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);    }

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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;        }    }}

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// 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:@Overrideprotected 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;}

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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;}