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在Android系统中,为什么需要广播机制呢?广播机制,本质上它就是一种组件间的通信方式,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。在软件工程中,是非常强调模块之间的高内聚低耦合性的,不然的话,随着系统越来越庞大,就会面临着越来越难维护的风险,最后导致整个项目的失败。Android应用程序的组织方式,可以说是把这种高内聚低耦合性的思想贯彻得非常透彻,在任何一个Activity中,都可以使用一个简单的Intent,通过startActivity或者startService,就可以把另外一个Activity或者Service启动起来为它服务,而且它根本上不依赖这个Activity或者Service的实现,只需要知道它的字符串形式的名字即可,而广播机制更绝,它连接收者的名字都不需要知道。
在分析之前,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。
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。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) {
//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) {
......
}
}
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;
}
}
......
}
下面详细介绍下面代码的含义。
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。
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