【goAsync帮你在onReceive中简便地进行异步操作】广播回调onReceive是在主线程跑的，所以我们不能在里面搞耗时操作，不然秒秒钟ANR。
又因为onReceive中的代码在执行完后，BroadcastReceiver对象就无效了，生命周期结束。
所以我们不能直接在里面起子线程，若应用进程被回收掉，线程的任务就可能无法完成。徒增不可控因素。
解决：
普遍的处理方式是在onReceive中再起一个IntentService去执行异步操作。这样就有了Service组件的保障，进程不会被轻易杀掉，但同时这个操作也比较重，代码实现上还得再去写个处理特殊业务的IntentService。
比如说我在onReceive里面只做一些很简单的耗时操作，可能就一两行代码我也要去写个Service？显然谷歌爹已经想到了这一点。
在API 11以后，BroadcastReceiver新增了一个静态内部类PendingResult，我们可以通过调用goAsync()方法来获取这个PendingResult对象，然后就可以愉快地进行子线程操作了，最后通过调用它的finish方法来结束广播接收者的生命。一切就变得可控了（相当于强行给Receiver续命）。
...
@Override
public void onReceive(final Context context, final Intent intent) {
final PendingResult result = goAsync();
AsyncHandler.post(new Runnable() {
@Override
public void run() {
// 在这里搞事情
...
// 成功续命，可以手动结束了
result.finish();
}
});
}
...
要注意的是，尽管这里可以异步操作，但耗时超过10s依然会爆ANR。
其中AsyncHandler是一个简单封装的可复用的异步操作类（来自Android源码）：
/**
* Helper class for managing the background thread used to perform io operations
* and handle async broadcasts.
*/
public final class AsyncHandler {
private static final HandlerThread sHandlerThread = new HandlerThread("AsyncHandler");
private static final Handler sHandler;
static {
sHandlerThread.start();
sHandler = new Handler(sHandlerThread.getLooper());
}
public static void post(Runnable r) {
sHandler.post(r);
}
public static void postDelayed(Runnable r, long delayedMills) {
sHandler.postDelayed(r, delayedMills);
}
public static Message obtain(Runnable r) {
return Message.obtain(sHandler, r);
}
public static void sendMessageDelayed(Message message, long delayedMills) {
sHandler.sendMessageDelayed(message, delayedMills);
}
public static void removeCallbacks(int what) {
sHandler.removeMessages(what);
}
private AsyncHandler() {}
}
最后，我们可以回味一下BroadcastReceiver源码是怎么描述PendingResult和goAsync的：
/**
* State for a result that is pending for a broadcast receiver.Returned
* by {@link BroadcastReceiver#goAsync() goAsync()}
* while in {@link BroadcastReceiver#onReceive BroadcastReceiver.onReceive()}.
* This allows you to return from onReceive() without having the broadcast
* terminate;
you must call {@link #finish()} once you are done with the
* broadcast.This allows you to process the broadcast off of the main
* thread of your app.
*
* Note on threading: the state inside of this class is not itself
* thread-safe, however you can use it from any thread if you properly
* sure that you do not have races.Typically this means you will hand
* the entire object to another thread, which will be solely responsible
* for setting any results and finally calling {@link #finish()}.
*/
public static class PendingResult {
...
/**
* This can be called by an application in {@link #onReceive} to allow
* it to keep the broadcast active after returning from that function.
* This does not change the expectation of being relatively
* responsive to the broadcast (finishing it within 10s), but does allow
* the implementation to move work related to it over to another thread
* to avoid glitching the main UI thread due to disk IO.
*
* @return Returns a {@link PendingResult} representing the result of
* the active broadcast.The BroadcastRecord itself is no longer active;
* all data and other interaction must go through {@link PendingResult}
* APIs.The {@link PendingResult#finish PendingResult.finish()} method
* must be called once processing of the broadcast is done.
*/
public final PendingResult goAsync() {
PendingResult res = mPendingResult;
mPendingResult = null;
return res;
}
