AsyncTask实现机制

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

    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;
    }

execute先调用onPreExecute()（可见，onPreExecute是自动调用的）然后调用exec.execute(mFuture)

    public interface Executor {
        void execute(Runnable command);
    }

这是一个接口，具体实现在

    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);
            }
        }
    }

从上面可知，AsyncTask执行过程如下：先执行onPreExecute，然后交给SerialExecutor执行。在SerialExecutor中，先把Runnable添加到mTasks中。
如果没有Runnable正在执行，那么就调用SerialExecutor的scheduleNext。同时当一个Runnable执行完以后，继续执行下一个任务

AsyncTask中有两个线程池，THREAD_POOL_EXECUTOR和SERIAL_EXECUTOR，以及一个Handler–InternalHandler

    /**
     * An {@link Executor} that can be used to execute tasks in parallel.
     */
    public static final Executor THREAD_POOL_EXECUTOR
            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);

    /**
     * An {@link Executor} that executes tasks one at a time in serial
     * order.  This serialization is global to a particular process.
     */
    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();

    private static InternalHandler sHandler;

SERIAL_EXECUTOR用于任务的排列，THREAD_POOL_EXECUTOR真正执行线程，InternalHandler用于线程切换
先看构造函数

    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);
                }
            }
        };
    }

看到了熟悉的doInBackground了吧，然后调用postResult

    private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }

主线程中创建InternalHandler并发送MESSAGE_POST_RESULT消息，然后调用finish函数

    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;
            }
        }
    }

    private void finish(Result result) {
        if (isCancelled()) {
            onCancelled(result);
        } else {
            onPostExecute(result);
        }
        mStatus = Status.FINISHED;
    }

finish中调用onPostExecute。

AsyncTask工作流程：new MyThread().execute(1);
先构造函数，然后execute
构造函数只是准备了mWorker和mFuture这两个变量
execute中调用onPreExecute，然后exec.execute(mFuture)，其中响应了call函数，call中调用doInBackground，然后将结果传给Handler然后finish掉，finish函数调用onPostExecute
你可能会奇怪，为什么没有onProgressUpdate，有注解可以解释

    /**
     * Runs on the UI thread after {@link #publishProgress} is invoked.
     * The specified values are the values passed to {@link #publishProgress}.
     *
     * @param values The values indicating progress.
     *
     * @see #publishProgress
     * @see #doInBackground
     */
    @SuppressWarnings({"UnusedDeclaration"})
    protected void onProgressUpdate(Progress... values) {
    }

也就是说必须调用publishProgress才会自动调用onProgressUpdate。
那如何调用publishProgress呢？

    /**
     * Override this method to perform a computation on a background thread. The
     * specified parameters are the parameters passed to {@link #execute}
     * by the caller of this task.
     *
     * This method can call {@link #publishProgress} to publish updates
     * on the UI thread.
     *
     * @param params The parameters of the task.
     *
     * @return A result, defined by the subclass of this task.
     *
     * @see #onPreExecute()
     * @see #onPostExecute
     * @see #publishProgress
     */
    protected abstract Result doInBackground(Params... params);

doInBackground说的很明确，在doInBackground函数里面显示调用publishProgress即可。
publishProgress源码：

    protected final void publishProgress(Progress... values) {
        if (!isCancelled()) {
            getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
                    new AsyncTaskResult<Progress>(this, values)).sendToTarget();
        }
    }

    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:
                    //****************************************在这里调用
                    result.mTask.onProgressUpdate(result.mData);
                    break;
            }
        }
    }