AsyncTask原理分析

AsyncTask在安卓中常用于线程间通信。在子线程执行耗时任务，在主线程更新UI。

AsyncTask内部封装了Handler和线程池。

1、AsyncTask主要靠InternalHandler去跟主线程通信，InternalHandler继承自Handler类，在handleMessage方法里面处理了消息。

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;
            }
        }
    }

2、有两个线程池THREAD_POOL_EXECUTOR和SERIAL_EXECUTOR
SERIAL_EXECUTOR负责队列管理，负责执行任务的是THREAD_POOL_EXECUTOR

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);
            }
        }
    }

回过头来，咱们再来看看executeOnExecutor方法

@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;
    }

这个方法里面做了下状态判断和状态修改，所以可以看出，一个AsyncTask实例只能执行一次。然后分别调用了onPreExecute和exec.execute(mFuture)，这里的exec就是SERIAL_EXECUTOR，调用exec.execute(mFuture)就是把任务插入到任务队列中。然后依次执行任务。