AsyncTask - 基本原理 后台线程和UI线程的交互

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。