AsyncTask原理解析

第一次写博客，写的不好请大家见谅，话说做Android开发，AsyncTask大家应该都不陌生，今天在这里给大家理一理AsyncTask的原理，先来个时序图

1. CountService中调用了task.execute(0)方法

2. 我们到AsyncTask中看下具体实现
   public final AsyncTask execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}

3. 继续查看executeOnExecutor方法的具体实现：

  public final AsyncTask 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;
    }

其中可以发现，如果线程正在Running或者线程已结束运行都会抛出异常，如果线程为PENDING状态的话，会将状态设置为RUNNING，调用onPreExecute方法，如果需要在doInBackground方法执行之前作一些准备的话，可以重写这个方法
4. exec的实例化对象为SerialExecutor，其execute方法的具体实现如下：

 public synchronized void execute(final Runnable r) {
            mTasks.offer(new Runnable() {
                public void run() {
                    try {
                        r.run();
                    } finally {
                        scheduleNext();
                    }
                }
            });
            if (mActive == null) {
                scheduleNext();
            }
        }

其中传进来的r对象为mFuture，在AsycTask构造方法中进行初始化

 mWorker = new WorkerRunnable() {
            public Result call() throws Exception {
                mTaskInvoked.set(true);

                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                //noinspection unchecked
                return postResult(doInBackground(mParams));
            }
        };

        mFuture = new FutureTask(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);
                }
            }
        };

 5. FutureTask中的run方法具体实现如下： 

  public void run() {
        sync.innerRun();
  }

sync的实力对象为Sync（FutureTask的内部类）

void innerRun() {
    if (!compareAndSetState(READY, RUNNING))
        return;

    runner = Thread.currentThread();
    if (getState() == RUNNING) { // recheck after setting thread
        V result;
        try {
            result = callable.call();
        } catch (Throwable ex) {
            setException(ex);
            return;
        }
        set(result);
    } else {
        releaseShared(0); // cancel
    }
}

其中callable其实就是mFuture 初始化中传进去的mWorker

6.callable.call()的实现也就是初始化过程中重写的那个方法

@Override
            public Result call() throws Exception {
                mTaskInvoked.set(true);             Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                return postResult(doInBackground(mParams));
            }

此时doInBackground方法就会被调用，执行完之后，会将result返回，其中的mParams在第3步中已经被初始化

7.再来看postResult实现

    private Result postResult(Result result){
        Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult(this, result));
        message.sendToTarget();
        return result;
    }   

其中getHandler会获取当前线程中对应的handler对象
8. 再来看看handMessage方法

private static class InternalHandler extends Handler{
        public InternalHandler(){
            super(Looper.getMainLooper());
        }

        @SuppressWarnings("unchecked")
        @Override
        public void handleMessage(Message msg) {
            AsyncTaskResult result = (AsyncTaskResult)msg.obj;
            switch(msg.what){
            case MESSAGE_POST_RESULT:
                result.mTask.finish(result.mData[0]);
                break;
            case MESSAGE_POST_PROGRESS:
                result.mTask.onProgressUpdate(result.mData);
                break;
            }

        }


    }

其中mTask对象就是AsyncTask本身，其finish方法如下：

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

如果是用户停止的就调用onCancelled方法，否则就是任务正常结束，调用onPostExecute方法，一般都要被重写实现自己的相关业务逻辑，mStatus 状态置为FINISHED，如有问题，欢迎指出，谢谢！