广播接收器的注册
广播接收器，是用来接收系统和应用发出的广播，常见的是开机广播，可以用于实现开机启动服务的功能，还有网络变化，电池电量变化等等均会发出相应的广播。Android系统中的广播设计的很好，对于开发者而言非常容易上手。
静态注册
不管该应用程序是否处于活动状态，都会进行监听，比如某个程序是监听内存的使用情况的，当在手机上安装好后，不管该应用程序是处于什么状态，都会执行该监听方法中的内容。
静态注册即在AndroidManifest中注册广播接收器
//TODO 解释各个属性的意义
<receiver android:enabled=["true" | "false"]
android:exported=["true" | "false"]
android:icon="drawable resource"
android:label="string resource"
android:name="string"
android:permission="string"
android:process="string" >
. . .
</receiver>实例：
<!-- AndroidManifest.xml -->
<receiver android:name=".MyReceiver" android:enabled="true" android:exported="true">
<intent-filter>
<action android:name="MLY" />
</intent-filter>
</receiver>动态注册
在代码中进行注册后，当应用程序关闭后，就不再进行监听，因此一般在Activity创建的时候注册，在Activity销毁的时候取消注册。
注册
IntentFilter filter = new IntentFilter("");
BroadcastReceiver receiver = new BroadcastReceiver() {
@Override
public void onReceive(Context context, Intent intent) {
}
};
registerReceiver(receiver, filter);取消注册
unregisterReceiver(receiver);静态广播接收器的注册流程
Android系统启动后，PackageManagerService会扫描各个APK的AndroidManifest.xml，并解析其中的Receiver标签，最后将Receiver信息保存在PackageManagerService的receivers中。AMS就可以调用PMS的queryIntentReceivers函数获取到ResolveInfo列表，一个ResolveInfo代表一个BroadcastReceiver，AMS就可以通过ResolveInfo启动未启动的BroadcastReceiver所在的进程，然后将广播分发给静态注册的BroadcastReceiver。
动态广播接收器注册流程
从序列图上看，动态广播的注册流程还是很简单的，在Activity里调用registerReceiver实际是调用了ComtextImpl的registerReceiver函数，该函数会调用registerReceiverInternal函数
Step 1
ComtextImpl.registerReceiverInternal
该函数定义在frameworks/base/core/java/android/app/ContextImpl.java
private Intent registerReceiverInternal(BroadcastReceiver receiver, int userId,
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 {
//调用了ActivityManagerNative的registerReceiver函数
return ActivityManagerNative.getDefault().registerReceiver(
mMainThread.getApplicationThread(), mBasePackageName,
rd, filter, broadcastPermission, userId);
} catch (RemoteException e) {
return null;
}
}
该函数主要是获取一个IIntentReceiver对象，最后AMS在分发广播的时候会通过这个对象回调到当前这个进程，一般情况下Receiver，mPackageInfo和context不为空，所以该对象是通过mPackageInfo.getReceiverDispatcher来获取的。同时scheduler指定了最后回调的函数运行的Handler，如果未指定，则是在主线程中。最后调用ActivityManagerNative的registerReceiver函数，该函数通过binder调用到AMS的registerReceiver。
Step 2
LoadedApk.getReceiverDispatcher函数
该函数定义在frameworks/base/core/java/android/app/LoadedApk.java函数中
public IIntentReceiver getReceiverDispatcher(BroadcastReceiver r,
Context context, Handler handler,
Instrumentation instrumentation, boolean registered) {
synchronized (mReceivers) {
LoadedApk.ReceiverDispatcher rd = null;
ArrayMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher> map = null;
if (registered) {
map = mReceivers.get(context);
if (map != null) {
rd = map.get(r);
}
}
//一个BroadcastReceiver可以注册多次，如果是多次注册，这里的rd不为空，返回的IIntentReceiver也是相同的
if (rd == null) {
rd = new ReceiverDispatcher(r, context, handler,
instrumentation, registered);
if (registered) {
if (map == null) {
map = new ArrayMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher>();
mReceivers.put(context, map);
}
map.put(r, rd);
}
} else {
rd.validate(context, handler);
}
rd.mForgotten = false;
return rd.getIIntentReceiver();
}
}- registered参数表示是否是注册receiver函数调用。mReceivers是一个ArrayMap类型，key是Context，value也是ArrayMap类型，其中key是BroadcastReceiver，value是ReceiverDispatcher。
- 如果registered为true，函数首先会检查mReceivers是否已经注册了相同的BroadcastReceiver对象，如果是则直接返回该对象对应的ReceiverDispatcher中的IIntentReceiver。如果没有注册，则实例化一个ReceiverDispatcher对象，并将其加入到mReceivers中，然后其中的IIntentReceiver对象。
从这里可以看到一个BroadcastReceiver是可以注册多次的，这里可以指定不同的IntentFilter，但是每个BroadcastReceiver只能指定一个Handler。 - 如果registered为false，则直接实例化一个ReceiverDispatcher并返回，这里并不会将ReceiverDispatcher对象保存在mReceivers里。
Step 3
ActivityManagerService.registerReceiver
该函数定义在frameworks/base/core/java/com/android/server/am/ActivityManagerService.java
synchronized (this) {
ReceiverList rl = mRegisteredReceivers.get(receiver.asBinder());
//同一个BroadcastReceiver第一次注册时，rl为null
if (rl == null) {
rl = new ReceiverList(this, callerApp, callingPid, callingUid,
userId, receiver);
mRegisteredReceivers.put(receiver.asBinder(), rl);
} else if (rl.uid != callingUid) {
throw new IllegalArgumentException(
"Receiver requested to register for uid " + callingUid
+ " was previously registered for uid " + rl.uid);
} else if (rl.pid != callingPid) {
throw new IllegalArgumentException(
"Receiver requested to register for pid " + callingPid
+ " was previously registered for pid " + rl.pid);
} else if (rl.userId != userId) {
throw new IllegalArgumentException(
"Receiver requested to register for user " + userId
+ " was previously registered for user " + rl.userId);
}
BroadcastFilter bf = new BroadcastFilter(filter, rl, callerPackage,
permission, callingUid, userId);
rl.add(bf);
mReceiverResolver.addFilter(bf);
}这里只截取主要的代码
1. 如果一个BroadcastReceiver是第一次注册，则rl为空，会实例化一个ReceiverList，该类型为ArrayList，用于保存同一个BroadcastReceiver注册的多个IntentFilter，然后将实例化ReceiverList和IIntentReceiver的binder对象保存mRegisteredReceivers Map中。
2. 如果非第一次注册，则会检查pid,uid,userId是否一致，若不一致则抛出异常
3. 实例化一个BroadcastFilter，并加入ReceiverList,同时会保存在mReceiverResolver中
至此，动态广播的注册结束。AMS中mReceiverResolver里包含了BroadcastFilter，这个与IntentFilter一一对应，BroadcastFilter里包含了IntentFilter和IIntentReceiver。 AMS中mRegisteredReceivers中，Key是IIntentReceiver的binder对象，Value是一个列表，包含了该对象对应的多个BroadcastFilter（与IntentFilter一一对应）。
动态广播接收器取消注册流程
Activity调用unregisterReceiver实际调用了ContextImpl的unregisterReceiver函数，该函数中调用LoadedApk的forgetReceiverDispatcher函数以及调用ActivityManagerNative的unregisterReceiver
Step 1
LoadedApk.forgetReceiverDispatcher
该函数定义在frameworks/base/core/java/android/app/LoadedApk.java函数中
public IIntentReceiver forgetReceiverDispatcher(Context context,
BroadcastReceiver r) {
synchronized (mReceivers) {
ArrayMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher> map = mReceivers.get(context);
LoadedApk.ReceiverDispatcher rd = null;
if (map != null) {
rd = map.get(r);
if (rd != null) {
map.remove(r);
if (map.size() == 0) {
mReceivers.remove(context);
}
rd.mForgotten = true;
return rd.getIIntentReceiver();
}
}
}
}该函数主要是将mReceivers中保存的BroadcastReceiver删除，并返回IIntentReceiver对象
Step 2
ActivityManagerService.unregisterReceiver
该函数定义在frameworks/base/core/java/com/android/server/am/ActivityManagerService.java
public void unregisterReceiver(IIntentReceiver receiver) {
final long origId = Binder.clearCallingIdentity();
try {
boolean doTrim = false;
synchronized(this) {
ReceiverList rl = mRegisteredReceivers.get(receiver.asBinder());
if (rl != null) {
final BroadcastRecord r = rl.curBroadcast;
if (r != null && r == r.queue.getMatchingOrderedReceiver(r)) {
final boolean doNext = r.queue.finishReceiverLocked(
r, r.resultCode, r.resultData, r.resultExtras,
r.resultAbort, false);
if (doNext) {
doTrim = true;
r.queue.processNextBroadcast(false);
}
}
if (rl.app != null) {
rl.app.receivers.remove(rl);
}
removeReceiverLocked(rl);
if (rl.linkedToDeath) {
rl.linkedToDeath = false;
rl.receiver.asBinder().unlinkToDeath(rl, 0);
}
}
}
} finally {
Binder.restoreCallingIdentity(origId);
}
}该函数首先判断接收器当前是否正在接收广播，若正在接收广播，需要处理finishReceiverLocked处理。然后调用removeReceiverLocked函数移除IIntentReceiver
Step 3
ActivityManagerService.removeReceiverLocked
该函数定义在frameworks/base/core/java/com/android/server/am/ActivityManagerService.java
void removeReceiverLocked(ReceiverList rl) {
mRegisteredReceivers.remove(rl.receiver.asBinder());
for (int i = rl.size() - 1; i >= 0; i--) {
mReceiverResolver.removeFilter(rl.get(i));
}
}从前面动态广播的注册流程来看，当一个IIntentReceiver注册后，mRegisteredReceivers保存了IIntentReceiver对象及其所对应的BroadcastFilter列表。 mReceiverResolver会保存BroadcastFilter。因此这里取消注册时候，需要对应删除两个列表。
