Android应用程序发送广播(sendBroadcast)的过程分析

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前面我们分析了Android应用程序注册广播接收器的过程,这个过程只完成了万里长征的第一步,接下来它还要等待ActivityManagerService将广播分发过来。ActivityManagerService是如何得到广播并把它分发出去的呢?这就是本文要介绍的广播发送过程了。

广播的发送过程比广播接收器的注册过程要复杂得多了,不过这个过程仍然是以ActivityManagerService为中心。广播的发送者将广播发送到ActivityManagerService,ActivityManagerService接收到这个广播以后,就会在自己的注册中心查看有哪些广播接收器订阅了该广播,然后把这个广播逐一发送到这些广播接收器中,但是ActivityManagerService并不等待广播接收器处理这些广播就返回了,因此,广播的发送和处理是异步的。概括来说,广播的发送路径就是从发送者到ActivityManagerService,再从ActivityManagerService到接收者,这中间的两个过程都是通过Binder进程间通信机制来完成的,因此,希望读者在继续阅读本文之前,对Android系统的Binder进程间通信机制有所了解,具体可以参考Android进程间通信(IPC)机制Binder简要介绍和学习计划一文。

本文继续以Android系统中的广播(Broadcast)机制简要介绍和学习计划一文中所开发的应用程序为例子,并且结合上文Android应用程序注册广播接收器(registerReceiver)的过程分析的内容,一起来分析Android应用程序发送广播的过程。

回顾一下Android系统中的广播(Broadcast)机制简要介绍和学习计划一文中所开发的应用程序的组织架构,MainActivity向ActivityManagerService注册了一个CounterService.BROADCAST_COUNTER_ACTION类型的计数器服务广播接收器,计数器服务CounterService在后台线程中启动了一个异步任务(AsyncTask),这个异步任务负责不断地增加计数,并且不断地将当前计数值通过广播的形式发送出去,以便MainActivity可以将当前计数值在应用程序的界面线程中显示出来。

计数器服务CounterService发送广播的代码如下所示:

public class CounterService extends Service implements ICounterService {  
        ...... 

        public void startCounter(int initVal) {  
            AsyncTask<Integer, Integer, Integer> task = new AsyncTask<Integer, Integer, Integer>() {      
                @Override  
                protected Integer doInBackground(Integer... vals) {  
                    ......  
                }  

                @Override   
                protected void onProgressUpdate(Integer... values) {  
                    super.onProgressUpdate(values);  

                    int counter = values[0];  

                    Intent intent = new Intent(BROADCAST_COUNTER_ACTION);  
                    intent.putExtra(COUNTER_VALUE, counter);  

                    sendBroadcast(intent);  
                }  

                @Override  
                protected void onPostExecute(Integer val) {  
                    ...... 
                }  

            };  

            task.execute(0);      
        }  

        ......
    }

在onProgressUpdate函数中,创建了一个BROADCAST_COUNTER_ACTION类型的Intent,并且在这里个Intent中附加上当前的计数器值,然后通过CounterService类的成员函数sendBroadcast将这个Intent发送出去。CounterService类继承了Service类,Service类又继承了ContextWrapper类,成员函数sendBroadcast就是从ContextWrapper类继承下来的,因此,我们就从ContextWrapper类的sendBroadcast函数开始,分析广播发送的过程。

在继承分析广播的发送过程前,我们先来看一下广播发送过程的序列图,然后按照这个序图中的步骤来一步一步分析整个过程。

点击查看大图

Step 1. ContextWrapper.sendBroadcast

这个函数定义在frameworks/base/core/java/android/content/ContextWrapper.java文件中:

public class ContextWrapper extends Context {
        Context mBase;

        ......

        @Override
        public void sendBroadcast(Intent intent) {
            mBase.sendBroadcast(intent);
        }

        ......

    }

这里的成员变量mBase是一个ContextImpl实例,这里只简单地调用ContextImpl.sendBroadcast进一行操作。

Step 2. ContextImpl.sendBroadcast

这个函数定义在frameworks/base/core/java/android/app/ContextImpl.java文件中:

class ContextImpl extends Context {
        ......

        @Override
        public void sendBroadcast(Intent intent) {
            String resolvedType = intent.resolveTypeIfNeeded(getContentResolver());
            try {
                ActivityManagerNative.getDefault().broadcastIntent(
                    mMainThread.getApplicationThread(), intent, resolvedType, null,
                    Activity.RESULT_OK, null, null, null, false, false);
            } catch (RemoteException e) {
            }
        }

        ......

    }

这里的resolvedType表示这个Intent的MIME类型,我们没有设置这个Intent的MIME类型,因此,这里的resolvedType为null。接下来就调用ActivityManagerService的远程接口ActivityManagerProxy把这个广播发送给ActivityManagerService了。

Step 3. ActivityManagerProxy.broadcastIntent

这个函数定义在frameworks/base/core/java/android/app/ActivityManagerNative.java文件中:

class ActivityManagerProxy implements IActivityManager
    {
        ......

        public int broadcastIntent(IApplicationThread caller,
            Intent intent, String resolvedType,  IIntentReceiver resultTo,
            int resultCode, String resultData, Bundle map,
            String requiredPermission, boolean serialized,
            boolean sticky) throws RemoteException
        {
            Parcel data = Parcel.obtain();
            Parcel reply = Parcel.obtain();
            data.writeInterfaceToken(IActivityManager.descriptor);
            data.writeStrongBinder(caller != null ? caller.asBinder() : null);
            intent.writeToParcel(data, 0);
            data.writeString(resolvedType);
            data.writeStrongBinder(resultTo != null ? resultTo.asBinder() : null);
            data.writeInt(resultCode);
            data.writeString(resultData);
            data.writeBundle(map);
            data.writeString(requiredPermission);
            data.writeInt(serialized ? 1 : 0);
            data.writeInt(sticky ? 1 : 0);
            mRemote.transact(BROADCAST_INTENT_TRANSACTION, data, reply, 0);
            reply.readException();
            int res = reply.readInt();
            reply.recycle();
            data.recycle();
            return res;
        }

        ......

    }

这里的实现比较简单,把要传递的参数封装好,然后通过Binder驱动程序进入到ActivityManagerService的broadcastIntent函数中。

Step 4. ctivityManagerService.broadcastIntent

这个函数定义在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 final int broadcastIntent(IApplicationThread caller,
                Intent intent, String resolvedType, IIntentReceiver resultTo,
                int resultCode, String resultData, Bundle map,
                String requiredPermission, boolean serialized, boolean sticky) {
            synchronized(this) {
                intent = verifyBroadcastLocked(intent);

                final ProcessRecord callerApp = getRecordForAppLocked(caller);
                final int callingPid = Binder.getCallingPid();
                final int callingUid = Binder.getCallingUid();
                final long origId = Binder.clearCallingIdentity();
                int res = broadcastIntentLocked(callerApp,
                    callerApp != null ? callerApp.info.packageName : null,
                    intent, resolvedType, resultTo,
                    resultCode, resultData, map, requiredPermission, serialized,
                    sticky, callingPid, callingUid);
                Binder.restoreCallingIdentity(origId);
                return res;
            }
        }

        ......
    }

这里调用broadcastIntentLocked函数来进一步处理。

Step 5. ActivityManagerService.broadcastIntentLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中:

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

        private final int broadcastIntentLocked(ProcessRecord callerApp,
                String callerPackage, Intent intent, String resolvedType,
                IIntentReceiver resultTo, int resultCode, String resultData,
                Bundle map, String requiredPermission,
                boolean ordered, boolean sticky, int callingPid, int callingUid) {
            intent = new Intent(intent);

            ......

            // Figure out who all will receive this broadcast.
            List receivers = null;
            List<BroadcastFilter> registeredReceivers = null;
            try {
                if (intent.getComponent() != null) {
                    ......
                } else {
                    ......
                    registeredReceivers = mReceiverResolver.queryIntent(intent, resolvedType, false);
                }
            } catch (RemoteException ex) {
                ......
            }

            final boolean replacePending =
                (intent.getFlags()&Intent.FLAG_RECEIVER_REPLACE_PENDING) != 0;

            int NR = registeredReceivers != null ? registeredReceivers.size() : 0;
            if (!ordered && NR > 0) {
                // If we are not serializing this broadcast, then send the
                // registered receivers separately so they don't wait for the
                // components to be launched.
                BroadcastRecord r = new BroadcastRecord(intent, callerApp,
                    callerPackage, callingPid, callingUid, requiredPermission,
                    registeredReceivers, resultTo, resultCode, resultData, map,
                    ordered, sticky, false);
                ......
                boolean replaced = false;
                if (replacePending) {
                    for (int i=mParallelBroadcasts.size()-1; i>=0; i--) {
                        if (intent.filterEquals(mParallelBroadcasts.get(i).intent)) {
                            ......
                            mParallelBroadcasts.set(i, r);
                            replaced = true;
                            break;
                        }
                    }
                }

                if (!replaced) {
                    mParallelBroadcasts.add(r);

                    scheduleBroadcastsLocked();
                }

                registeredReceivers = null;
                NR = 0;
            }

            ......

        }

        ......
    }

这个函数首先是根据intent找出相应的广播接收器:

    // Figure out who all will receive this broadcast.
        List receivers = null;
        List<BroadcastFilter> registeredReceivers = null;
        try {
        if (intent.getComponent() != null) {
                ......
        } else {
            ......
            registeredReceivers = mReceiverResolver.queryIntent(intent, resolvedType, false);
        }
        } catch (RemoteException ex) {
        ......
        }

回忆一下前面一篇文章Android应用程序注册广播接收器(registerReceiver)的过程分析中的Step 6(ActivityManagerService.registerReceiver)中,我们将一个filter类型为BROADCAST_COUNTER_ACTION类型的BroadcastFilter实例保存在了ActivityManagerService的成员变量mReceiverResolver中,这个BroadcastFilter实例包含了我们所注册的广播接收器,这里就通过mReceiverResolver.queryIntent函数将这个BroadcastFilter实例取回来。由于注册一个广播类型的接收器可能有多个,所以这里把所有符合条件的的BroadcastFilter实例放在一个List中,然后返回来。在我们这个场景中,这个List就只有一个BroadcastFilter实例了,就是MainActivity注册的那个广播接收器。

继续往下看:

   final boolean replacePending =
            (intent.getFlags()&Intent.FLAG_RECEIVER_REPLACE_PENDING) != 0;

这里是查看一下这个intent的Intent.FLAG_RECEIVER_REPLACE_PENDING位有没有设置,如果设置了的话,ActivityManagerService就会在当前的系统中查看有没有相同的intent还未被处理,如果有的话,就有当前这个新的intent来替换旧的intent。这里,我们没有设置intent的Intent.FLAG_RECEIVER_REPLACE_PENDING位,因此,这里的replacePending变量为false。

再接着往下看:

   int NR = registeredReceivers != null ? registeredReceivers.size() : 0;
       if (!ordered && NR > 0) {
        // If we are not serializing this broadcast, then send the
        // registered receivers separately so they don't wait for the
        // components to be launched.
        BroadcastRecord r = new BroadcastRecord(intent, callerApp,
            callerPackage, callingPid, callingUid, requiredPermission,
            registeredReceivers, resultTo, resultCode, resultData, map,
            ordered, sticky, false);
        ......
        boolean replaced = false;
        if (replacePending) {
            for (int i=mParallelBroadcasts.size()-1; i>=0; i--) {
                if (intent.filterEquals(mParallelBroadcasts.get(i).intent)) {
                    ......
                    mParallelBroadcasts.set(i, r);
                    replaced = true;
                    break;
                }
            }
        }

        if (!replaced) {
            mParallelBroadcasts.add(r);

            scheduleBroadcastsLocked();
        }

        registeredReceivers = null;
        NR = 0;
        }

前面我们说到,这里得到的列表registeredReceivers的大小为1,且传进来的参数ordered为false,表示要将这个广播发送给所有注册了BROADCAST_COUNTER_ACTION类型广播的接收器,因此,会执行下面的if语句。这个if语句首先创建一个广播记录块BroadcastRecord,里面记录了这个广播是由谁发出的以及要发给谁等相关信息。由于前面得到的replacePending变量为false,因此,不会执行接下来的if语句,即不会检查系统中是否有相同类型的未处理的广播。

这样,这里得到的replaced变量的值也为false,于是,就会把这个广播记录块r放在ActivityManagerService的成员变量mParcelBroadcasts中,等待进一步处理;进一步处理的操作由函数scheduleBroadcastsLocked进行。

Step 6. ActivityManagerService.scheduleBroadcastsLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中:

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

        private final void scheduleBroadcastsLocked() {
            ......

            if (mBroadcastsScheduled) {
                return;
            }

            mHandler.sendEmptyMessage(BROADCAST_INTENT_MSG);
            mBroadcastsScheduled = true;
        }

        ......
    }

这里的mBroadcastsScheduled表示ActivityManagerService当前是不是正在处理其它广播,如果是的话,这里就先不处理直接返回了,保证所有广播串行处理。

注意这里处理广播的方式,它是通过消息循环来处理,每当ActivityManagerService接收到一个广播时,它就把这个广播放进自己的消息队列去就完事了,根本不管这个广播后续是处理的,因此,这里我们可以看出广播的发送和处理是异步的。

这里的成员变量mHandler是一个在ActivityManagerService内部定义的Handler类变量,通过它的sendEmptyMessage函数把一个类型为BROADCAST_INTENT_MSG的空消息放进ActivityManagerService的消息队列中去。这里的空消息是指这个消息除了有类型信息之外,没有任何其它额外的信息,因为前面已经把要处理的广播信息都保存在mParcelBroadcasts中了,等处理这个消息时,从mParcelBroadcasts就可以读回相关的广播信息了,因此,这里不需要把广播信息再放在消息内容中。

Step 7. Handler.sendEmptyMessage

这个自定义的Handler类实现在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中,它是ActivityManagerService的内部类,调用了它的sendEmptyMessage函数来把一个消息放到消息队列后,一会就会调用它的handleMessage函数来真正处理这个消息:

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

        final Handler mHandler = new Handler() {
            public void handleMessage(Message msg) {
                switch (msg.what) {
                ......
                case BROADCAST_INTENT_MSG: {
                    ......
                    processNextBroadcast(true);
                } break;
                ......
                }
            }
        }

        ......
    } 

这里又调用了ActivityManagerService的processNextBroadcast函数来处理下一个未处理的广播。

Step 8. ActivityManagerService.processNextBroadcast

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中:

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

        private final void processNextBroadcast(boolean fromMsg) {
            synchronized(this) {
                BroadcastRecord r;

                ......

                if (fromMsg) {
                    mBroadcastsScheduled = false;
                }

                // First, deliver any non-serialized broadcasts right away.
                while (mParallelBroadcasts.size() > 0) {
                    r = mParallelBroadcasts.remove(0);
                    ......
                    final int N = r.receivers.size();
                    ......
                    for (int i=0; i<N; i++) {
                        Object target = r.receivers.get(i);
                        ......

                        deliverToRegisteredReceiverLocked(r, (BroadcastFilter)target, false);
                    }
                    addBroadcastToHistoryLocked(r);
                    ......
                }

                ......

            }
        }

        ......
    }

这里传进来的参数fromMsg为true,于是把mBroadcastScheduled重新设为false,这样,下一个广播就能进入到消息队列中进行处理了。前面我们在Step 5中,把一个广播记录块BroadcastRecord放在了mParallelBroadcasts中,因此,这里就把它取出来进行处理了。广播记录块BroadcastRecord的receivers列表中包含了要接收这个广播的目标列表,即前面我们注册的广播接收器,用BroadcastFilter来表示,这里while循环中的for循环就是把这个广播发送给每一个订阅了该广播的接收器了,通过deliverToRegisteredReceiverLocked函数执行。

Step 9. ActivityManagerService.deliverToRegisteredReceiverLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中:

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

        private final void deliverToRegisteredReceiverLocked(BroadcastRecord r,
                BroadcastFilter filter, boolean ordered) {
            boolean skip = false;
            if (filter.requiredPermission != null) {
                ......
            }
            if (r.requiredPermission != null) {
                ......
            }

            if (!skip) {
                // If this is not being sent as an ordered broadcast, then we
                // don't want to touch the fields that keep track of the current
                // state of ordered broadcasts.
                if (ordered) {
                    ......
                }

                try {
                    ......
                    performReceiveLocked(filter.receiverList.app, filter.receiverList.receiver,
                        new Intent(r.intent), r.resultCode,
                        r.resultData, r.resultExtras, r.ordered, r.initialSticky);
                    ......
                } catch (RemoteException e) {
                    ......
                }
            }

        }

        ......
    }

函数首先是检查一下广播发送和接收的权限,在我们分析的这个场景中,没有设置权限,因此,这个权限检查就跳过了,这里得到的skip为false,于是进入下面的if语句中。由于上面传时来的ordered参数为false,因此,直接就调用performReceiveLocked函数来进一步执行广播发送的操作了。

Step 10. ActivityManagerService.performReceiveLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中:

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

        static void performReceiveLocked(ProcessRecord app, IIntentReceiver receiver,
                Intent intent, int resultCode, String data, Bundle extras,
                boolean ordered, boolean sticky) throws RemoteException {
            // Send the intent to the receiver asynchronously using one-way binder calls.
            if (app != null && app.thread != null) {
                // If we have an app thread, do the call through that so it is
                // correctly ordered with other one-way calls.
                app.thread.scheduleRegisteredReceiver(receiver, intent, resultCode,
                        data, extras, ordered, sticky);
            } else {
                ......
            }
        }

        ......
    }

注意,这里传进来的参数app是注册广播接收器的Activity所在的进程记录块,在我们分析的这个场景中,由于是MainActivity调用registerReceiver函数来注册这个广播接收器的,因此,参数app所代表的ProcessRecord就是MainActivity所在的进程记录块了;而参数receiver也是注册广播接收器时传给ActivityManagerService的一个Binder对象,它的类型是IIntentReceiver,具体可以参考上一篇文章Android应用程序注册广播接收器(registerReceiver)的过程分析中的Step 2。

MainActivity在注册广播接收器时,已经把自己的ProcessRecord记录下来了,所以这里的参数app和app.thread均不为null,于是,ActivityManagerService就调用app.thread.scheduleRegisteredReceiver函数来把这个广播分发给MainActivity了。这里的app.thread是一个Binder远程对象,它的类型是ApplicationThreadProxy,我们在前面介绍应用程序的Activity启动过程时,已经多次看到了,具体可以参考主题Android应用程序的Activity启动过程简要介绍和学习计划

Step 11. ApplicationThreadProxy.scheduleRegisteredReceiver 这个函数定义在frameworks/base/core/java/android/app/ApplicationThreadNative.java文件中:

class ApplicationThreadProxy implements IApplicationThread {
        ......

        public void scheduleRegisteredReceiver(IIntentReceiver receiver, Intent intent,
                int resultCode, String dataStr, Bundle extras, boolean ordered, boolean sticky)
                throws RemoteException {
            Parcel data = Parcel.obtain();
            data.writeInterfaceToken(IApplicationThread.descriptor);
            data.writeStrongBinder(receiver.asBinder());
            intent.writeToParcel(data, 0);
            data.writeInt(resultCode);
            data.writeString(dataStr);
            data.writeBundle(extras);
            data.writeInt(ordered ? 1 : 0);
            data.writeInt(sticky ? 1 : 0);
            mRemote.transact(SCHEDULE_REGISTERED_RECEIVER_TRANSACTION, data, null,
                IBinder.FLAG_ONEWAY);
            data.recycle();
        }

        ......
    }

这里通过Binder驱动程序就进入到ApplicationThread.scheduleRegisteredReceiver函数去了。ApplicationThread是ActivityThread的一个内部类,具体可以参考Activity启动主题Android应用程序的Activity启动过程简要介绍和学习计划

Step 12. ApplicaitonThread.scheduleRegisteredReceiver 这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中:

public final class ActivityThread {
        ......

        private final class ApplicationThread extends ApplicationThreadNative {
            ......

            // This function exists to make sure all receiver dispatching is
            // correctly ordered, since these are one-way calls and the binder driver
            // applies transaction ordering per object for such calls.
            public void scheduleRegisteredReceiver(IIntentReceiver receiver, Intent intent,
                    int resultCode, String dataStr, Bundle extras, boolean ordered,
                    boolean sticky) throws RemoteException {
                receiver.performReceive(intent, resultCode, dataStr, extras, ordered, sticky);
            }

            ......
        }

        ......

    }

这里的receiver是在前面一篇文章Android应用程序注册广播接收器(registerReceiver)的过程分析中的Step 4中创建的,它的具体类型是LoadedApk.ReceiverDispatcher.InnerReceiver,即定义在LoadedApk类的内部类ReceiverDispatcher里面的一个内部类InnerReceiver,这里调用它的performReceive函数。

Step 13. InnerReceiver.performReceive

这个函数定义在frameworks/base/core/java/android/app/LoadedApk.java文件中:

final class LoadedApk {  
        ...... 

        static final class ReceiverDispatcher {  

            final static class InnerReceiver extends IIntentReceiver.Stub { 
                ......

                public void performReceive(Intent intent, int resultCode,
                        String data, Bundle extras, boolean ordered, boolean sticky) {

                    LoadedApk.ReceiverDispatcher rd = mDispatcher.get();
                    ......
                    if (rd != null) {
                        rd.performReceive(intent, resultCode, data, extras,
                                ordered, sticky);
                    } else {
                        ......
                    }
                }
            }

            ......
        }

        ......
    }

这里,它只是简单地调用ReceiverDispatcher的performReceive函数来进一步处理,这里的ReceiverDispatcher类是LoadedApk类里面的一个内部类。

Step 14. ReceiverDispatcher.performReceive

这个函数定义在frameworks/base/core/java/android/app/LoadedApk.java文件中:

final class LoadedApk {  
        ...... 

        static final class ReceiverDispatcher {  
            ......

            public void performReceive(Intent intent, int resultCode,
                    String data, Bundle extras, boolean ordered, boolean sticky) {
                ......

                Args args = new Args();
                args.mCurIntent = intent;
                args.mCurCode = resultCode;
                args.mCurData = data;
                args.mCurMap = extras;
                args.mCurOrdered = ordered;
                args.mCurSticky = sticky;
                if (!mActivityThread.post(args)) {
                    ......
                } 
            }

            ......
        }

        ......
    }

这里mActivityThread成员变量的类型为Handler,它是前面MainActivity注册广播接收器时,从ActivityThread取得的,具体可以参考前面一篇文章Android应用程序注册广播接收器(registerReceiver)的过程分析中的Step 3。这里ReceiverDispatcher借助这个Handler,把这个广播以消息的形式放到MainActivity所在的这个ActivityThread的消息队列中去,因此,ReceiverDispatcher不等这个广播被MainActivity处理就返回了,这里也体现了广播的发送和处理是异步进行的。

注意这里处理消息的方式是通过Handler.post函数进行的,post函数的参数是Runnable类型的,这个消息最终会调用这个这个参数的run成员函数来处理。这里的Args类是LoadedApk类的内部类ReceiverDispatcher的一个内部类,它继承于Runnable类,因此,可以作为mActivityThread.post的参数传进去,代表这个广播的intent也保存在这个Args实例中。

Step 15. Hanlder.post

这个函数定义在frameworks/base/core/java/android/os/Handler.java文件中,这个函数我们就不看了,有兴趣的读者可以自己研究一下,它的作用就是把消息放在消息队列中,然后就返回了,这个消息最终会在传进来的Runnable类型的参数的run成员函数中进行处理。

Step 16. Args.run

这个函数定义在frameworks/base/core/java/android/app/LoadedApk.java文件中:

final class LoadedApk {  
        ...... 

        static final class ReceiverDispatcher {
            ......

            final class Args implements Runnable {
                ......

                public void run() {
                    BroadcastReceiver receiver = mReceiver;

                    ......

                    Intent intent = mCurIntent;

                    ......

                    try {
                        ClassLoader cl =  mReceiver.getClass().getClassLoader();
                        intent.setExtrasClassLoader(cl);
                        if (mCurMap != null) {
                            mCurMap.setClassLoader(cl);
                        }
                        receiver.setOrderedHint(true);
                        receiver.setResult(mCurCode, mCurData, mCurMap);
                        receiver.clearAbortBroadcast();
                        receiver.setOrderedHint(mCurOrdered);
                        receiver.setInitialStickyHint(mCurSticky);
                        receiver.onReceive(mContext, intent);
                    } catch (Exception e) {
                        ......
                    }

                    ......
                }

                ......
            }

            ......
        }

        ......
    }

这里的mReceiver是ReceiverDispatcher类的成员变量,它的类型是BroadcastReceiver,这里它就是MainActivity注册广播接收器时创建的BroadcastReceiver实例了,具体可以参考前面一篇文章Android应用程序注册广播接收器(registerReceiver)的过程分析中的Step 2。

有了这个ReceiverDispatcher实例之后,就可以调用它的onReceive函数把这个广播分发给它处理了。

Step 17. BroadcastReceiver.onReceive

这个函数定义Android系统中的广播(Broadcast)机制简要介绍和学习计划一文中所介绍的Android应用程序Broadcast的工程目录下的src/shy/luo/broadcast/MainActivity.java文件中:

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

        private BroadcastReceiver counterActionReceiver = new BroadcastReceiver(){  
            public void onReceive(Context context, Intent intent) {  
                int counter = intent.getIntExtra(CounterService.COUNTER_VALUE, 0);  
                String text = String.valueOf(counter);  
                counterText.setText(text);  

                Log.i(LOG_TAG, "Receive counter event");  
            }    
        }

        ......  

    }

这样,MainActivity里面的定义的BroadcastReceiver实例counterActionReceiver就收到这个广播并进行处理了。 至此,Android应用程序发送广播的过程就分析完成了,结合前面这篇分析广播接收器注册过程的文章Android应用程序注册广播接收器(registerReceiver)的过程分析,就会对Android系统的广播机制且个更深刻的认识和理解了。

最后,我们总结一下这个Android应用程序发送广播的过程:

1. Step 1 - Step 7,计数器服务CounterService通过sendBroadcast把一个广播通过Binder进程间通信机制发送给ActivityManagerService,ActivityManagerService根据这个广播的Action类型找到相应的广播接收器,然后把这个广播放进自己的消息队列中去,就完成第一阶段对这个广播的异步分发了;

2. Step 8 - Step 15,ActivityManagerService在消息循环中处理这个广播,并通过Binder进程间通信机制把这个广播分发给注册的广播接收分发器ReceiverDispatcher,ReceiverDispatcher把这个广播放进MainActivity所在的线程的消息队列中去,就完成第二阶段对这个广播的异步分发了;

3. Step 16 - Step 17, ReceiverDispatcher的内部类Args在MainActivity所在的线程消息循环中处理这个广播,最终是将这个广播分发给所注册的BroadcastReceiver实例的onReceive函数进行处理。

这样,Android系统广播机制就学习完成了,希望对读者有所帮助。重新学习Android系统的广播机制,请回到Android系统中的广播(Broadcast)机制简要介绍和学习计划一文中。

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