AsyncTask - 基本原理 后台线程和UI线程的交互

Posted 2022-12-07 zj510

前面一个文章大概描述了一下任务是怎么被执行的？http://blog.csdn.net/zj510/article/details/51485120

其实也就是AsyncTask里面的doInBackground怎么通过FutureTask和WorkerRunnable在ThreadPoolExecutor的某个线程里面执行的。

那么执行过程中和执行完毕了总要通知主调线程（主线程）吧？

AsyncTask的事件

基本上，AsyncTask有如下一些事件通知

onPreExecute()

onPostExecute()

onProgressUpdate()

onCancelled()

下面的代码来自AsyncTask。

    protected abstract Result doInBackground(Params... params);

    /**
     * Runs on the UI thread before @link #doInBackground.
     *
     * @see #onPostExecute
     * @see #doInBackground
     */
    protected void onPreExecute() 
    

    /**
     * <p>Runs on the UI thread after @link #doInBackground. The
     * specified result is the value returned by @link #doInBackground.</p>
     * 
     * <p>This method won't be invoked if the task was cancelled.</p>
     *
     * @param result The result of the operation computed by @link #doInBackground.
     *
     * @see #onPreExecute
     * @see #doInBackground
     * @see #onCancelled(Object) 
     */
    @SuppressWarnings("UnusedDeclaration")
    protected void onPostExecute(Result result) 
    

    /**
     * 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) 
    

    /**
     * <p>Runs on the UI thread after @link #cancel(boolean) is invoked and
     * @link #doInBackground(Object[]) has finished.</p>
     * 
     * <p>The default implementation simply invokes @link #onCancelled() and
     * ignores the result. If you write your own implementation, do not call
     * <code>super.onCancelled(result)</code>.</p>
     *
     * @param result The result, if any, computed in
     *               @link #doInBackground(Object[]), can be null
     * 
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    @SuppressWarnings("UnusedParameters")
    protected void onCancelled(Result result) 
        onCancelled();
        
    
    /**
     * <p>Applications should preferably override @link #onCancelled(Object).
     * This method is invoked by the default implementation of
     * @link #onCancelled(Object).</p>
     * 
     * <p>Runs on the UI thread after @link #cancel(boolean) is invoked and
     * @link #doInBackground(Object[]) has finished.</p>
     *
     * @see #onCancelled(Object) 
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    protected void onCancelled() 
    

那么我们就来看看每个事件回调是怎么被调用的？

onPreExecute()

AsyncTask的调用者需要调用execute来启动一个AsyncTask，如：

        AsyncTask<Void, Void, String> asyncTask = new AsyncTask<Void, Void, String>()
            @Override
            protected String doInBackground(Void... param)
            
                Log.v("AsyncTask", "doInBackground");
                return "hello asyncTask";
            

            @Override
            public void onPostExecute(String response) 
                //    callback.onSendRequestFinished(JsonUtil.jsonToBean(response, mBeanType));
                Toast.makeText(MainActivity.this, "result: " + response, Toast.LENGTH_LONG).show();
            
        ;
        asyncTask.execute();

那么execute里面做了什么呢？看：

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

    /**
     * Executes the task with the specified parameters. The task returns
     * itself (this) so that the caller can keep a reference to it.
     * 
     * <p>This method is typically used with @link #THREAD_POOL_EXECUTOR to
     * allow multiple tasks to run in parallel on a pool of threads managed by
     * AsyncTask, however you can also use your own @link Executor for custom
     * behavior.
     * 
     * <p><em>Warning:</em> Allowing multiple tasks to run in parallel from
     * a thread pool is generally <em>not</em> what one wants, because the order
     * of their operation is not defined.  For example, if these tasks are used
     * to modify any state in common (such as writing a file due to a button click),
     * there are no guarantees on the order of the modifications.
     * Without careful work it is possible in rare cases for the newer version
     * of the data to be over-written by an older one, leading to obscure data
     * loss and stability issues.  Such changes are best
     * executed in serial; to guarantee such work is serialized regardless of
     * platform version you can use this function with @link #SERIAL_EXECUTOR.
     *
     * <p>This method must be invoked on the UI thread.
     *
     * @param exec The executor to use.  @link #THREAD_POOL_EXECUTOR is available as a
     *              convenient process-wide thread pool for tasks that are loosely coupled.
     * @param params The parameters of the task.
     *
     * @return This instance of AsyncTask.
     *
     * @throws IllegalStateException If @link #getStatus() returns either
     *         @link AsyncTask.Status#RUNNING or @link AsyncTask.Status#FINISHED.
     *
     * @see #execute(Object[])
     */
    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;
    

很明显，onPreExecute()直接在调用线程里面被调用了，然后才调用Executor的execute来执行任务（mFuture）。onPreExecute很简单，没啥好说的。

onPostExecute

onPostExecute这个就有点讲究了，之前也讲到了，AsyncTask的构造函数里面会创建一个WorkerRunnable的子类，重写了call函数。而call函数将在ThreadPoolExecutor的一个线程里面被调用。

    public AsyncTask() 
        mWorker = new WorkerRunnable<Params, Result>() 
            public Result call() throws Exception 
                mTaskInvoked.set(true);

                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                //noinspection unchecked
                Result result = doInBackground(mParams);
                Binder.flushPendingCommands();
                return postResult(result);
            
        ;

        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 occurred while executing doInBackground()",
                            e.getCause());
                 catch (CancellationException e) 
                    postResultIfNotInvoked(null);
                
            
        ;
    

从上面的代码可以看到doInBackground是在ThreadPoolExecutor的一个线程里面被调用的。当doInBackground调用结束，注意postResult(result)这行。先看看代码：

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

很清楚，通过一个handler，把MESSAGE_POST_RESULT消息发给了handler的拥有者，同时通过AsyncTaskResult对象把doInBackground返回值一并给handler的拥有者。

MESSAGE_POST_RESULT是AsyncTask定义的一个消息ID，如下：

    private static final int MESSAGE_POST_RESULT = 0x1;
    private static final int MESSAGE_POST_PROGRESS = 0x2;

那么现在来看看sHandler，

    private static final InternalHandler sHandler = new InternalHandler();

sHandler是AsyncTask的一个静态数据成员。

InternalHandler定义如下：

    private static class InternalHandler extends Handler 
        @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;
            
        
    

超简单的一个类，就是继承自Handler，然后重写handleMessage，处理了MESSAGE_POST_RESULT和MESSAGE_POST_PROGRESS两个消息。

在case MESSAGE_POST_RESULT里面看到result.mTask.finish(result.mData[0]);

result是由postResult发过来的，它是一个AsyncTaskResult对象，它的定义如下：

    private static class AsyncTaskResult<Data> 
        final AsyncTask mTask;
        final Data[] mData;

        AsyncTaskResult(AsyncTask task, Data... data) 
            mTask = task;
            mData = data;
        
    

其实第一个数据成员其实就是当前AsyncTask对象，可以看postResult里面把this当作了AsyncTaskResult构造函数的第一个参数。所以我们只需要看AsyncTask的finish函数。

finish函数在handler的处理函数里面被调用，而实际上handlerMessage实在handler所在线程被调用，那么finish也将在Asynctask的创建线程里面被调用。

看实现：

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

very simple，如果调用者调用了AsyncTask的cancel，那么就调用onCancel(result),否则就调用onPostExecute(result)；

画了个示意图，不是很准确，但大致就这么个意思。

对于onCancelled和onPostExecute，这两个消息响应函数都是通过发送MESSAGE_POST_RESULT给handler，在handler的handleMessage里面被处理的。

onProgressUpdate

对于onProgressUpdate，就得先看看publishProgress了。

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

这个一般是用来显示后台线程处理进度了。我们使用AsyncTask的时候，都是会派生一个子类，然后重写doInBackground来处理事情。比如在doInBackground里面需要进行网络访问，那么doInBackground就会在后台线程里面被执行。当我们想知道进度的时候，就需要自己在doInBackground里面调用publishProgress了。

比如：

       AsyncTask<Void, Void, String> asyncTask = new AsyncTask<Void, Void, String>()
            @Override
            protected String doInBackground(Void... param)
            
                publishProgress(10%);
                Log.v("AsyncTask", "doInBackground");
                publishProgress(50%);
                do something;
                publishProgress(100%);
                return "hello asyncTask";
            

            @Override
            protected onProgressUpdate()
            
                update UI progress bar;
            
            @Override
            public void onPostExecute(String response) 
                //    callback.onSendRequestFinished(JsonUtil.jsonToBean(response, mBeanType));
                Toast.makeText(MainActivity.this, "result: " + response, Toast.LENGTH_LONG).show();
            
        ;
        asyncTask.execute();

publishProgress其实就是通过handler发送消息，这里发送的是MESSAGE_POST_PROGRESS。然后InternalExecutor会处理这个消息。

    private static class InternalHandler extends Handler 
        @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;
            
        
    

直接调用AsyncTask的onProgressUpdate，因为调用者重写了AsyncTask的onProgressUpdate函数，所以派生类的这个函数就被调用了（在主线程）。然后我们就可以更新一些UI什么的。

本质上onProgressUpdate和onPostExecute，onCancelled一样，都是通过handler从后台线程发送到主调线程的，用了不同的MESSAGE而已。