Android Broadcast原理分析之registerReceiver（一）

1. BroadcastReceiver概述

广播作为四大组件之一，在平时开发过程中会大量使用到，使用方式也是多种多样的，既可以自己在manifest中注册，也可以在java代码中动态注册，既可以接收由系统发出的广播，也可以接受自己定义并发送的广播。广播可以实现进程内以及跨进程之间的通信。从本文开始将分别介绍广播的注册，广播的派发，本地广播（LocalBroadcast）以及Android O上对广播的限制，本文主要介绍广播动态注册。

2. BroadcastReceiver分类

从注册方式上区分：动态注册以及静态注册（显示广播和隐式广播）
从发送方式上区分：无序广播和有序广播
从处理类型上区分：前台广播和后台广播
从运行方式上区分：普通广播和Sticky广播（已弃用）
从发送者区分：系统广播和自定义广播
此外还有protect broadcast（只允许指定应用可以发送）
sticky广播：系统保存了一部分广播的状态，当你注册的时候，不需要等到下次这个广播发出来，直接把最近上一次发送的这个广播返回给你

以上的这些概念在接下来的介绍中都会逐个涉及。

3. registerReceiver流程图

其中的APP，ContextImpl，LoadedApk，ActivityManagerProxy都在APP本身的进程中，ActivityManagerService在system_server进程中。

首先在APP的进程中发起广播的注册，通过registerReceiver接口，这个方法有很多重载方法，但是最终的入口都是在ContextImpl中，后面会详细介绍
从之前的Context的学习可以知道，registerReceiver最终调用的实现在ContextImpl
如果没有指定处理广播的handler，则默认使用主线程的handler
获取要注册的ReceiverDispatcher，在注册的Context相同的情况下，每个Receiver对应一个ReceiverDispatcher
通过binder call到systemserver进行广播注册

4. 源码解析

4.1 ContextImpl.registerReceiverInternal

    private Intent registerReceiverInternal(BroadcastReceiver receiver, int userId,IntentFilter filter, String broadcastPermission,Handler scheduler, Context context, int flags) {IIntentReceiver rd = null;if (receiver != null) {if (mPackageInfo != null && context != null) {if (scheduler == null) {// 注册receiver的时候可以指定接受recover的Handler// 如果没有指定，则默认用主线程的handler处理scheduler = mMainThread.getHandler();}// 获取IIntentReceiver// 这个是一个Binder对象，当广播来临时，用于AMS向客户端发起回调rd = mPackageInfo.getReceiverDispatcher(receiver, context, scheduler,mMainThread.getInstrumentation(), true);} else {if (scheduler == null) {scheduler = mMainThread.getHandler();}rd = new LoadedApk.ReceiverDispatcher(receiver, context, scheduler, null, true).getIIntentReceiver();}}try {// binder call至AMS，进行广播注册final Intent intent = ActivityManager.getService().registerReceiver(mMainThread.getApplicationThread(), mBasePackageName, rd, filter,broadcastPermission, userId, flags);if (intent != null) {intent.setExtrasClassLoader(getClassLoader());intent.prepareToEnterProcess();}return intent;} catch (RemoteException e) {throw e.rethrowFromSystemServer();}}

参数解析：
receiver：将要注册的receiver
userId：用户空间标志，默认情况下我们都只有一个user，现在一些手机推出的分身，其实就是用的第二个user，这种情况下userid会变，否则默认主空间的都是0
IntentFilter：要注册的广播的filter
broadcastPermission：指定要注册的广播的权限
scheduler：指定广播接受（也就是onReceive）所在的线程，也就是说注册的时候就可以指定好广播处理放在哪个线程，如果receiver中事情太多，可以放在另外一个线程，这样可以避免主线程被卡住
context：通过getOuterContext获取到，前面在介绍context的时候有提到，application/service/activity中获取到的是不一样的
flags:注册广播所携带的flag

4.2 LoadedApk.getReceiverDispatcher

    public IIntentReceiver getReceiverDispatcher(BroadcastReceiver r,Context context, Handler handler,Instrumentation instrumentation, boolean registered) {synchronized (mReceivers) {// 如果Context相同，每个receiver对应一个ReceiverDispatcherLoadedApk.ReceiverDispatcher rd = null;ArrayMap<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 ArrayMap<BroadcastReceiver, LoadedApk.ReceiverDispatcher>();mReceivers.put(context, map);}map.put(r, rd);}} else {rd.validate(context, handler);}rd.mForgotten = false;return rd.getIIntentReceiver();}}

mReceivers是一个二级map，一级key是context，二级key是BroadcastReceiver，value是ReceiverDispatcher。
这里的ReceiverDispatcher又是什么呢？
它是LoadedApk中的一个内部类，保存了这个receiver的信息，用于在广播派发到本进程的时候执行，上面这方法最重要的是getIIntentReceiver，这个就非常重要了，它是一个Binder对象，说在广播注册的时候将这个binder对象传递到了AMS，然后当广播派发到本进程的时候，通过这个binder对象再会调回来，它在ReceiverDispatcher创建的时候创建。

    static final class ReceiverDispatcher {// 是一个binder对象final static class InnerReceiver extends IIntentReceiver.Stub {final WeakReference<LoadedApk.ReceiverDispatcher> mDispatcher;final LoadedApk.ReceiverDispatcher mStrongRef;InnerReceiver(LoadedApk.ReceiverDispatcher rd, boolean strong) {mDispatcher = new WeakReference<LoadedApk.ReceiverDispatcher>(rd);mStrongRef = strong ? rd : null;}@Overridepublic void performReceive(Intent intent, int resultCode, String data,Bundle extras, boolean ordered, boolean sticky, int sendingUser) {// 这里就是广播真正派发到本进程的时候从systemserver binder call过来执行的...}final IIntentReceiver.Stub mIIntentReceiver;final BroadcastReceiver mReceiver;final Context mContext;final Handler mActivityThread;final Instrumentation mInstrumentation;final boolean mRegistered;final IntentReceiverLeaked mLocation;RuntimeException mUnregisterLocation;boolean mForgotten;...}

到这里，广播注册在APP进程的流程就走完了，主要做了几件事：

获取handler
获取ReceiverDispatcher
获取InnerReceiver
将上面这些连带receiver的相关信息，发起binder call到ActivityManagerService

4.3 ActivityManagerService.registerReceiver

    public Intent registerReceiver(IApplicationThread caller, String callerPackage,IIntentReceiver receiver, IntentFilter filter, String permission, int userId,int flags) {enforceNotIsolatedCaller("registerReceiver");ArrayList<Intent> stickyIntents = null;ProcessRecord callerApp = null;final boolean visibleToInstantApps= (flags & Context.RECEIVER_VISIBLE_TO_INSTANT_APPS) != 0;int callingUid;int callingPid;boolean instantApp;synchronized(this) {if (caller != null) {// 正常来讲caller是发起binder call的客户端进程对应的ApplicationThread对象// 如果为null则抛异常callerApp = getRecordForAppLocked(caller);if (callerApp == null) {throw new SecurityException("Unable to find app for caller " + caller+ " (pid=" + Binder.getCallingPid()+ ") when registering receiver " + receiver);}if (callerApp.info.uid != SYSTEM_UID &&!callerApp.pkgList.containsKey(callerPackage) &&!"android".equals(callerPackage)) {throw new SecurityException("Given caller package " + callerPackage+ " is not running in process " + callerApp);}callingUid = callerApp.info.uid;callingPid = callerApp.pid;} else {callerPackage = null;callingUid = Binder.getCallingUid();callingPid = Binder.getCallingPid();}// 判断caller是否为instant appinstantApp = isInstantApp(callerApp, callerPackage, callingUid);userId = mUserController.handleIncomingUser(callingPid, callingUid, userId, true,ALLOW_FULL_ONLY, "registerReceiver", callerPackage);// 获取广播注册的filter中的action封装到list中Iterator<String> actions = filter.actionsIterator();if (actions == null) {ArrayList<String> noAction = new ArrayList<String>(1);noAction.add(null);actions = noAction.iterator();}// mStickyBroadcasts是一个二级map// 一级key是userId，二级key是广播对应的action，value是广播对应intent的list（一般只有一个intent）// 这里是为了查询对于当前user，本次注册的所有action对应的sticky广播的intentint[] userIds = { UserHandle.USER_ALL, UserHandle.getUserId(callingUid) };while (actions.hasNext()) {String action = actions.next();for (int id : userIds) {ArrayMap<String, ArrayList<Intent>> stickies = mStickyBroadcasts.get(id);if (stickies != null) {ArrayList<Intent> intents = stickies.get(action);if (intents != null) {if (stickyIntents == null) {stickyIntents = new ArrayList<Intent>();}stickyIntents.addAll(intents);}}}}}ArrayList<Intent> allSticky = null;// 这里不为null表示本次注册的广播中有sticky广播if (stickyIntents != null) {final ContentResolver resolver = mContext.getContentResolver();// 查找匹配的sticky广播for (int i = 0, N = stickyIntents.size(); i < N; i++) {Intent intent = stickyIntents.get(i);// 如果caller是instant app，且intent的flag不允许对instant可见，则跳过if (instantApp &&(intent.getFlags() & Intent.FLAG_RECEIVER_VISIBLE_TO_INSTANT_APPS) == 0) {continue;}// If intent has scheme "content", it will need to acccess// provider that needs to lock mProviderMap in ActivityThread// and also it may need to wait application response, so we// cannot lock ActivityManagerService here.if (filter.match(resolver, intent, true, TAG) >= 0) {if (allSticky == null) {allSticky = new ArrayList<Intent>();}allSticky.add(intent);}}}// 直接把最近的一个匹配到的sticky广播返回Intent sticky = allSticky != null ? allSticky.get(0) : null;// 广播注册的时候receiver是可以为null的，这种情况下这里直接returnif (receiver == null) {return sticky;}synchronized (this) {// 校验caller进程是否正常if (callerApp != null && (callerApp.thread == null|| callerApp.thread.asBinder() != caller.asBinder())) {// Original caller already diedreturn null;}// mRegisteredReceivers中存放了所有的已注册的receiver// 每个BroadcastReceiver对应一个InnerReceiver，即Binder对象// binder对象做key，value是ReceiverList// ReceiverList是一个ArrayListReceiverList rl = mRegisteredReceivers.get(receiver.asBinder());if (rl == null) {rl = new ReceiverList(this, callerApp, callingPid, callingUid,userId, receiver);if (rl.app != null) {rl.app.receivers.add(rl);} else {try {// 如果是新创建的receiver，还需要linkToDeathreceiver.asBinder().linkToDeath(rl, 0);} catch (RemoteException e) {return sticky;}rl.linkedToDeath = true;}// 放入mRegisteredReceiversmRegisteredReceivers.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+ " callerPackage is " + callerPackage);} else if (rl.pid != callingPid) {throw new IllegalArgumentException("Receiver requested to register for pid " + callingPid+ " was previously registered for pid " + rl.pid+ " callerPackage is " + callerPackage);} else if (rl.userId != userId) {throw new IllegalArgumentException("Receiver requested to register for user " + userId+ " was previously registered for user " + rl.userId+ " callerPackage is " + callerPackage);}// 每一个IntentFilter对应一个BroadcastFilterBroadcastFilter bf = new BroadcastFilter(filter, rl, callerPackage,permission, callingUid, userId, instantApp, visibleToInstantApps);// receiverList中存放了通过这个receiver注册的所有的filter// 每调用一次register就会add一次rl.add(bf);// mReceiverResolver中存放所有的BroadcastFiltermReceiverResolver.addFilter(bf);// 有匹配的sticky广播，则直接开始调度派发if (allSticky != null) {ArrayList receivers = new ArrayList();receivers.add(bf);// 对于每一个sticky广播，创建BroadcastRecord并入队（并行）final int stickyCount = allSticky.size();for (int i = 0; i < stickyCount; i++) {Intent intent = allSticky.get(i);// 根据flag是否有FLAG_RECEIVER_FOREGROUND判断入队是前台还是后台队列BroadcastQueue queue = broadcastQueueForIntent(intent);BroadcastRecord r = new BroadcastRecord(queue, intent, null,null, -1, -1, false, null, null, AppOpsManager.OP_NONE, null, receivers,null, 0, null, null, false, true, true, -1);// 入队，并行队列queue.enqueueParallelBroadcastLocked(r);// 启动广播的调度，也就是开始派发广播queue.scheduleBroadcastsLocked();}}return sticky;}}

上面主要做了几件事情：

对caller的判断
遍历action，查询是否有匹配的sticky广播
将本次注册的广播放到mRegisteredReceivers中记录
如果是sticky广播，开始派发

5. 总结

本文从App的一次广播注册发起开始，到systemserver的注册流程，整体上流程还是比较简单的，顾名思义，注册，正是把要注册的广播在systemserver中进行登记，等到这个事件真正来临的时候，从登记中取出需要被通知的receiver，这也就是后面广播的派发了。
从设计模式的角度讲，这正是经典的观察者模式。