廣播機(jī)制是Android系統(tǒng)中的一種消息傳播機(jī)制，通過觀察者模式實(shí)現(xiàn)了消息發(fā)送者與消息接收者之間的解耦吵冒。BroadcastReceiver的使用方式有兩種招盲，一種是靜態(tài)注冊(cè)，即在Manifest文件中注冊(cè)堪旧，然后在需要發(fā)送廣播時(shí)調(diào)用context.sendBroadcast(intent)；奖亚；第二種是動(dòng)態(tài)注冊(cè)淳梦。BroadcastReceiver的使用不是本文的重點(diǎn)，本文將著重講解廣播的注冊(cè)過程和消息發(fā)送及接收過程昔字。

1 廣播注冊(cè)過程

廣播的靜態(tài)注冊(cè)是通過PMS(PackageManagerService)來完成的爆袍，其余三大組件也是通過PMS來完成注冊(cè)的。這里重點(diǎn)講一下BroadcastReceiver的動(dòng)態(tài)啟動(dòng)方法。和Activity以及Service一樣螃宙，其啟動(dòng)過程也是通過ContextWrapper-->ContextImpl來完成的蛮瞄。其主要的啟動(dòng)函數(shù)是ContextImpl.registerReceiver:

private Intent registerReceiverInternal(BroadcastReceiver receiver, int userId, IntentFilter filter, String broadcastPermission, Handler scheduler, Context context) {
    IIntentReceiver rd = null;
    ......
    rd = mPackageInfo.getReceiverDispatcher(receiver, context, scheduler, mMainThread.getInstrumentation, true);
    ......
    return ActivityManagerNatvice.getDefault().registerReceiver(mMainThread.getApplicationThread(), mBasePackageName, rd, filter, boradcastPermission, userId);
    ......
}

從上面的代碼可以看出主要做了兩件事：

• 從mPackageInfo獲取IIntentReceiver對(duì)象，之所以這樣和bindService是一樣的谆扎，因?yàn)樯鲜龅淖?cè)過程是一個(gè)跨進(jìn)程的通信方式挂捅，而BroadcastReceiver作為Android的一個(gè)組件是不能直接跨進(jìn)程傳遞的，所以需要使用IIntentReceiver來中轉(zhuǎn)堂湖。其具體是由LoadedApk.ReceiverDispatcher.InnerReceiver闲先，ReceiverDispatcher內(nèi)部同時(shí)保存了BroadcastReceiver和InnerReceiver，所以當(dāng)接收到廣播時(shí)无蜂，ReceiverDispatcher可以很方便地調(diào)用BroadcastReceiver.onReceive()方法伺糠。

• 通過ActivityManagerNative.getDefault()獲取ActivityManagerService,然后通過AMS來完成廣播的注冊(cè)過程。

接下來具體看一下AMS的registerReceiver具體的實(shí)現(xiàn):

public Intent registerReceiver(IApplicationThread caller, String callerPackage, IIntentReceiver receiver, IntentFilter filter, String permission, int userId) {
    ......
    mRegisterReceivers.put(receiver.asBinder(), rl);
    
    BroadcastFilter bf = new BroadcastFilter(filter, rl, callerPackage, permission, callingUid, userId);
    rl.add(bf);
    mReceiverResolver.addFilter(bf);
}

2 廣播的發(fā)送和接收過程

廣播的發(fā)送通過ContextImpl.sendBroadcast方法：

public void sendBroadcast(Intent intent) {
    ......
    ActivityManagerNative.getDefault().broadcastIntent(mMainThread.getApplicationThread(), intent, resolvedType, null, Activity.RESULT_OK, null, null, null, AppOpsManager.OP_NONE, false, false, getUserId());
    ......
}

不出意料斥季，任務(wù)又轉(zhuǎn)到了AMS中训桶，AMS在接收到這個(gè)指令會(huì)調(diào)用內(nèi)部的broadcastIntentLocked方法，在該方法中酣倾，AMS會(huì)根據(jù)intent-filter查找出匹配的廣播接收者舵揭，并通過一系列的條件過濾，并將最終滿足條件的廣播接收者添加到BroadcastQueue中躁锡，然后BroadcastQueue會(huì)將廣播發(fā)送到相應(yīng)的廣播接收者,核心代碼如下：

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);
......
queue.enqueOrderedBroadcastLocked(r);
queue.scheduleBroadcastsLocked();

下面再看一下在BroadcastQueue中發(fā)送廣播scheduleBroadcastsLocked的實(shí)現(xiàn)：

public void scheduleBroadcastsLocked(){
    ......
    mHandler.sendMessage(mHandler.obtainMessage(BROADCAST_INTENT_MSG, this));
    ......
}

實(shí)際上BroadcastQueue的scheduleBroadcastsLocked方法沒有立即發(fā)送廣播午绳，而是發(fā)送了一個(gè)BROADCAST_INTENT_MSG類型的消息，BroadcastQueue收到該消息后會(huì)調(diào)用processNestBroadcast方法：

while(mParallelBroadcasts.size() > 0) {
    r = mParallelBroadcasts.remove(0);
    r.dispatchTime = SystemClock.uptimeMillis();
    r.dispatchClockTime = System.currentTimeMillis();
    final int N = r.recivers.size();
    for(int i = 0; i < N; i++) {
        Object target = r.receivers.get(i);
        deliverToRegisteredReceiverLocked(r, (BroadcastFilter)target, false);
    }
    addBroadcastToHistoryLocked(r);
}

可以看到無序廣播存儲(chǔ)在mParallelBroadcasts中映之，系統(tǒng)遍歷該隊(duì)列拦焚，并將廣播發(fā)送給它所有的接收者。具體的發(fā)送工作通過deliverToRegisteredReceiverLocked完成杠输，在該函數(shù)內(nèi)部通過performReceivedLocked來完成：

private static void performReceiveLocked(ProcessRecord app, IIntentReceiver receiver, Intent intent, int resultCode, String data, Bundle extras, boolean ordered, boolean sticky, int sendingUser) throws RemoteException {
    ......
    app.thread.scheduleRegisteredReceiver(receiver, intent, resultCode, data, extras, ordered, sticky, sendingUser, app.repProcState);
    ......
}

ApplicationThread的scheduleRegisteredReceiver會(huì)調(diào)用InnerReceiver.performReceive來實(shí)現(xiàn)廣播的接收赎败。而在這個(gè)方法中會(huì)調(diào)用LoadedApk.ReceiverDispatcher.performReceive方法：

public void performReceive(Intent intent, int resultCode, String data, Bundle extras, boolean ordered, boolean sticky, int sendingUser) {
    ......
    Args args = new Args(intent, resultCode, data, extras, ordered, sticky, sendingUser);
    if(!mActivityThread.post(args)) {
        if(mRistered && ordered) {
            IActivityManager mgr = ActivityManagerNative.getDefault();
            args.sendFinished(mgr);
        }
    }
}

在上面的代碼中，會(huì)創(chuàng)建一個(gè)Args對(duì)象抬伺，并通過mActivityThread的post方法來執(zhí)行Args的邏輯螟够，Args實(shí)際上是一個(gè)Runnable接口。mActivityThread是一個(gè)Handler峡钓，其實(shí)就是ActivityThread中的mH，類型是H若河。Args中的run方法有如下幾行代碼：

final BroadcastReceiver receiver = mReceiver;
receiver.setPendingResult(this);
receiver.onReceive(mContext, intent);

這個(gè)時(shí)候BroadcastReceiver的onReceive方法才被執(zhí)行能岩，也就接收到廣播了。