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Android应用程序注冊广播接收器(registerReceiver)的过程分析

        前面我们介绍了Android系统的广播机制,从本质来说,它是一种消息订阅/公布机制,因此,使用这样的消息驱动模型的第一步便是订阅消息;而对Android应用程序来说,订阅消息事实上就是注冊广播接收器,本文将探讨Android应用程序是怎样注冊广播接收器以及把广播接收器注冊到哪里去的。

        在Android的广播机制中,ActivityManagerService扮演着广播中心的角色,负责系统中全部广播的注冊和公布操作,因此,Android应用程序注冊广播接收器的过程就把是广播接收器注冊到ActivityManagerService的过程。Android应用程序是通过调用ContextWrapper类的registerReceiver函数来把广播接收器BroadcastReceiver注冊到ActivityManagerService中去的,而ContextWrapper类本身又借助ContextImpl类来注冊广播接收器。

        在Android应用程序框架中,Activity和Service类都继承了ContextWrapper类,因此,我们能够在Activity或者Service的子类中调用registerReceiver函数来注冊广播接收器。Activity、Service、ContextWrapper和ContextImpl这四个类的关系能够參考前面Android系统在新进程中启动自己定义服务过程(startService)的原理分析一文中描写叙述的Activity类图。

        这篇文章还是继续以实例来进行情景分析,所用到的样例便是上一篇文章Android系统中的广播(Broadcast)机制简要介绍和学习计划里面介绍的应用程序了,所以希望读者在继续阅读本文之前,先看看这篇文章;又因为Android应用程序是把广播接器注冊到ActivityManagerService中去的,因此,这里又会涉入到Binder进程间通信机制,所以希望读者对Android系统的Binder进程间通信机制有所了解,具体请參考Android进程间通信(IPC)机制Binder简要介绍和学习计划一文。

        開始进入主题了,在Android系统中的广播(Broadcast)机制简要介绍和学习计划一文所介绍的样例中,注冊广播接收器的操作是MainActivity发起的,我们先来看看注冊过程的序列图:


        在分析这个序列图之前,我们先来看一下MainActivity是怎样调用registerReceiver函数来注冊广播接收器的:

public class MainActivity extends Activity implements OnClickListener {  
	......

	@Override   
	public void onResume() {  
		super.onResume();  

		IntentFilter counterActionFilter = new IntentFilter(CounterService.BROADCAST_COUNTER_ACTION);  
		registerReceiver(counterActionReceiver, counterActionFilter);  
	} 

	......

}
        MainActivity在onResume函数里,通过其父类ContextWrapper的registerReceiver函数注冊了一个BroadcastReceiver实例counterActionReceiver,而且通过IntentFilter实例counterActionFilter告诉ActivityManagerService,它要订阅的广播是CounterService.BROADCAST_COUNTER_ACTION类型的,这样,ActivityManagerService在收到CounterService.BROADCAST_COUNTER_ACTION类型的广播时,就会分发给counterActionReceiver实例的onReceive函数。

        接下来,就開始分析注冊过程中的每个步骤了。

        Step 1. ContextWrapper.registerReceiver

        这个函数实如今frameworks/base/core/java/android/content/ContextWrapper.java文件里:

public class ContextWrapper extends Context {
	Context mBase;
	......

	@Override
	public Intent registerReceiver(
		BroadcastReceiver receiver, IntentFilter filter) {
		return mBase.registerReceiver(receiver, filter);
	}

	......

}
        这里的成员变量mBase是一个ContextImpl实例,想知道为什么,能够回过头去看看Android应用程序启动过程源码分析这篇文章>~<。

        Step 2. ContextImpl.registerReceiver

        这个函数实如今frameworks/base/core/java/android/app/ContextImpl.java文件里:

class ContextImpl extends Context {
	......

	@Override
	public Intent registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
		return registerReceiver(receiver, filter, null, null);
	}

	@Override
	public Intent registerReceiver(BroadcastReceiver receiver, IntentFilter filter,
			String broadcastPermission, Handler scheduler) {
		return registerReceiverInternal(receiver, filter, broadcastPermission,
			scheduler, getOuterContext());
	}

	private Intent registerReceiverInternal(BroadcastReceiver receiver,
			IntentFilter filter, String broadcastPermission,
			Handler scheduler, Context context) {
		IIntentReceiver rd = null;
		if (receiver != null) {
			if (mPackageInfo != null && context != null) {
				if (scheduler == null) {
					scheduler = mMainThread.getHandler();
				}
				rd = mPackageInfo.getReceiverDispatcher(
					receiver, context, scheduler,
					mMainThread.getInstrumentation(), true);
			} else {
				......
			}
		}
		try {
			return ActivityManagerNative.getDefault().registerReceiver(
					mMainThread.getApplicationThread(),
					rd, filter, broadcastPermission);
		} catch (RemoteException e) {
				return null;
		}
	}

	......

}
        通过两个函数的中转,终于就进入到ContextImpl.registerReceiverInternal这个函数来了。这里的成员变量mPackageInfo是一个LoadedApk实例,它是用来负责处理广播的接收的,在后面一篇文章讲到广播的发送时(sendBroadcast),会具体描写叙述。參数broadcastPermission和scheduler都为null,而參数context是上面的函数通过调用函数getOuterContext得到的,这里它就是指向MainActivity了,因为MainActivity是继承于Context类的,因此,这里用Context类型来引用。

        因为条件mPackageInfo != null和context != null都成立,而且条件scheduler == null也成立,于是就调用mMainThread.getHandler来获得一个Handler了,这个Hanlder是后面用来分发ActivityManagerService发送过的广播用的。这里的成员变量mMainThread是一个ActivityThread实例,在前面Android应用程序启动过程源码分析这篇文章也描写叙述过了。我们先来看看ActivityThread.getHandler函数的实现,然后再回过头来继续分析ContextImpl.registerReceiverInternal函数。

        Step 3. ActivityThread.getHandler

        这个函数实如今frameworks/base/core/java/android/app/ActivityThread.java文件里:

public final class ActivityThread {
	......

	final H mH = new H();

	private final class H extends Handler {
		......

		public void handleMessage(Message msg) {
			......

			switch (msg.what) {
			......
			}

			......
		}

		......

	}

	......

	final Handler getHandler() {
		return mH;
	}

	......

}
        有了这个Handler之后,就能够分发消息给应用程序处理了。

        再回到上一步的ContextImpl.registerReceiverInternal函数中,它通过mPackageInfo.getReceiverDispatcher函数获得一个IIntentReceiver接口对象rd,这是一个Binder对象,接下来会把它传给ActivityManagerService,ActivityManagerService在收到对应的广播时,就是通过这个Binder对象来通知MainActivity来接收的。

        我们也是先来看一下mPackageInfo.getReceiverDispatcher函数的实现,然后再回过头来继续分析ContextImpl.registerReceiverInternal函数。

        Step 4. LoadedApk.getReceiverDispatcher

        这个函数实如今frameworks/base/core/java/android/app/LoadedApk.java文件里:

final class LoadedApk {
	......

	public IIntentReceiver getReceiverDispatcher(BroadcastReceiver r,
			Context context, Handler handler,
			Instrumentation instrumentation, boolean registered) {
		synchronized (mReceivers) {
			LoadedApk.ReceiverDispatcher rd = null;
			HashMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher> map = null;
			if (registered) {
				map = mReceivers.get(context);
				if (map != null) {
					rd = map.get(r);
				}
			}
			if (rd == null) {
				rd = new ReceiverDispatcher(r, context, handler,
					instrumentation, registered);
				if (registered) {
					if (map == null) {
						map = new HashMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher>();
						mReceivers.put(context, map);
					}
					map.put(r, rd);
				}
			} else {
				rd.validate(context, handler);
			}
			return rd.getIIntentReceiver();
		}
	}

	......

	static final class ReceiverDispatcher {

		final static class InnerReceiver extends IIntentReceiver.Stub {
			final WeakReference<LoadedApk.ReceiverDispatcher> mDispatcher;
			......

			InnerReceiver(LoadedApk.ReceiverDispatcher rd, boolean strong) {
				mDispatcher = new WeakReference<LoadedApk.ReceiverDispatcher>(rd);
				......
			}

			......
		}

		......

		final IIntentReceiver.Stub mIIntentReceiver;
		final Handler mActivityThread;

		......

		ReceiverDispatcher(BroadcastReceiver receiver, Context context,
				Handler activityThread, Instrumentation instrumentation,
				boolean registered) {
			......

			mIIntentReceiver = new InnerReceiver(this, !registered);
			mActivityThread = activityThread;
			
			......
		}

		......

		IIntentReceiver getIIntentReceiver() {
			return mIIntentReceiver;
		}

	}

	......

}

        在LoadedApk.getReceiverDispatcher函数中,首先看一下參数r是不是已经有对应的ReceiverDispatcher存在了,如果有,就直接返回了,否则就新建一个ReceiverDispatcher,而且以r为Key值保在一个HashMap中,而这个HashMap以Context,这里即为MainActivity为Key值保存在LoadedApk的成员变量mReceivers中,这样,仅仅要给定一个Activity和BroadcastReceiver,就能够查看LoadedApk里面是否已经存在对应的广播接收公布器ReceiverDispatcher了。

        在新建广播接收公布器ReceiverDispatcher时,会在构造函数里面创建一个InnerReceiver实例,这是一个Binder对象,实现了IIntentReceiver接口,能够通过ReceiverDispatcher.getIIntentReceiver函数来获得,获得后就会把它传给ActivityManagerService,以便接收广播。在ReceiverDispatcher类的构造函数中,还会把传进来的Handle类型的參数activityThread保存下来,以便后面在分发广播的时候使用。

        如今,再回到ContextImpl.registerReceiverInternal函数,在获得了IIntentReceiver类型的Binder对象后,就開始要把它注冊到ActivityManagerService中去了。

        Step 5. ActivityManagerProxy.registerReceiver

        这个函数实如今frameworks/base/core/java/android/app/ActivityManagerNative.java文件里:

class ActivityManagerProxy implements IActivityManager
{
	......

	public Intent registerReceiver(IApplicationThread caller,
			IIntentReceiver receiver,
			IntentFilter filter, String perm) throws RemoteException
	{
		Parcel data = http://www.mamicode.com/Parcel.obtain();>         这个函数通过Binder驱动程序就进入到ActivityManagerService中的registerReceiver函数中去了。

         Step 6. ActivityManagerService.registerReceiver

         这个函数实如今frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件里:

public final class ActivityManagerService extends ActivityManagerNative
		implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
	......

	public Intent registerReceiver(IApplicationThread caller,
			IIntentReceiver receiver, IntentFilter filter, String permission) {
		synchronized(this) {
			ProcessRecord callerApp = null;
			if (caller != null) {
				callerApp = getRecordForAppLocked(caller);
				if (callerApp == null) {
					......
				}
			}

			List allSticky = null;

			// Look for any matching sticky broadcasts...
			Iterator actions = filter.actionsIterator();
			if (actions != null) {
				while (actions.hasNext()) {
					String action = (String)actions.next();
					allSticky = getStickiesLocked(action, filter, allSticky);
				}
			} else {
				......
			}

			// The first sticky in the list is returned directly back to
			// the client.
			Intent sticky = allSticky != null ? (Intent)allSticky.get(0) : null;

			......

			if (receiver == null) {
				return sticky;
			}

			ReceiverList rl
				= (ReceiverList)mRegisteredReceivers.get(receiver.asBinder());
			if (rl == null) {
				rl = new ReceiverList(this, callerApp,
					Binder.getCallingPid(),
					Binder.getCallingUid(), receiver);

				if (rl.app != null) {
					rl.app.receivers.add(rl);
				} else {
					......
				}
				mRegisteredReceivers.put(receiver.asBinder(), rl);
			}

			BroadcastFilter bf = new BroadcastFilter(filter, rl, permission);
			rl.add(bf);
			......
			mReceiverResolver.addFilter(bf);

			// Enqueue broadcasts for all existing stickies that match
			// this filter.
			if (allSticky != null) {
				......
			}

			return sticky;
		}
	}

	......

}
         函数首先是获得调用registerReceiver函数的应用程序进程记录块:

    ProcessRecord callerApp = null;
    if (caller != null) {
	callerApp = getRecordForAppLocked(caller);
	if (callerApp == null) {
	    ......
        }
    }
        这里得到的便是上一篇文章Android系统中的广播(Broadcast)机制简要介绍和学习计划里面介绍的应用程序Broadcast的进程记录块了,MainActivity就是在里面启动起来的。

        

    List allSticky = null;

    // Look for any matching sticky broadcasts...
    Iterator actions = filter.actionsIterator();
    if (actions != null) {
	while (actions.hasNext()) {
		String action = (String)actions.next();
		allSticky = getStickiesLocked(action, filter, allSticky);
	}
    } else {
	......
    }

    // The first sticky in the list is returned directly back to
    // the client.
    Intent sticky = allSticky != null ? (Intent)allSticky.get(0) : null;
        这里传进来的filter仅仅有一个action,就是前面描写叙述的CounterService.BROADCAST_COUNTER_ACTION了,这里先通过getStickiesLocked函数查找一下有没有对应的sticky intent列表存在。什么是Sticky Intent呢?我们在最后一次调用sendStickyBroadcast函数来发送某个Action类型的广播时,系统会把代表这个广播的Intent保存下来,这样,后来调用registerReceiver来注冊同样Action类型的广播接收器,就会得到这个最后发出的广播。这就是为什么叫做Sticky Intent了,这个最后发出的广播尽管被处理完了,可是仍然被粘住在ActivityManagerService中,以便下一个注冊对应Action类型的广播接收器还能继承处理。

        这里,如果我们不使用sendStickyBroadcast来发送CounterService.BROADCAST_COUNTER_ACTION类型的广播,于是,这里得到的allSticky和sticky都为null了。

        继续往下看,这里传进来的receiver不为null,于是,继续往下运行:

    ReceiverList rl
	= (ReceiverList)mRegisteredReceivers.get(receiver.asBinder());
    if (rl == null) {
	rl = new ReceiverList(this, callerApp,
		Binder.getCallingPid(),
		Binder.getCallingUid(), receiver);

	if (rl.app != null) {
		rl.app.receivers.add(rl);
	} else {
		......
	}
	mRegisteredReceivers.put(receiver.asBinder(), rl);
    }
        这里事实上就是把广播接收器receiver保存一个ReceiverList列表中,这个列表的宿主进程是rl.app,这里就是MainActivity所在的进程了,在ActivityManagerService中,用一个进程记录块来表示这个应用程序进程,它里面有一个列表receivers,专门用来保存这个进程注冊的广播接收器。接着,又把这个ReceiverList列表以receiver为Key值保存在ActivityManagerService的成员变量mRegisteredReceivers中,这些都是为了方便在收到广播时,高速找到对应的广播接收器的。

        再往下看:

    BroadcastFilter bf = new BroadcastFilter(filter, rl, permission);
    rl.add(bf);
    ......
    mReceiverResolver.addFilter(bf);
        上面仅仅是把广播接收器receiver保存起来了,可是还没有把它和filter关联起来,这里就创建一个BroadcastFilter来把广播接收器列表rl和filter关联起来,然后保存在ActivityManagerService中的成员变量mReceiverResolver中去。

        这样,广播接收器注冊的过程就介绍完了,比較简单,可是工作又比較琐碎,主要就是将广播接收器receiver及其要接收的广播类型filter保存在ActivityManagerService中,以便以后能够接收到对应的广播并进行处理,在下一篇文章,我们将具体分析这个过程,敬请关注。

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Android应用程序注冊广播接收器(registerReceiver)的过程分析