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

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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/发布机制,因此,使用这种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,订阅消息其实就是注册广播接收器,本文将探讨Android应用程序是如何注册广播接收器以及把广播接收器注册到哪里去的。

在Android的广播机制中,ActivityManagerService扮演着广播中心的角色,负责系统中所有广播的注册和发布操作,因此,Android应用程序注册广播接收器的过程就把是广播接收器注册到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注册到ActivityManagerService中去的,而ContextWrapper类本身又借助ContextImpl类来注册广播接收器。

在Android应用程序框架中,Activity和Service类都继承了ContextWrapper类,因此,我们可以在Activity或者Service的子类中调用registerReceiver函数来注册广播接收器。Activity、Service、ContextWrapper和ContextImpl这四个类的关系可以参考前面Android系统在新进程中启动自定义服务过程(startService)的原理分析一文中描述的Activity类图。

这篇文章还是继续以实例来进行情景分析,所用到的例子便是上一篇文章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;又由于Android应用程序是把广播接器注册到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,具体请参考Android进程间通信(IPC)机制Binder简要介绍和学习计划一文。

开始进入主题了,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例子中,注册广播接收器的操作是MainActivity发起的,我们先来看看注册过程的序列图:

在分析这个序列图之前,我们先来看一下MainActivity是如何调用registerReceiver函数来注册广播接收器的:

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函数注册了一个BroadcastReceiver实例counterActionReceiver,并且通过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,就开始分析注册过程中的每一个步骤了。

Step 1. ContextWrapper.registerReceiver

这个函数实现在frameworks/base/core/java/android/content/ContextWrapper.java文件中:

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,可以回过头去看看Android应用程序启动过程源代码分析这篇文章>~<。

Step 2. ContextImpl.registerReceiver

这个函数实现在frameworks/base/core/java/android/app/ContextImpl.java文件中:

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,最终就进入到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),会详细描述。参数broadcastPermission和scheduler都为null,而参数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。

由于条件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代码分析这篇文章也描述过了。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

这个函数实现在frameworks/base/core/java/android/app/ActivityThread.java文件中:

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,就可以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在收到相应的广播时,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

这个函数实现在frameworks/base/core/java/android/app/LoadedApk.java文件中:

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,首先看一下参数r是不是已经有相应的ReceiverDispatcher存在了,如果有,就直接返回了,否则就新建一个ReceiverDispatcher,并且以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,只要给定一个Activity和BroadcastReceiver,就可以查看LoadedApk里面是否已经存在相应的广播接收发布器ReceiverDispatcher了。

在新建广播接收发布器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,可以通过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类型的参数activityThread保存下来,以便后面在分发广播的时候使用。

现在,再回到ContextImpl.registerReceiverInternal函数,在获得了IIntentReceiver类型的Binder对象后,就开始要把它注册到ActivityManagerService中去了。

Step 5. ActivityManagerProxy.registerReceiver

这个函数实现在frameworks/base/core/java/android/app/ActivityManagerNative.java文件中:

class ActivityManagerProxy implements IActivityManager
    {
        ......

        public Intent registerReceiver(IApplicationThread caller,
                IIntentReceiver receiver,
                IntentFilter filter, String perm) throws RemoteException
        {
            Parcel data = Parcel.obtain();
            Parcel reply = Parcel.obtain();
            data.writeInterfaceToken(IActivityManager.descriptor);
            data.writeStrongBinder(caller != null ? caller.asBinder() : null);
            data.writeStrongBinder(receiver != null ? receiver.asBinder() : null);
            filter.writeToParcel(data, 0);
            data.writeString(perm);
            mRemote.transact(REGISTER_RECEIVER_TRANSACTION, data, reply, 0);
            reply.readException();
            Intent intent = null;
            int haveIntent = reply.readInt();
            if (haveIntent != 0) {
                intent = Intent.CREATOR.createFromParcel(reply);
            }
            reply.recycle();
            data.recycle();
            return intent;
        }

        ......

    }

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

这个函数实现在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中:

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只有一个action,就是前面描述的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来注册相同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虽然被处理完了,但是仍然被粘住在ActivityManagerService中,以便下一个注册相应Action类型的广播接收器还能继承处理。

这里,假设我们不使用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,继续往下执行:

    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);
        }

这里其实就是把广播接收器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册的广播接收器。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,快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);

上面只是把广播接收器receiver保存起来了,但是还没有把它和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,广播接收器注册的过程就介绍完了,比较简单,但是工作又比较琐碎,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相应的广播并进行处理,在下一篇文章,我们将详细分析这个过程,敬请关注。

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