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。
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。
