AsyncTask工作原理

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

@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();             //首先执行onPreExecute()
    mWorker.mParams = params;   //将参数封装为mWorker的相关参数
    exec.execute(mFuture);      //串行执行Task
    return this;
}

public static final Executor SERIAL_EXECUTOR = new SerialExecutor();

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() {   //将FutureTask对象插入到任务队列中
            public void run() {
                try {
                    r.run();
                } finally {
                    scheduleNext();
                }
            }
        });
        if (mActive == null) { //如果没有正在活动的任务，那么就调用scheduleNext()来执行下一个任务
            scheduleNext();
        }
    }

    protected synchronized void scheduleNext() {
        if ((mActive = mTasks.poll()) != null) {
            THREAD_POOL_EXECUTOR.execute(mActive);
        }
    }
}

mWorker = new WorkerRunnable<Params, Result>() {
    //FutureTask的run方法会调用mWorker的call()方法，因此mWorker的call方法最终会在线程池中执行。
    public Result call() throws Exception {
        mTaskInvoked.set(true); //当前任务已经被调用
        Result result = null;   //
        try {
            Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
            //noinspection unchecked
            result = doInBackground(mParams);
            Binder.flushPendingCommands();
        } catch (Throwable tr) {
            mCancelled.set(true);
            throw tr;
        } finally {
            postResult(result);
        }
        return result;
    }
};