关于BroadcastReceiver与Service交互时调用peekService返回null的疑问及解析

大家都清楚由于onReceive()函数是由进程的主线程调用的，生命期在10s左右，如果遇上了耗时操作,官网的BroadcastReceiver中onReceive(context,intent)部分建议如下:

in particular, for interacting with services, you should use startService(Intent) instead of bindService(Intent, ServiceConnection, int). If you wish to interact with a service that is already running, you can use peekService(Context, Intent)

就是说涉及到跟service交互的话，不要用bindService方法，因为可能binder对象还没返回回来这个receiver就killed掉了。建议使用peekService(context,intent)方法，这个方法还是比较少用到的，也不清楚这个函数返回的Ibinder对象来自于哪儿。所以下面写了个demo来看看这个peekService怎么用.

从官网提供来看:

Provide a binder to an already-running service. This method is synchronous and will not start the target service if it is not present, so it is safe to call from onReceive(Context, Intent).

可见我们在调用peekService前需要保证intent参数内的service是一个已经启动的service，peekService是一个同步的操作，所以在onReceive函数内部执行时安全的。

写了一个小demo如下:

activity{
...
...
//绑定一个button的onClick: sendBroadcast
public void sendBroadcast(View view) {
        //先启动一个service
        Intent intent1 = new Intent();
        intent1.setClass(MainActivity.this, MyService.class);
        bindService(intent1, connection, BIND_AUTO_CREATE);
        //发送广播
        Intent intent = new Intent();
        intent.setAction("com.example.zhangxiaang.broadcastreceiverdemo.intent.MyReceiver");
        intent.putExtra("name", "zhangxiang");
        sendBroadcast(intent);
    }
    private ServiceConnection connection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            //do nothing
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {
        }
    };
}

MyService{
...
...
	@Override
    public IBinder onBind(Intent intent) {
        MyBinder binder = new MyBinder();
        return binder;
    }
    class MyBinder extends Binder {
        public void downloadImg() {
            //耗时操作开始
            new Thread(new Runnable() {
                @Override
                public void run() {
                    SystemClock.sleep(1000 * 13);
                    Log.e(TAG, "13s已经过去了,但是onReceive函数依旧可以接收到任务返回的结果");
                }
            }).start();
        }
    }
}
MyReceive{
...
...
	@Override
    public void onReceive(Context context, Intent intent) {
        Intent intent1 = new Intent();
        intent1.setClass(context, MyService.class);
        //确保MyService已经启动了，否则会导致peekService()返回null
        MyService.MyBinder binder = (MyService.MyBinder) peekService(context, intent1);
        if (binder != null) {
            binder.downloadImg();
        } else Toast.makeText(context, "binder == null", Toast.LENGTH_LONG).show();
    }
}

当我触发sendBroadcast()函数后，打印的内容如下

E/MyService: onCreate()
E/MyService: onBind()…//停止13s
E/MyService: 13s已经过去了，但是onReceive函数依旧可以接收任务返回的结果
可见在onReceive 函数里调用peekService函数并没有再次调用已经启动的service的onBind()函数。那么在peekService函数这个binder是如何不调用onBind()而获取到的呢?
我尝试看源码

public IBinder peekService(Context myContext, Intent service) {
        IActivityManager am = ActivityManagerNative.getDefault();
        IBinder binder = null;
        try {
            service.prepareToLeaveProcess();
            binder = am.peekService(service, service.resolveTypeIfNeeded(
                    myContext.getContentResolver()), myContext.getOpPackageName());
        } catch (RemoteException e) {
        }
        return binder;
    }

到了am.peekService()这里就无法继续的深入寻找原理，很惭愧。还请大神们多多指教！！！

---

更新：good tips for this problem
提示我们需要注意，如果再次启动这个service导致执行的是reStart()那么MyService.MyBinder binder = (MyService.MyBinder) peekService(context, intent1);返回的binder就是一个null对象，因为跳过了onBind()函数，无法生成一个binder.