#### **参考文章**
[Android应用程序注册广播接收器（registerReceiver）的过程分析](https://ihavenolimitations.xyz/alex_wsc/androids/477750)
#### **观察者模式的深入探讨**
BroadcastReceiver是Android 的四大组件之一，它作为应用内、进程间的一种重要通信手段，能够将某个消息通过广播的形式传递给它注册的对应广播接收器的对象，接收对象需要通过Context的registerReceiver函数注册到AMS中，当通过sendBroadcast发送广播时，所有注册了对应的lntentfilter的BroadcastReceiver对象就会接收到这个消息，BroadcastReceiver 的onReceive方法就会被调用，这就是一个典型的发布——订阅模式，也就是我们的观察者模式。广播接收器的注册调用时序图如图1所示。
:-: 
图1 注册广播接收器（registerReceiver）的过程
#### **实例**：
例：我们在MainActivity中注册一个广播接收器。
~~~
public class MainActivity extends Activity{
......
@Override
public void onResume() {
super.onResume();
IntentFilter updateFilter = new IntentFilter("info.update");
registerReceiver(updateReceiver, updateFilter);
}
BrocastReceiver updateReceiver=new BrocastReceiver(){
@Override
public void onReceive(Context context,Intent intent) {
Toast.makeText(getApplicationContext,"update",Toast.LENGTH_SHORT).show();
}
};
......
}
~~~
我们在MainActivity的onResume里注册了一个只接收Action为“info.update”的广播接收器，应用内的其他地方发布一个Action为“info.update”的广播时，就会触发updateReceiver的onReceive函数。下面我们就来一步一步分析广播接收器的基本原理。
我们发现registerReceiver函数并不是在Activity中实现，因此，我们把目标移向Activity的父类ContextWrapper, registerReceiver 函数如下：
**Step 1. [ContextWrapper](https://www.androidos.net.cn/android/2.3.7_r1/xref/frameworks/base/core/java/android/content/ContextWrapper.java).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，继续转移到Contextlmpl 的registerReceiver 函数， 具体代码如下：
**Step 2. [ContextImpl](https://www.androidos.net.cn/android/2.3.7_r1/xref/frameworks/base/core/java/android/app/ContextImpl.java).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(); //获取Handler 来投递消息
}
//获取 IIntentReceiver 对象，通过它与AMS交互，并且通过Handler传递消息
rd = mPackageInfo.getReceiverDispatcher(
receiver, context, scheduler,
mMainThread.getInstrumentation(), true);
} else {
......
}
}
try {
//调用ActivityManagerNative 的registerReceiver
return ActivityManagerNative.getDefault().registerReceiver(
mMainThread.getApplicationThread(),
rd, filter, broadcastPermission);
} catch (RemoteException e) {
return null;
}
}
......
}
~~~
注册广播接收器的函数调用最终就进入到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，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](https://www.androidos.net.cn/android/2.3.7_r1/xref/frameworks/base/core/java/android/app/ActivityThread.java).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](https://www.androidos.net.cn/android/2.3.7_r1/xref/frameworks/base/core/java/android/app/LoadedApk.java).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) {
// BroadcastReceiver 为key , ReceiverDispatcher 为value
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](https://www.androidos.net.cn/android/2.3.7_r1/xref/frameworks/base/core/java/android/app/ActivityManagerNative.java).registerReceiver**
这个函数实现在frameworks/base/core/java/android/app/ActivityManagerNative.java-ActivityManagerProxy.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);
// 向AMS 提交注册广播接收器的请求
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](https://www.androidos.net.cn/android/2.3.7_r1/xref/frameworks/base/services/java/com/android/server/am/ActivityManagerService.java).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) {
// 1 . 获取ProcessRecord
ProcessRecord callerApp = null;
if (caller != null) {
callerApp = getRecordForAppLocked(caller);
if (callerApp == null) {
......
}
}
List allSticky = null;
// 2. 根据Action 查找匹配的sticky 接收器
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;
}
// 3 . 获取ReceiverList
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);
}
// 4 . 构建BroadcastFilter 对象，并且添加到ReceiverList 中
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，即注释1 处，然后获取这个进程对应的pid、uida。注释2 处获取到IntentFilter的所有Action，在MainActivity 中构建的filter只有一个Action，就是前面描述的“info.update”，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，假设不使用sendStickyBroadcast来发送“info.update”类型的广播，于是，得到的allSticky和sticky都为null。
继续往下看，这里传进来的receiver不为null，于是，继续往下执行到注释3处。这里其实就是把广播接收器receiver保存到一个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，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mRegistered Receivers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。
再往下看注释4，只是把广播接收器receiver保存起来，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的成员变量mReceiverResolver中，这样，就将广播接收器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。
### **广播的发送过程**
广播的发送过程比广播接收器的注册过程要复杂得多，不过这个过程仍然是以ActivityManagerService 为中心。具体详情可参考[Android应用程序发送广播（sendBroadcast）的过程分析](https://ihavenolimitations.xyz/alex_wsc/androids/477751)
这里，我们简单总结一下Android 应用程序发送广播的过程。
（1）通过send.Broadcast 把一个广播通过Binder 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，就完成第一阶段对这个广播的异步分发：
~~~
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 {
............
// 查询到该Intent 对应的BroadcastFilter，也就是接收器列表
registeredReceivers = mReceiverResolver.queryIntent(intent, resolvedType, false);
}
} catch (RemoteException ex) {
// pm is in same process, this will never happen.
}
.........
if (!ordered && NR > 0) {
..........
if (!replaced) {
mParallelBroadcasts.add(r);
scheduleBroadcastsLocked();// 处理广播的异步分发
}
registeredReceivers = null;
NR = 0;
}
.........
}
.........
}
~~~
（2）ActivityManagerService 在消息循环中处理这个广播， 并通过Binder 机制把这个广播分发给注册的ReceiverDispatcher, ReceiverDispatcher 把这个广播放进MainActivity 所在线程的消息队列中，就完成第二阶段对这个广播的异步分发：
~~~
public final class ActivityManagerService extends ActivityManagerNative
implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
.............
private final void scheduleBroadcastsLocked() {
if (DEBUG_BROADCAST) Slog.v(TAG, "Schedule broadcasts: current="
+ mBroadcastsScheduled);
if (mBroadcastsScheduled) {
return;
}
// mHandler 发送一个空消息
mHandler.sendEmptyMessage(BROADCAST_INTENT_MSG);
mBroadcastsScheduled = true;
}
............
//mHandler的类型为Handler，是AMS的内部类
final Handler mHandler = new Handler() {
public void handleMessage(Message msg) {
switch (msg.what) {
..........
case BROADCAST_INTENT_MSG: {
if (DEBUG_BROADCAST) Slog.v(
TAG, "Received BROADCAST_INTENT_MSG");
processNextBroadcast(true);//处理下一个广播
} break;
..........
}
}
};
...........
}
~~~
（3）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：
~~~
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);
// 调用接收器的onReceive方法
receiver.onReceive(mContext, intent);
} catch (Exception e) {
.........
}
...........
}
}
..........
}
.........
}
~~~
简单来说， 广播这就是一个订阅一一发布的过程，通过一些map 存储BroadcastReceiver, key就是封装了这些广播的信息类，如Action 之类的。当发布一个广播时通过AMS 到这个map 中查询注册了这个广播的IntentFilter 的BroadcastReceiver，然后通过ReceiverDispatcher 将广播分发给各个订阅对象，从而完成这个发布一一订阅过程。
