在Android系统中，广播（Broadcast）是在组件之间传播数据（Intent）的一种机制；这些组件甚至是可以位于不同的进程中，这样它就像Binder机制一样，起到进程间通信的作用。
在Android系统中，为什么需要广播机制呢？如果是两个组件位于不同的进程当中，那么可以用Binder机制来实现，如果两个组件是在同一个进程中，那么它们之间可以用来通信的方式就更多了，这样看来，广播机制似乎是多余的。
然而，广播机制却是不可替代的，它和Binder机制不一样的地方在于，广播的发送者和接收者事先是不需要知道对方的存在的，这样带来的好处便是，系统的各个组件可以松耦合地组织在一起，这样系统就具有高度的可扩展性，容易与其它系统进行集成。
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使用广播的两个步骤： 1、广播的接收者需要通过调用registerReceiver函数告诉系统，它对什么样的广播有兴趣，即指定IntentFilter，并且向系统注册广播接收器，即指定BroadcastReceiver：
IntentFilter counterActionFilter = new IntentFilter(CounterService.BROADCAST_COUNTER_ACTION); registerReceiver(counterActionReceiver, counterActionFilter);
这里，指定感兴趣的广播就是CounterService.BROADCAST_COUNTER_ACTION了，而指定的广播接收器就是counterActonReceiver，它是一个BroadcastReceiver类型的实例。
2、广播的发送者通过调用sendBroadcast函数来发送一个指定的广播，并且可以指定广播的相关参数：
Intent intent = new Intent(BROADCAST_COUNTER_ACTION); intent.putExtra(COUNTER_VALUE, counter); sendBroadcast(intent)
这里，指定的广播为CounterService.BROADCAST_COUNTER_ACTION，并且附带的带参数当前的计数器值counter。调用了sendBroadcast函数之后，所有注册了CounterService.BROADCAST_COUNTER_ACTION广播的接收者便可以收到这个广播了。
在第1步中，广播的接收者把广播接收器注册到ActivityManagerService中；在第2步中，广播的发送者同样是把广播发送到ActivityManagerService中，由ActivityManagerService去查找注册了这个广播的接收者，然后把广播分发给它们。
在第2步的分发的过程，其实就是把这个广播转换成一个消息，然后放入到接收器所在的线程消息队列中去，最后就可以在消息循环中调用接收器的onReceive函数了。这里有一个要非常注意的地方是，由于ActivityManagerService把这个广播放进接收器所在的线程消息队列后，就返回了，它不关心这个消息什么时候会被处理，因此，对广播的处理是异步的，即调用sendBroadcast时，这个函数不会等待这个广播被处理完后才返回。
虚线上面Step 1到Step 4步是注册广播接收器的过程，其中Step 2通过LoadedApk.getReceiverDispatcher在LoadedApk内部创建了一个IIntentReceiver接口，并且传递给ActivityManagerService；
虚线下面的Step 5到Step 11是发送广播的过程，在Step 8中，ActivityManagerService利用上面得到的IIntentReceiver远程接口，调用LoadedApk.performReceiver接口，LoadedApk.performReceiver接口通过ActivityThread.H接口的post函数将这个广播消息放入到ActivityThread的消息队列中去，最后这个消息在LoadedApk的Args.run函数中处理，LoadedApk.Args.run函数接着调用MainActivity.BroadcastReceiver的onReceive函数来最终处理这个广播。
注册广播接收器（registerReceiver）
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。
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); } ...... }Step 1. ContextWrapper.registerReceiver
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。
Step 2. ContextImpl.registerReceiver
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。
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。
Step 3. ActivityThread.getHandler
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。
Step 4. LoadedApk.getReceiverDispatcher
final class LoadedApk {...... public IIntentReceiver getReceiverDispatcher(BroadcastReceiver r, Context context, Handler handler, Instrumentation instrumentation, boolean registered) {synchronized (mReceivers) {LoadedApk.ReceiverDispatcher rd = null; HashMap 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(); 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 mDispatcher; ...... InnerReceiver(LoadedApk.ReceiverDispatcher rd, boolean strong) {mDispatcher = new WeakReference(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
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
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。
发送广播（sendBroadcast）的过程分析
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。
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。
在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。
Step 1. ContextWrapper.sendBroadcast
public class ContextWrapper extends Context {Context mBase; ...... @Override public void sendBroadcast(Intent intent) {mBase.sendBroadcast(intent); } ...... } Step 2. ContextImpl.sendBroadcast
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
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; } ...... } Step 4. ActivityManagerService.broadcastIntent
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
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 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; ListregisteredReceivers = null; try {if (intent.getComponent() != null) {...... } else {...... registeredReceivers = mReceiverResolver.queryIntent(intent, resolvedType, false); } } catch (RemoteException ex) {...... }
回忆一下前面过程分析中的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
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
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; iObject 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
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
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。
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。
Step 11. ApplicationThreadProxy.scheduleRegisteredReceiver
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。
Step 12. ApplicaitonThread.scheduleRegisteredReceiver
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
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
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。
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，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
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。
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
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。
最后，我们总结一下这个Android应用程序发送广播的过程：
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；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；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。