with one of the following states:
     39      * COMPLETE, CANCELED, or ERROR;
     40      */
     41     private static final int STATE_INITIAL = (1 << 0);
     42     private static final int STATE_EXECUTING = (1 << 1);
     43     private static final int STATE_CANCELING = (1 << 2);
     44     private static final int STATE_CANCELED = (1 << 3);
     45     private static final int STATE_ERROR = (1 << 4);
     46     private static final int STATE_COMPLETE = (1 << 5);
     47 
     48     private int mState = STATE_INITIAL;
     49 
     50     private Throwable mError;
     51     private T mResult;
     52     private Cancelable<?> mCurrentTask;
     53 
     54     protected abstract T execute() throws Exception;
     55 
     56     /**
     57      * Frees the result (which is not null) when the task has been canceled.
     58      */
     59     protected void freeCanceledResult(T result) {
     60         // Do nothing by default;
     61     }
     62 
     63     private boolean isInStates(int states) {
     64         return (states & mState) != 0;
     65     }
     66 
     67     private T handleTerminalStates() throws ExecutionException {
     68         if (mState == STATE_CANCELED) {
     69             throw new CancellationException();
     70         }
     71         if (mState == STATE_ERROR) {
     72             throw new ExecutionException(mError);
     73         }
     74         if (mState == STATE_COMPLETE)
     75             return mResult;
     76         throw new IllegalStateException();
     77     }
     78 
     79     public synchronized void await() throws InterruptedException {
     80         while (!isInStates(STATE_COMPLETE | STATE_CANCELED | STATE_ERROR)) {
     81             wait();
     82         }
     83     }
     84 
     85     public final T get() throws InterruptedException, ExecutionException {
     86         synchronized (this) {
     87             if (mState != STATE_INITIAL) {
     88                 await();
     89                 return handleTerminalStates();
     90             }
     91             mState = STATE_EXECUTING;
     92         }
     93         try {
     94             mResult = execute();
     95         } catch (CancellationException e) {
     96             mState = STATE_CANCELED;
     97         } catch (InterruptedException e) {
     98             mState = STATE_CANCELED;
     99         } catch (Throwable error) {
    100             synchronized (this) {
    101                 if (mState != STATE_CANCELING) {
    102                     mError = error;
    103                     mState = STATE_ERROR;
    104                 }
    105             }
    106         }
    107         synchronized (this) {
    108             if (mState == STATE_CANCELING)
    109                 mState = STATE_CANCELED;
    110             if (mState == STATE_EXECUTING)
    111                 mState = STATE_COMPLETE;
    112             notifyAll();
    113             if (mState == STATE_CANCELED && mResult != null) {
    114                 freeCanceledResult(mResult);
    115             }
    116             return handleTerminalStates();
    117         }
    118     }
    119 
    120     /**
    121      * Requests the task to be canceled.
    122      *
    123      * @return true if the task is running and has not been canceled; false
    124      *         otherwise
    125      */
    126 
    127     public synchronized boolean requestCancel() {
    128         if (mState == STATE_INITIAL) {
    129             mState = STATE_CANCELED;
    130             notifyAll();
    131             return false;
    132         }
    133         if (mState == STATE_EXECUTING) {
    134             if (mCurrentTask != null)
    135                 mCurrentTask.requestCancel();
    136             mState = STATE_CANCELING;
    137             return true;
    138         }
    139         return false;
    140     }
    141 }
    142