android AsynTask 实现原理
2013-04-13 13:38 java20130722 阅读(258) 评论(0) 编辑 收藏 举报android AsynTask 实现原理
android asynTask 是借助thread和handler一起结合起来实现的。从其他的Thread操作UI thread 中的view要借组Handler,asynTask封装了这部分的实现,通过ThreadPool实现,doInBackground()方法是在其他的线程中
运行, onPreExecute()、onProgressUpdate(...)和onPostExecute(...)都是运行的UI主线程中,onPreExecute()和其他两个方法还是有点区别的,它是直接在UI
thread 里面运行的,而其他的两个方法是AsynTask里面的IntentHandler里面运行的,从其他的线程里面通过Message将信息传回到主线程中。
从外部启动调用AsyncTask, 通过调用execute方法。
public final AsyncTask<Params, Progress, Result> execute(Params... params) { return executeOnExecutor(sDefaultExecutor, params); }
用指定的参数执行方法, 放回自身对象,以便调用者可以保持对它的引用。
注意:这个方法在任务队列上为一个后台线程或者线程池调度任务。默认情况是在单线程中完成任务的。
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;
}
这个方法必须是在主线程(UI thread)中运行.
这个方法可以用自定义的Executor,达到多个线程同时执行。
1. 方法首先是判断当前任务的状态;
2. 然后执行
onPreExecute()方法, 这个方法是空的, 如果用户重写了该方法,那么一般是些初始化的操作。 3. 然后把params参数传递给Callback类(Class:WorkerRunnable)的成 员变量mParams;这个参数会用到
FuthurTask中。
4.exec默认的情况下其实是SerialExecutor
5.返回自己
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); } } }
public AsyncTask() { mWorker = new WorkerRunnable<Params, Result>() { public Result call() throws Exception { mTaskInvoked.set(true); Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked return postResult(doInBackground(mParams)); } }; 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 occured while executing doInBackground()", e.getCause()); } catch (CancellationException e) { postResultIfNotInvoked(null); } } }; }
exec.execute(mFuture);方法调用的时候,SerialExecutor.execute(Runnable r)会被调用,然后
Executor THREAD_POOL_EXECUTOR=new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
THREAD_POOL_EXECUTOR调用execute(Runnable r), 下面这段代码会触发执行:
new Runnable() { public void run() { try { r.run(); } finally { scheduleNext(); } } }
r.run(); 先调用mWorker.call()方法,在mWorker.call()方法中会触发doInBackground(mParams)方法,然后调用mFuture.done()方法。通过Handler(IntentHandler),
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 result.mTask.finish(result.mData[0]); break; case MESSAGE_POST_PROGRESS: result.mTask.onProgressUpdate(result.mData); break; } } }整个调用过程就结束。
在doInBackground()方法中,如果主动调用了publishProgress(..) ,那么AsyncTask会通过IntentHandler调用onProgressUpdate()方法。