AsyncTask的原理分析

先看一张流程图

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public () { mWorker = new WorkerRunnable<Params, Result>() { public Result call() throws Exception { mTaskInvoked.set(true); Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked //后台执行任务 Result result = doInBackground(mParams); Binder.flushPendingCommands(); //postResult方法中会使用Handler把执行结果抛到主线程 return postResult(result); } }; //把上述执行体mWorker传进来 mFuture = new FutureTask<Result>(mWorker) { protected void done() { postResultIfNotInvoked(get()); } }; }

这就是new AsyncTask()所做的操作，可以想到肯定有一个地方执行了mFeture，然后调用了call方法回到WorkRunnable中开始任务的执行。

1 2	public static final Executor SERIAL_EXECUTOR = new SerialExecutor(); private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

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private static class SerialExecutor implements Executor { final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>(); Runnable mActive; //右下边的分析可以知道，这里的 r 就是mFuture实例 public synchronized void execute(final Runnable r) { mTasks.offer(new Runnable() { 大专栏  AsyncTask的原理分析pan class="comment">//mActive的run方法 public void run() { try { //线程会走到这里执行真正的执行体，也就是mFuture的run方法 //mFuture是一个FutureTask类型，在执行Task的run时会调用call方法 //result = c.call()这个c就是上述的mWorker实例 r.run(); } finally { scheduleNext(); } } }); if (mActive == null) { scheduleNext(); } } protected synchronized void scheduleNext() { //执行体出队-把上述的mTasks.offer的那个Runnable赋值为mActive if ((mActive = mTasks.poll()) != null) { //直到这里才开始真正执行任务，会执行mActive的run方法 THREAD_POOL_EXECUTOR.execute(mActive); } } }

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public final AsyncTask<Params, Progress, Result> execute(Params... params) { // return executeOnExecutor(sDefaultExecutor, params); }

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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)"); } } //置为运行状态 mStatus = Status.RUNNING; onPreExecute(); mWorker.mParams = params; /** * exec就是sDefaultExecutor，这里执行sDefaultExecutor也就是SerialExecutor的execute方法 * mFeture就是在构造方法里边初始化的那个FutureTask实例 */ exec.execute(mFuture); return this; }