//初始状态
private volatile Status mStatus = Status.PENDING;
public enum Status {
/**
* Indicates that the task has not been executed yet.
*/
PENDING,
/**
* Indicates that the task is running.
*/
RUNNING,
/**
* Indicates that {@link AsyncTask#onPostExecute} has finished.
*/
FINISHED,
}
/**
* Returns the current status of this task.
*
* @return The current status.
*/
public final Status getStatus() {
return mStatus;
}
public final AsyncTask execute(Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
//如果该任务正在被执行则抛出异常
//值得一提的是,在调用cancel取消任务后,状态仍未RUNNING
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
//如果该任务已经执行完成则抛出异常
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
//改变状态为RUNNING
mStatus = Status.RUNNING;
//调用onPreExecute方法
onPreExecute();
mWorker.mParams = params;
sExecutor.execute(mFuture);
return this;
}
private static final int CORE_POOL_SIZE = 5;
private static final int MAXIMUM_POOL_SIZE = 128;
private static final int KEEP_ALIVE = 10;
//新建一个队列用来存放线程
private static final BlockingQueue sWorkQueue =
new LinkedBlockingQueue(10);
//新建一个线程工厂
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1);
//新建一个线程
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
//新建一个线程池执行器,用于管理线程的执行
private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE,
MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);
/**
* A cancellable asynchronous computation.
* ...
*/
public class FutureTask implements RunnableFuture {
public interface RunnableFuture extends Runnable, Future {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();
}
可以看到FutureTask是一个可以中途取消的用于异步计算的类。
下面是mWorker和mFuture实例在AsyncTask中的体现：
private final WorkerRunnable mWorker;
private final FutureTask mFuture;
public AsyncTask() {
mWorker = new WorkerRunnable() {
//call方法被调用后,将设置优先级为后台级别,然后调用AsyncTask的doInBackground方法
public Result call() throws Exception {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
return doInBackground(mParams);
}
};
//在mFuture实例中,将会调用mWorker做后台任务,完成后会调用done方法
mFuture = new FutureTask(mWorker) {
@Override
protected void done() {
Message message;
Result result = null;
try {
result = get();
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occured while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
//发送取消任务的消息
message = sHandler.obtainMessage(MESSAGE_POST_CANCEL,
new AsyncTaskResult(AsyncTask.this, (Result[]) null));
message.sendToTarget();
return;
} catch (Throwable t) {
throw new RuntimeException("An error occured while executing "
+ "doInBackground()", t);
}
//发送显示结果的消息
message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult(AsyncTask.this, result));
message.sendToTarget();
}
};
}
private static final int MESSAGE_POST_RESULT = 0x1; //显示结果
private static final int MESSAGE_POST_PROGRESS = 0x2; //更新进度
private static final int MESSAGE_POST_CANCEL = 0x3; //取消任务
private static final InternalHandler sHandler = new InternalHandler();
private static class InternalHandler extends Handler {
@SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
@Override
public void handleMessage(Message msg) {
AsyncTaskResult result = (AsyncTaskResult) msg.obj;
switch (msg.what) {
case MESSAGE_POST_RESULT:
// There is only one result
//调用AsyncTask.finish方法
result.mTask.finish(result.mData[0]);
break;
case MESSAGE_POST_PROGRESS:
//调用AsyncTask.onProgressUpdate方法
result.mTask.onProgressUpdate(result.mData);
break;
case MESSAGE_POST_CANCEL:
//调用AsyncTask.onCancelled方法
result.mTask.onCancelled();
break;
}
}
}
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