Android四大组件系列8 Broadcast广播机制（下）
概述
广播 (Broadcast) 机制用于进程或线程间通信，广播分为广播发送和广播接收两个过程，其中广播接收者 BroadcastReceiver 是 Android 四大组件之一。BroadcastReceiver 分为两类：
- 静态广播接收者：通过 AndroidManifest.xml 的标签来声明的 BroadcastReceiver
- 动态广播接收者：通过 AMS.registerReceiver() 方式注册的 BroadcastReceiver，动态注册更为灵活，可在不需要时动态取消注册
PS：动态 BroadcastReceiver 比较简单，静态的就麻烦一些了，因为在广播发送之时，静态 receiver 所从属的进程可能还没有启动呢，这就需要先启动新的进程，费时费力。另一方面，有些时候用户希望广播能够按照一定顺序发送，为此，Android 又搞出了 ordered broadcast 的概念。
从广播发送方式可分为三类：
- 普通广播：通过 Context.sendBroadcast() 发送，并行处理
- 有序广播：通过 Context.sendOrderedBroadcast() 发送，串行处理
- Sticky 广播：通过 Context.sendStickyBroadcast() 发送
以上是对广播机制的概述，我们接上篇 Android四大组件系列7 Broadcast广播机制（上）继续分析
4.4 BroadcastQueue.processNextBroadcast
从 processNextBroadcast() 的代码，我们就可以看清楚前面说的 “平行广播”、“有序广播” 和 “动态 receiver”、“静态 receiver” 之间的关系了。
可以说：processNextBroadcast 函数是广播处理的核心。
我们在前文已经说过，所有的静态 receiver 都是串行处理的，而动态 receiver 则会按照发广播时指定的方式，进行 “并行” 或 “串行” 处理。
能够并行处理的广播，其对应的若干 receiver 一定都已经存在了，不会牵扯到启动新进程的操作，所以可以在一个 while 循环中，一次性全部 deliver。
而有序广播，则需要一个一个地处理，其循环处理的手段是发送事件，也就是说，在一个 receiver 处理完毕后，会利用广播队列（BroadcastQueue）的 mHandler，发送一个 BROADCAST_INTENT_MSG 事件，从而执行下一次的 processNextBroadcast()。
BroadcastQueue.java
final void processNextBroadcast(boolean fromMsg) {
synchronized (mService) {
processNextBroadcastLocked(fromMsg, false);
}
}
继续调用 processNextBroadcastLocked，这个函数很长，我们先看下全部的代码，然后下面进行详细分析。
final void processNextBroadcastLocked(boolean fromMsg, boolean skipOomAdj) {
BroadcastRecord r;
if (DEBUG_BROADCAST) Slog.v(TAG_BROADCAST, "processNextBroadcast ["
+ mQueueName + "]: "
+ mParallelBroadcasts.size() + " parallel broadcasts; "
+ mDispatcher.describeStateLocked());
mService.updateCpuStats();
if (fromMsg) {
mBroadcastsScheduled = false;
}
// First, deliver any non-serialized broadcasts right away.
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);
deliverToRegisteredReceiverLocked(r,
(BroadcastFilter)target, false, i);
}
addBroadcastToHistoryLocked(r);
}
// Now take care of the next serialized one...
// If we are waiting for a process to come up to handle the next
// broadcast, then do nothing at this point. Just in case, we
// check that the process we're waiting for still exists.
if (mPendingBroadcast != null) {
if (DEBUG_BROADCAST_LIGHT) Slog.v(TAG_BROADCAST,
"processNextBroadcast [" + mQueueName + "]: waiting for "
+ mPendingBroadcast.curApp);
boolean isDead;
if (mPendingBroadcast.curApp.pid > 0) {
synchronized (mService.mPidsSelfLocked) {
ProcessRecord proc = mService.mPidsSelfLocked.get(
mPendingBroadcast.curApp.pid);
isDead = proc == null || proc.isCrashing();
}
} else {
final ProcessRecord proc = mService.mProcessList.mProcessNames.get(
mPendingBroadcast.curApp.processName,
mPendingBroadcast.curApp.uid);
isDead = proc == null || !proc.pendingStart;
}
if (!isDead) {
// It's still alive, so keep waiting
return;
} else {
Slog.w(TAG, "pending app ["
+ mQueueName + "]" + mPendingBroadcast.curApp
+ " died before responding to broadcast");
mPendingBroadcast.state = BroadcastRecord.IDLE;
mPendingBroadcast.nextReceiver = mPendingBroadcastRecvIndex;
mPendingBroadcast = null;
}
}
boolean looped = false;
do {
final long now = SystemClock.uptimeMillis();
r = mDispatcher.getNextBroadcastLocked(now);
if (r == null) {
// No more broadcasts are deliverable right now, so all done!
mDispatcher.scheduleDeferralCheckLocked(false);
mService.scheduleAppGcsLocked();
if (looped) {
// If we had finished the last ordered broadcast, then
// make sure all processes have correct oom and sched
// adjustments.
mService.updateOomAdjLocked(
OomAdjuster.OOM_ADJ_REASON_START_RECEIVER);
}
// when we have no more ordered broadcast on this queue, stop logging
if (mService.mUserController.mBootCompleted &&
mLogLatencyMetrics) {
mLogLatencyMetrics = false;
}
return;
}
boolean forceReceive = false;
int numReceivers = (r.receivers != null) ? r.receivers.size() : 0;
if (mService.mProcessesReady && !r.timeoutExempt &&
r.dispatchTime > 0) {
if ((numReceivers > 0) &&
(now > r.dispatchTime +
(2 * mConstants.TIMEOUT * numReceivers))) {
Slog.w(TAG, "Hung broadcast ["
+ mQueueName + "] discarded after timeout failure:"
+ " now=" + now
+ " dispatchTime=" + r.dispatchTime
+ " startTime=" + r.receiverTime
+ " intent=" + r.intent
+ " numReceivers=" + numReceivers
+ " nextReceiver=" + r.nextReceiver
+ " state=" + r.state);
broadcastTimeoutLocked(false); // forcibly finish this broadcast
forceReceive = true;
r.state = BroadcastRecord.IDLE;
}
}
if (r.state != BroadcastRecord.IDLE) {
if (DEBUG_BROADCAST) Slog.d(TAG_BROADCAST,
"processNextBroadcast("
+ mQueueName + ") called when not idle (state="
+ r.state + ")");
return;
}
// Is the current broadcast is done for any reason?
if (r.receivers == null || r.nextReceiver >= numReceivers
|| r.resultAbort || forceReceive) {
// Send the final result if requested
if (r.resultTo != null) {
boolean sendResult = true;
if (r.splitToken != 0) {
int newCount = mSplitRefcounts.get(r.splitToken) - 1;
if (newCount == 0) {
mSplitRefcounts.delete(r.splitToken);
} else {
sendResult = false;
mSplitRefcounts.put(r.splitToken, newCount);
}
}
if (sendResult) {
try {
performReceiveLocked(r.callerApp, r.resultTo,
new Intent(r.intent), r.resultCode,
r.resultData, r.resultExtras, false,
false, r.userId);
r.resultTo = null;
} catch (RemoteException e) {
r.resultTo = null;
Slog.w(TAG, "Failure ["
+ mQueueName + "] sending broadcast result of "
+ r.intent, e);
}
}
}
cancelBroadcastTimeoutLocked();
if (DEBUG_BROADCAST_LIGHT) Slog.v(TAG_BROADCAST,
"Finished with ordered broadcast " + r);
// ... and on to the next...
addBroadcastToHistoryLocked(r);
if (r.intent.getComponent() == null && r.intent.getPackage() == null
&& (r.intent.getFlags()&
Intent.FLAG_RECEIVER_REGISTERED_ONLY) == 0) {
mService.addBroadcastStatLocked(r.intent.getAction(),
r.callerPackage, r.manifestCount, r.manifestSkipCount,
r.finishTime-r.dispatchTime);
}
mDispatcher.retireBroadcastLocked(r);
r = null;
looped = true;
continue;
}
if (!r.deferred) {
final int receiverUid = r.getReceiverUid(r.receivers.get(r.nextReceiver));
if (mDispatcher.isDeferringLocked(receiverUid)) {
BroadcastRecord defer;
if (r.nextReceiver + 1 == numReceivers) {
defer = r;
mDispatcher.retireBroadcastLocked(r);
} else {
defer = r.splitRecipientsLocked(receiverUid, r.nextReceiver);
if (DEBUG_BROADCAST_DEFERRAL) {
Slog.i(TAG_BROADCAST, "Post split:");
Slog.i(TAG_BROADCAST, "Original broadcast receivers:");
for (int i = 0; i < r.receivers.size(); i++) {
Slog.i(TAG_BROADCAST, " " + r.receivers.get(i));
}
Slog.i(TAG_BROADCAST, "Split receivers:");
for (int i = 0; i < defer.receivers.size(); i++) {
Slog.i(TAG_BROADCAST, " " + defer.receivers.get(i));
}
}
// Track completion refcount as well if relevant
if (r.resultTo != null) {
int token = r.splitToken;
if (token == 0) {
r.splitToken = defer.splitToken = nextSplitTokenLocked();
mSplitRefcounts.put(r.splitToken, 2);
} else {
final int curCount = mSplitRefcounts.get(token);
mSplitRefcounts.put(token, curCount + 1);
}
}
}
mDispatcher.addDeferredBroadcast(receiverUid, defer);
r = null;
looped = true;
continue;
}
}
} while (r == null);
// Get the next receiver...
int recIdx = r.nextReceiver++;
// Keep track of when this receiver started, and make sure there
// is a timeout message pending to kill it if need be.
r.receiverTime = SystemClock.uptimeMillis();
if (recIdx == 0) {
r.dispatchTime = r.receiverTime;
r.dispatchClockTime = System.currentTimeMillis();
if (mLogLatencyMetrics) {
StatsLog.write(
StatsLog.BROADCAST_DISPATCH_LATENCY_REPORTED,
r.dispatchClockTime - r.enqueueClockTime);
}
if (Trace.isTagEnabled(Trace.TRACE_TAG_ACTIVITY_MANAGER)) {
Trace.asyncTraceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER,
createBroadcastTraceTitle(r, BroadcastRecord.DELIVERY_PENDING),
System.identityHashCode(r));
Trace.asyncTraceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER,
createBroadcastTraceTitle(r, BroadcastRecord.DELIVERY_DELIVERED),
System.identityHashCode(r));
}
}
if (! mPendingBroadcastTimeoutMessage) {
long timeoutTime = r.receiverTime + mConstants.TIMEOUT;
if (DEBUG_BROADCAST) Slog.v(TAG_BROADCAST,
"Submitting BROADCAST_TIMEOUT_MSG ["
+ mQueueName + "] for " + r + " at " + timeoutTime);
setBroadcastTimeoutLocked(timeoutTime);
}
final BroadcastOptions brOptions = r.options;
final Object nextReceiver = r.receivers.get(recIdx);
if (nextReceiver instanceof BroadcastFilter) {
// Simple case: this is a registered receiver who gets
// a direct call.
BroadcastFilter filter = (BroadcastFilter)nextReceiver;
if (DEBUG_BROADCAST) Slog.v(TAG_BROADCAST,
"Delivering ordered ["
+ mQueueName + "] to registered "
+ filter + ": " + r);
deliverToRegisteredReceiverLocked(r, filter, r.ordered, recIdx);
if (r.receiver == null || !r.ordered) {
// The receiver has already finished