AsyncTask用来异步下载一些下载时间短的任务。为什么只适合异步下载适合下载时间短的任务,因为他是串行的,只有一个任务完成后才会进行下一个任务。 AsyncTask四个常用的方法 execute供外界调用的执行后台下载方法 doInbackground在后台进行异步下载操作的方法, onPreExecute下载前做准备的方法。 onProgressUpdate进度更新方法,在UI线程(Handler的handlerMessage)中被调用。 onPostResult下载完成执行UI更新的方法。 在分析这四个方法之前,先看下AsyncTask有哪些成员。 public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static final int MESSAGE_POST_RESULT = 0x1; private static final int MESSAGE_POST_PROGRESS = 0x2;
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR; private static InternalHandler sHandler;
private final WorkerRunnable<Params, Result> mWorker; private final FutureTask<Result> mFuture; AsyncTask有两个方式执行下载任务,一个是SerialExecutor对象SERIAL_EXECUTOR,另一个是线程池THREAD_POOL_EXECUTOR。AsyncTask提供的execute方法默认使用SERIAL_EXECUTOR下载。实际上都是使用线程池下载,看SerialExecutor类 @MainThread public static void execute(Runnable runnable) { sDefaultExecutor.execute(runnable); } private static class SerialExecutor implements Executor { final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>(); Runnable mActive;
public synchronized void execute(final Runnable r) { mTasks.offer(new Runnable() { public void run() { try { r.run(); } finally { scheduleNext(); } } }); if (mActive == null) { scheduleNext(); } }
protected synchronized void scheduleNext() { if ((mActive = mTasks.poll()) != null) { THREAD_POOL_EXECUTOR.execute(mActive); } } } 可以看见把下载任务添加到mTask后。当mActive为空即当前没有下载任务时,调用scheduleNext方法,线程池的execute方法执行所创建的Runnable任务。参数Runable r在新new出来的Runbale中被调用其run方法(这个run方法中调用doInBackground方法),除此之外,再调用sheduleNext方法,保证mTask的任务能被连续调用。只要mTask中还有任务的话。offer方法把new出来的runable方法加到task。当有多个execute方法被调用。就能保证每个任务都会被执行,这里采用递归调用方法sheduleNext而不是采用循环,设计得很好。保证了每个任务都是串行执行。 MESSAGE_POST_RESULT和MESSAGE_POST_PROGRESS分别是handler处理信息的判别码。sHandler是处理UI的handler。 private static class InternalHandler extends Handler { public InternalHandler() { super(Looper.getMainLooper()); }
@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 result.mTask.finish(result.mData[0]); break; case MESSAGE_POST_PROGRESS: result.mTask.onProgressUpdate(result.mData); break; } } } protected final void publishProgress(Progress... values) { if (!isCancelled()) { getHandler().obtainMessage(MESSAGE_POST_PROGRESS, new AsyncTaskResult<Progress>(this, values)).sendToTarget(); } } onProgressUpdate在Handler中被调用更新进度,postResult则在finish被调用处理下载完成的逻辑。并更新状态为finish。 private void finish(Result result) { if (isCancelled()) { onCancelled(result); } else { onPostExecute(result); } mStatus = Status.FINISHED; } WorkRunable是继承Call接口的接口,并保存参数,构建FutureTask。FutrueTask的run方法是线程池实际执行的接口方法,在在AsyncTask的构造函数中构建 private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> { Params[] mParams; }
public AsyncTask() { mWorker = new WorkerRunnable<Params, Result>() { public Result call() throws Exception { mTaskInvoked.set(true);
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked Result result = doInBackground(mParams); Binder.flushPendingCommands(); return postResult(result); } };
mFuture = new FutureTask<Result>(mWorker) { @Override protected void done() { try { postResultIfNotInvoked(get()); } catch (InterruptedException e) { android.util.Log.w(LOG_TAG, e); } catch (ExecutionException e) { throw new RuntimeException("An error occurred while executing doInBackground()", e.getCause()); } catch (CancellationException e) { postResultIfNotInvoked(null); } } }; }
可以看到call方法调用了doInBackground方法,暴露给外界的后台下载方法,call方法会在mFuture的run方法被调用,done方法是run方法调用后调用,如果call方法中mTaskInvoked没被赋值,就会通过postResultIfNotInvoked再次发给Handler消息postResult方法是向handler发送消息,postResultIfNotInvoked方法也调用了postResult方法。 private void postResultIfNotInvoked(Result result) { final boolean wasTaskInvoked = mTaskInvoked.get(); if (!wasTaskInvoked) { postResult(result); } }
private Result postResult(Result result) { @SuppressWarnings("unchecked") Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult<Result>(this, result)); message.sendToTarget(); return result; } 提供给外界执行的一个是execute方法一个是executeOnExecutor方法,execute间接调用了executeOnExecutor方法,不过execute是采用SerialExecutor来执行任务,executeOnExecutor直接采用ThreadPoolExecutor来执行任务。 @MainThread public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec, Params... params) { if (mStatus != Status.PENDING) { switch (mStatus) { case RUNNING: throw new IllegalStateException("Cannot execute task:" + " the task is already running."); case FINISHED: throw new IllegalStateException("Cannot execute task:" + " the task has already been executed " + "(a task can be executed only once)"); } }
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params; exec.execute(mFuture);
return this; } @MainThread public final AsyncTask<Params, Progress, Result> execute(Params... params) { return executeOnExecutor(sDefaultExecutor, params); } 可以看见无论是execute方法还是executeOnExecutor方法都需要判断mStatus是否为Pending状态,Pending状态才执行SerialExecutor的execute方法或ThreadPool的execute方法。调用后只是将mStatus设置为Finish状态。所以一个AsyncTask只能执行一次execute方法。而SerialExecutor和ThreadPoollExecutor是静态的被所有对象共享。通过不断new asyncTask,添加任务到SerialExecutor或ThreadPoolExecutor中。 Params参数设置给WorkRunable保存的mParams。doInBackground用到这些参数。doInBackground调用后得到result再包装成AsyncTaskResult通过postResult方法发给Handler。
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