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AsyncTask原理

new AsyncTask<String, String, String>() {
    
    // 2. 运行在主线程中, 做一些准备操作.
    public void onPreExecute() {

    }

    // 3. 运行在子线程中, 做一些耗时的任务.
    public String doInBackground(String... params) {
        return null;
    }

    // 4. 运行主线程中, result就是doInBackground方法返回的值. 做一些收尾操作.
    public void onPostExecute(String result) {

    }
}.execute(String... params);    // 1. 开始执行异步任务.

AsyncTask的execute的方法, 代码如下:

public final AsyncTask<Params, Progress, Result> execute(Params... params) {
    ...

    mStatus = Status.RUNNING;

    // 调用用户的实现方法, 让用户做一些准备操作. 运行在主线程中.
    onPreExecute();

    // 把mWorker中的mParams赋值为用户传递进来的参数.
    mWorker.mParams = params;
    // 使用线程池, 执行任务. 这时进入到子线程中.
    sExecutor.execute(mFuture);

    return this;
}

查看mWorker的初始化过程, 代码如下:

// AsyncTask构造函数在一开始使用异步任务时, 已经调用.
public AsyncTask() {
    mWorker = new WorkerRunnable<Params, Result>() {
        public Result call() throws Exception {
            ...
        }
    };

    // 1. 把mWorker对象传递给了FutureTask的构造函数
    mFuture = new FutureTask<Result>(mWorker) {
        @Override
        protected void done() {
            ...
        }
    };
}

// FutureTask类中接收的是Callable的类型, 其实WorkerRunnable类型实现了Callable的类型.
public FutureTask(Callable<V> callable) {
    if (callable == null)
        throw new NullPointerException();
    // 2. 把callable(mWorker)传递给了Sync的构造函数.
    sync = new Sync(callable);
}

Sync(Callable<V> callable) {
     // 3. 把接收过来的callable(mWorker)对象赋值给Sync类中的成员变量callable
    // 总结: Sync类中的成员变量callable就是AsyncTask中的mWorker对象.
    this.callable = callable;
}

mFuture对象中的run方法如下:

public void run() {
    // 1. 调用了innerRun方法.
    sync.innerRun();
}

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

    runner = Thread.currentThread();
    if (getState() == RUNNING) { // recheck after setting thread
        V result;
        try {
            // 2. 调用了callable(mWorker)的call方法. 获取一个返回结果.
            result = callable.call();
        } catch (Throwable ex) {
            setException(ex);
            return;
        }
        // 4. 把结果传递给set方法.
        set(result);
    } else {
        releaseShared(0); // cancel
    }
}

// 在AsyncTask的构造函数中, mWorker对象初始化时, 已经覆盖了call方法, 代码如下
public Result call() throws Exception {
    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
    // 3. 调用了用户实现的doInBackground方法, 去执行一个耗时的任务. 运行在子线程中.
    // doInBackground接收的参数就是execute方法接收的参数.
    return doInBackground(mParams);    
}

---

protected void set(V v) {
    // 5. 继续把参数传递给innerSet方法.
    sync.innerSet(v);
}

void innerSet(V v) {
    for (;;) {
        int s = getState();
        if (s == RAN)
            return;
        if (s == CANCELLED) {
            // aggressively release to set runner to null,
            // in case we are racing with a cancel request
            // that will try to interrupt runner
            releaseShared(0);
            return;
        }
        if (compareAndSetState(s, RAN)) {
            // 6. 把doInBackground返回的结果赋值给成员变量result
            result = v;
            releaseShared(0);
            // 7. 调用FutureTask中的done方法.
            done();
            return;
        }
    }
}

---

// 此方法定义在AsyncTask的构造函数中, 初始化mFuture时,已经覆盖了done方法, 代码如下:
@Override
protected void done() {
    Message message;
    Result result = null;

    try {
        // 8. 调用FutureTask中的get方法获取result(doInBackground返回的结果).
        result = 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) {
        message = sHandler.obtainMessage(MESSAGE_POST_CANCEL,
                new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null));
        message.sendToTarget();
        return;
    } catch (Throwable t) {
        throw new RuntimeException("An error occured while executing "
                + "doInBackground()", t);
    }

    // 11. 创建一个消息对象.
    // msg.what = MESSAGE_POST_RESULT;
    // msg.obj = new AsyncTaskResult<Result>(AsyncTask.this, result);
    message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
            new AsyncTaskResult<Result>(AsyncTask.this, result));
    // 12. 把当前消息使用sHandler发送出去.
    message.sendToTarget();
}

public V get() throws InterruptedException, ExecutionException {
    // 9. 转调innerGet方法.
    return sync.innerGet();
}

V innerGet() throws InterruptedException, ExecutionException {
    ...

    // 10. 把result(doInBackground的结果)返回回去. result成员变量是在innerSet方法中赋值. 详情看第6步.
    return result;
}

---

private static class InternalHandler extends Handler {
    @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
    @Override
    public void handleMessage(Message msg) {
        // 把obj转换成AsyncTaskResult类, 这个类中mTask对象就是当前的AsyncTask对象. mData对象就是doInBackground的返回结果.
        AsyncTaskResult result = (AsyncTaskResult) msg.obj;
        switch (msg.what) {
            case MESSAGE_POST_RESULT:
                // There is only one result
                // 13. 调用AsyncTask中的finish方法, 并且把doInBackground的返回结果传递进去.
                result.mTask.finish(result.mData[0]);
                break;
            case MESSAGE_POST_PROGRESS:
                result.mTask.onProgressUpdate(result.mData);
                break;
            case MESSAGE_POST_CANCEL:
                result.mTask.onCancelled();
                break;
        }
    }
}

private void finish(Result result) {
    if (isCancelled()) result = null;
    // 14. 把doInBackground的返回结果传递给用户实现的onPostExecute方法. 运行在主线程中, 用户可以做一些操作界面, 更新界面的操作.
    onPostExecute(result);
    mStatus = Status.FINISHED;
}