在 Android 里面有各种各样的广播，比如电池的使用状态，电话的接收和短信的接收都会产生一个广播，应用程序开发者也可以监听这些广播并做出程序逻辑的处理;
今天我们就来分析下广播的运行机制
静态广播是通过PackageManagerService在启动的时候扫描已安装的应用去注册的;
在PackageManagerService的构造方法中,会去扫描应用安装目录,顺序是先扫描系统应用安装目录再扫描第三方应用安装目录;
PackageManagerService.scanDirLI就是用于扫描目录的方法:
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![[[436974]]](https://s2.51cto.com/oss/202111/26/9d4cd84ebf7705289a56b294bce182e0.png){kind=link}
private void scanDirLI(File dir, int flags, int scanMode, long currentTime) { String[] files = dir.list(); if (files == null) { return; } int i; for (i=0; i<files.length; i++) { File file = new File(dir, files[i]); if (!isPackageFilename(files[i])) { continue; } PackageParser.Package pkg = scanPackageLI(file, flags|PackageParser.PARSE_MUST_BE_APK, scanMode, currentTime, null); if (pkg == null && (flags & PackageParser.PARSE_IS_SYSTEM) == 0 && mLastScanError == PackageManager.INSTALL_FAILED_INVALID_APK) { file.delete(); } } } private static final boolean isPackageFilename(String name) { return name != null && name.endsWith(".apk"); }
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可以看到,它通过File.list方法列出目录下的所有后缀为".apk"的文件传给scanPackageLI去处理;
而scanPackageLI(File scanFile,int parseFlags, int scanMode, long currentTime, UserHandle user)内部会调用它的重载方法;
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scanPackageLI(PackageParser.Package pkg,int parseFlags, int scanMode, long currentTime, UserHandle user): private PackageParser.Package scanPackageLI(File scanFile,int parseFlags, int scanMode, long currentTime, UserHandle user) { ... final PackageParser.Package pkg = pp.parsePackage(scanFile,scanPath, mMetrics, parseFlags); ... PackageParser.Package scannedPkg = scanPackageLI(pkg, parseFlags, scanMode | SCAN_UPDATE_SIGNATURE, currentTime, user); ... }
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在这个scanPackageLIl里面会解析Package并且将AndroidManifest.xml中注册的BroadcastReceiver保存下来:
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... N = pkg.receivers.size(); r = null; for (i=0; i<N; i++) { PackageParser.Activity a = pkg.receivers.get(i); a.info.processName = fixProcessName(pkg.applicationInfo.processName, a.info.processName, pkg.applicationInfo.uid); mReceivers.addActivity(a, "receiver"); ... } ...
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系统应用的广播先于第三方应用的广播注册;
而安装在同一个目录下的应用的静态广播的注册顺序是按照File.list列出来的apk的顺序注册的;
他们的注册顺序就决定了它们接收广播的顺序;
通过静态广播的注册流程,我们已经将静态广播注册到了PackageManagerService的mReceivers中;
而我们可以使用PackageManagerService.queryIntentReceivers方法查询intent对应的静态广播
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public List<ResolveInfo> queryIntentReceivers(Intent intent, String resolvedType, int flags, int userId) { if (!sUserManager.exists(userId)) return Collections.emptyList(); ComponentName comp = intent.getComponent(); if (comp == null) { if (intent.getSelector() != null) { intent = intent.getSelector(); comp = intent.getComponent(); } } if (comp != null) { List<ResolveInfo> list = new ArrayList<ResolveInfo>(1); ActivityInfo ai = getReceiverInfo(comp, flags, userId); if (ai != null) { ResolveInfo ri = new ResolveInfo(); ri.activityInfo = ai; list.add(ri); } return list; } synchronized (mPackages) { String pkgName = intent.getPackage(); if (pkgName == null) { return mReceivers.queryIntent(intent, resolvedType, flags, userId); } final PackageParser.Package pkg = mPackages.get(pkgName); if (pkg != null) { return mReceivers.queryIntentForPackage(intent, resolvedType, flags, pkg.receivers, userId); } return null; } }
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我们调用Context.registerReceiver最后会调到ActivityManagerService.registerReceiver:
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public Intent registerReceiver(IApplicationThread caller, String callerPackage, IIntentReceiver receiver, IntentFilter filter, String permission, int userId) { ... ReceiverList rl = (ReceiverList)mRegisteredReceivers.get(receiver.asBinder()); ... BroadcastFilter bf = new BroadcastFilter(filter, rl, callerPackage, permission, callingUid, userId); ... mReceiverResolver.addFilter(bf); ... }
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所以通过mReceiverResolver.queryIntent就能获得intent对应的动态广播了;
ContextImpl.sendBroadcast中会调用ActivityManagerNative.getDefault().broadcastIntent()
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public void sendBroadcast(Intent intent) { warnIfCallingFromSystemProcess(); String resolvedType = intent.resolveTypeIfNeeded(getContentResolver()); try { intent.prepareToLeaveProcess(); ActivityManagerNative.getDefault().broadcastIntent( mMainThread.getApplicationThread(), intent, resolvedType, null, Activity.RESULT_OK, null, null, null, AppOpsManager.OP_NONE, false, false,getUserId()); } catch (RemoteException e) { } }
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实际是调用ActivityManagerService.broadcastIntent:
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public final int broadcastIntent(IApplicationThread caller, Intent intent, String resolvedType, IIntentReceiver resultTo, int resultCode, String resultData, Bundle map, String requiredPermission, int appOp, boolean serialized, boolean sticky, int userId) { enforceNotIsolatedCaller("broadcastIntent"); synchronized(this) { intent = verifyBroadcastLocked(intent); final ProcessRecord callerApp = getRecordForAppLocked(caller); final int callingPid = Binder.getCallingPid(); final int callingUid = Binder.getCallingUid(); final long origId = Binder.clearCallingIdentity(); int res = broadcastIntentLocked(callerApp, callerApp != null ? callerApp.info.packageName : null, intent, resolvedType, resultTo, resultCode, resultData, map, requiredPermission, appOp, serialized, sticky, callingPid, callingUid, userId); Binder.restoreCallingIdentity(origId); return res; } }
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调用ActivityManagerService.broadcastIntentLocked,而broadcastIntentLocked中的关键代码如下:
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broadcastIntentLocked // 静态广播 List receivers = null; // 动态广播 List<BroadcastFilter> registeredReceivers = null; if ((intent.getFlags()&Intent.FLAG_RECEIVER_REGISTERED_ONLY) == 0) { // 查询静态广播 receivers = collectReceiverComponents(intent, resolvedType, users); } if (intent.getComponent() == null) { // 查询动态广播 registeredReceivers = mReceiverResolver.queryIntent(intent, resolvedType, false, userId); } final boolean replacePending = (intent.getFlags()&Intent.FLAG_RECEIVER_REPLACE_PENDING) != 0; int NR = registeredReceivers != null ? registeredReceivers.size() : 0; if (!ordered && NR > 0) { final BroadcastQueue queue = broadcastQueueForIntent(intent); BroadcastRecord r = new BroadcastRecord(queue, intent, callerApp, callerPackage, callingPid, callingUid, resolvedType, requiredPermission, appOp, registeredReceivers, resultTo, resultCode, resultData, map, ordered, sticky, false, userId); final boolean replaced = replacePending && queue.replaceParallelBroadcastLocked(r); if (!replaced) { // 发送动态广播 queue.enqueueParallelBroadcastLocked(r); queue.scheduleBroadcastsLocked(); } registeredReceivers = null; NR = 0; } ... if ((receivers != null && receivers.size() > 0) || resultTo != null) { BroadcastQueue queue = broadcastQueueForIntent(intent); BroadcastRecord r = new BroadcastRecord(queue, intent, callerApp, callerPackage, callingPid, callingUid, resolvedType, requiredPermission, appOp, receivers, resultTo, resultCode, resultData, map, ordered, sticky, false, userId); boolean replaced = replacePending && queue.replaceOrderedBroadcastLocked(r); if (!replaced) { // 发送静态广播 queue.enqueueOrderedBroadcastLocked(r); queue.scheduleBroadcastsLocked(); } }
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动态广播会优先于静态广播,从上面的代码我们可以看到;
实际上静态广播静态就是从PackageManagerService中查询的:
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private List<ResolveInfo> collectReceiverComponents(Intent intent, String resolvedType, int[] users) { ... List<ResolveInfo> newReceivers = AppGlobals.getPackageManager() .queryIntentReceivers(intent, resolvedType, STOCK_PM_FLAGS, user); ...
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从ActivityManagerService.broadcastIntentLocked中我们可以看到,实际上它不是直接将广播发送到BroadcastReceiver中;
而是将他包装到BroadcastRecord中,再放进BroadcastQueue:
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BroadcastQueue queue = broadcastQueueForIntent(intent); BroadcastRecord r = new BroadcastRecord(queue, intent, null, null, -1, -1, null, null, AppOpsManager.OP_NONE, receivers, null, 0, null, null, false, true, true, -1); queue.enqueueParallelBroadcastLocked(r); queue.scheduleBroadcastsLocked(); enqueueParallelBroadcastLocked方法用于并发执行广播的发送.它很简单,就是将BroadcastRecord放到了mParallelBroadcasts中: public void enqueueParallelBroadcastLocked(BroadcastRecord r) { mParallelBroadcasts.add(r); }
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scheduleBroadcastsLocked方法同样很简单,就是向mHandler发送了个BROADCAST_INTENT_MSG消息:
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public void scheduleBroadcastsLocked() { if (mBroadcastsScheduled) { return; } mHandler.sendMessage(mHandler.obtainMessage(BROADCAST_INTENT_MSG, this)); mBroadcastsScheduled = true; }
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mHandler在接收到BROADCAST_INTENT_MSG消息的时候会做些什么:
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final Handler mHandler = new Handler() { public void handleMessage(Message msg) { switch (msg.what) { case BROADCAST_INTENT_MSG: { processNextBroadcast(true); } break; case BROADCAST_TIMEOUT_MSG: { synchronized (mService) { broadcastTimeoutLocked(true); } } break; } } };
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processNextBroadcast方法用于从队列中获取广播消息并发送给BroadcastReceiver,它内部有两个分支,并行处理和串行处理;
动态注册的非有序广播等就是使用并行处理:
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final void processNextBroadcast(boolean fromMsg) { synchronized(mService) { BroadcastRecord r; mService.updateCpuStats(); if (fromMsg) { mBroadcastsScheduled = false; } while (mParallelBroadcasts.size() > 0) { r = mParallelBroadcasts.remove(0); r.dispatchTime = SystemClock.uptimeMillis(); r.dispatchClockTime = System.currentTimeMillis(); final int N = r.receivers.size(); for (int i=0; i<N; i++) { Object target = r.receivers.get(i); // 发送消息给Receiver deliverToRegisteredReceiverLocked(r, (BroadcastFilter)target, false); } addBroadcastToHistoryLocked(r); } ... } ... } private final void deliverToRegisteredReceiverLocked(BroadcastRecord r, BroadcastFilter filter, boolean ordered) { ... // 获取BroadcastReceiver的Binder r.receiver = filter.receiverList.receiver.asBinder(); ... // 使用Binder机制将消息传递给BroadcastReceiver performReceiveLocked(filter.receiverList.app, filter.receiverList.receiver, new Intent(r.intent), r.resultCode, r.resultData, r.resultExtras, r.ordered, r.initialSticky, r.userId); ... } void performReceiveLocked(ProcessRecord app, IIntentReceiver receiver, Intent intent, int resultCode, String data, Bundle extras, boolean ordered, boolean sticky, int sendingUser) throws RemoteException { ...... //通过Binder将消息处理传到应用进程,应用进程内部再使用Handler机制,将消息处理放到主线程中 app.thread.scheduleRegisteredReceiver(receiver, intent, resultCode, data, extras, ordered, sticky, sendingUser, app.repProcState); ...... } }
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有序广播和静态广播等,会通过enqueueOrderedBroadcastLocked传给BroadcastQueue:
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public void enqueueOrderedBroadcastLocked(BroadcastRecord r) { mOrderedBroadcasts.add(r); }
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然后在processNextBroadcast里面会对mOrderedBroadcasts进行特殊处理;
广播队列传送广播给Receiver的原理其实就是将BroadcastReceiver和消息都放到BroadcastRecord里面,然后通过Handler机制遍历BroadcastQueue里面的BroadcastRecord,将消息发送给BroadcastReceiver;
广播作为Android组件间的通信方式，可以使用的场景如下：
同一app内部的同一组件(Component)内的消息通信(单个或多个线程之间);
同一app内部的不同组件之间的消息通信(单个进程);
同一app具有多个进程的不同组件之间的消息通信;
不同app之间的组件之间消息通信;
Android系统在特定情况下与App之间的消息通信;
从实现原理上看，Android中的广播使用了观察者模式，基于消息的发布/订阅事件模型。因此，从实现的角度来看，Android中的广播将广播的发送者和接受者极大程度上解耦，使得系统能够方便集成，更易扩展。具体实现流程要点粗略概括如下：
广播接收者BroadcastReceiver通过Binder机制向AMS(Activity Manager Service)进行注册;
广播发送者通过binder机制向AMS发送广播;
AMS查找符合相应条件(IntentFilter/Permission等)的BroadcastReceiver，将广播发送到BroadcastReceiver相应的消息循环队列中;
消息循环队列拿到此广播后，回调BroadcastReceiver中的onReceive()方法;
广播发送者和广播接收者分别属于观察者模式中的消息发布和订阅两端，AMS属于中间的处理中心;
广播发送者和广播接收者的执行是异步的，发出去的广播不会关心有无接收者接收，也不确定接收者到底是何时才能接收到;
学如逆水行舟，不进则退。心似平原跑马，易放难收;
本文转载自微信公众号「Android开发编程」
