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ReceiverDispatcher
LoadedApk.ReceiverDispatcher rd = null;
ArrayMap 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();
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 mDispatcher;
final LoadedApk.ReceiverDispatcher mStrongRef;
InnerReceiver(LoadedApk.ReceiverDispatcher rd, boolean strong) {
mDispatcher = new WeakReference(rd);
mStrongRef = strong ? rd : null;
}
@Override
public 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 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 app
instantApp = 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 actions = filter.actionsIterator();
if (actions == null) {
ArrayList noAction = new ArrayList(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intent
int[] userIds = { UserHandle.USER_ALL, UserHandle.getUserId(callingUid) };
while (actions.hasNext()) {
String action = actions.next();
for (int id : userIds) {
ArrayMap> stickies = mStickyBroadcasts.get(id);
if (stickies != null) {
ArrayList intents = stickies.get(action);
if (intents != null) {
if (stickyIntents == null) {
stickyIntents = new ArrayList();
}
stickyIntents.addAll(intents);
}
}
}
}
}
ArrayList 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();
}
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return
if (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 died
return 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ArrayList
ReceiverList 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linkToDeath
receiver.asBinder().linkToDeath(rl, 0);
} catch (RemoteException e) {
return sticky;
}
rl.linkedToDeath = true;
}
// 放入mRegisteredReceivers
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
+ " 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BroadcastFilter
BroadcastFilter 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BroadcastFilter
mReceiverResolver.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。
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