Android P 显示流程分析（三）---EventThread MessageQueue 交互分析

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上篇分析SurfaceFlinger的init()里创建了几个线程，主要用于界面刷新。里面涉及了一个EventThread和MessageQueue。我们来看看像界面刷新这种高频的事件通知及处理，Google是如何设计的。

EventThread的初始化

EventThread::EventThread(VSyncSource* src, ResyncWithRateLimitCallback resyncWithRateLimitCallback,InterceptVSyncsCallback interceptVSyncsCallback, const char* threadName){//创建一个线程，实际做事的也是这个线程处理的mThread = std::thread(&EventThread::threadMain, this);...                    
}

创建connection

其它一些线程需要将事件推给EventThread去处理，需要中间有一个连接者与EventThread交互，EventThread里面就创建出来了一个内部类Connection，负责连接交互的工作

sp<BnDisplayEventConnection> EventThread::createEventConnection() const {return new Connection(const_cast<EventThread*>(this));
}
class Connection : public BnDisplayEventConnection {
public:virtual status_t postEvent(const DisplayEventReceiver::Event& event);
private:void requestNextVsync() override; 
}

上篇SurfaceFlinger.init()中实例化一个EventThread之后，就调用了mEventQueue->setEventThread(mSFEventThread.get()),

void MessageQueue::setEventThread(android::EventThread* eventThread) {
...mEventThread = eventThread;mEvents = eventThread->createEventConnection();
...
}
sp<BnDisplayEventConnection> EventThread::createEventConnection() const {return new Connection(const_cast<EventThread*>(this));
}

MessageQueue中的mEvents就是一个connection对象。就是它就将MessageQueue与EventThread连接起来了。
我们再来看看EventHandler 的那个线程启动之后，做了哪些事情：

void EventThread::threadMain() NO_THREAD_SAFETY_ANALYSIS {std::unique_lock<std::mutex> lock(mMutex);while (mKeepRunning) {DisplayEventReceiver::Event event;Vector<sp<EventThread::Connection> > signalConnections;signalConnections = waitForEventLocked(&lock, &event);// dispatch events to listeners...const size_t count = signalConnections.size();for (size_t i = 0; i < count; i++) { const sp<Connection>& conn(signalConnections[i]);// now see if we still need to report this eventstatus_t err = conn->postEvent(event);if (err == -EAGAIN || err == -EWOULDBLOCK) {ALOGW("EventThread: dropping event (%08x) for connection %p", event.header.type,conn.get());} else if (err < 0) {removeDisplayEventConnectionLocked(signalConnections[i]);}}}
}

总体来说是获取事件，将获取到的事件放到BitTube中去，具体的获取事件的waitForEventLocked是如何做的呢？

Vector<sp<EventThread::Connection> > EventThread::waitForEventLocked(std::unique_lock<std::mutex>* lock, DisplayEventReceiver::Event* event) {Vector<sp<EventThread::Connection> > signalConnections;while (signalConnections.isEmpty() && mKeepRunning) {for (int32_t i = 0; i < DisplayDevice::NUM_BUILTIN_DISPLAY_TYPES; i++) {timestamp = mVSyncEvent[i].header.timestamp;if (timestamp) {// we have a vsync event to dispatchif (mInterceptVSyncsCallback) {mInterceptVSyncsCallback(timestamp);}*event = mVSyncEvent[i];mVSyncEvent[i].header.timestamp = 0;vsyncCount = mVSyncEvent[i].vsync.count;break;}} if (!timestamp) {// no vsync event, see if there are some other eventeventPending = !mPendingEvents.isEmpty();if (eventPending) {// we have some other event to dispatch*event = mPendingEvents[0];mPendingEvents.removeAt(0);}}        // find out connections waiting for eventssize_t count = mDisplayEventConnections.size();for (size_t i = 0; i < count;) {sp<Connection> connection(mDisplayEventConnections[i].promote()); if (connection != nullptr) {bool added = false;if (connection->count >= 0) {// we need vsync events because at least// one connection is waiting for itwaitForVSync = true;if (timestamp) {// we consume the event only if it's time// (ie: we received a vsync event)if (connection->count == 0) {// fired this time aroundconnection->count = -1;signalConnections.add(connection);added = true;} else if (connection->count == 1 ||(vsyncCount % connection->count) == 0) {// continuous event, and time to report itsignalConnections.add(connection);added = true;}}}if (eventPending && !timestamp && !added) {// we don't have a vsync event to process// (timestamp==0), but we have some pending// messages.signalConnections.add(connection);}++i; } else {// we couldn't promote this reference, the connection has// died, so clean-up!mDisplayEventConnections.removeAt(i);--count;}}   ...// note: !timestamp implies signalConnections.isEmpty(), because we// don't populate signalConnections if there's no vsync pendingif (!timestamp && !eventPending) {// wait for something to happenif (waitForVSync) {// This is where we spend most of our time, waiting// for vsync events and new client registrations.//// If the screen is off, we can't use h/w vsync, so we// use a 16ms timeout instead.  It doesn't need to be// precise, we just need to keep feeding our clients.//// We don't want to stall if there's a driver bug, so we// use a (long) timeout when waiting for h/w vsync, and// generate fake events when necessary.bool softwareSync = mUseSoftwareVSync;auto timeout = softwareSync ? 16ms : 1000ms;if (mCondition.wait_for(*lock, timeout) == std::cv_status::timeout) {if (!softwareSync) {ALOGW("Timed out waiting for hw vsync; faking it");}// FIXME: how do we decide which display id the fake// vsync came from ?mVSyncEvent[0].header.type = DisplayEventReceiver::DISPLAY_EVENT_VSYNC;mVSyncEvent[0].header.id = DisplayDevice::DISPLAY_PRIMARY;mVSyncEvent[0].header.timestamp = systemTime(SYSTEM_TIME_MONOTONIC);mVSyncEvent[0].vsync.count++;}} else {// Nobody is interested in vsync, so we just want to sleep.// h/w vsync should be disabled, so this will wait until we// get a new connection, or an existing connection becomes// interested in receiving vsync again.mCondition.wait(*lock);}}}// here we're guaranteed to have a timestamp and some connections to signal// (The connections might have dropped out of mDisplayEventConnections// while we were asleep, but we'll still have strong references to them.)return signalConnections;
}

waitForEventLocked 就如果vsyncEvent和pendingEvent里已经存在事件，就将其取出指定给event，然后遍历出所有的mDisplayEventConnections, 找且需要的connection, 将他们一一添加到signalConnections中, 如果没有找到需要connection，就设置mCondition.wait(*lock)，条件加锁等待，直到此条件被唤醒。
我们上面EventThread的threadMain分析到将获取的signalConnections中的connection遍历出来，然后将通过调用 connection的postEvent 将event事件加入到BitTube中。

MessageQueue的作用

surfaceFlinger很多事件都是通过MessageQueue来处理的，SurfaceFlinger里的mEventQueue就是MessageQueue的对象指针。

void SurfaceFlinger::waitForEvent() {mEventQueue->waitMessage();
}
void SurfaceFlinger::signalTransaction() {mEventQueue->invalidate();
}
void SurfaceFlinger::signalLayerUpdate() {mEventQueue->invalidate();
}
void SurfaceFlinger::signalRefresh() {mRefreshPending = true;mEventQueue->refresh();
}
void SurfaceFlinger::run() {do {waitForEvent();} while (true);
}

以上这些都是通过MessageQueue去调用实现的。那我们再具体看看MessageQueue里是如何实现的。

void MessageQueue::invalidate() {mEvents->requestNextVsync();
}
void EventThread::Connection::requestNextVsync() {mEventThread->requestNextVsync(this);
}
void EventThread::requestNextVsync(const sp<EventThread::Connection>& connection) {std::lock_guard<std::mutex> lock(mMutex);if (connection->count < 0) {connection->count = 0;mCondition.notify_all();}
}

invalidate 主要是为了唤醒waitForEventLocked , 让EventThread继续执行。

void MessageQueue::refresh() {mHandler->dispatchRefresh();
}
void MessageQueue::Handler::dispatchRefresh() {if ((android_atomic_or(eventMaskRefresh, &mEventMask) & eventMaskRefresh) == 0) {mQueue.mLooper->sendMessage(this, Message(MessageQueue::REFRESH));}
}
void MessageQueue::Handler::handleMessage(const Message& message) {switch (message.what) {...case REFRESH:android_atomic_and(~eventMaskRefresh, &mEventMask);mQueue.mFlinger->onMessageReceived(message.what);break;}
}
void SurfaceFlinger::onMessageReceived(int32_t what) {... case MessageQueue::REFRESH: {handleMessageRefresh();break;}
}
void SurfaceFlinger::handleMessageRefresh() {ATRACE_CALL();mRefreshPending = false;nsecs_t refreshStartTime = systemTime(SYSTEM_TIME_MONOTONIC);preComposition(refreshStartTime);rebuildLayerStacks();setUpHWComposer();doDebugFlashRegions();doTracing("handleRefresh");logLayerStats();doComposition();postComposition(refreshStartTime);mPreviousPresentFence = getBE().mHwc->getPresentFence(HWC_DISPLAY_PRIMARY);mHadClientComposition = false;for (size_t displayId = 0; displayId < mDisplays.size(); ++displayId) {const sp<DisplayDevice>& displayDevice = mDisplays[displayId];mHadClientComposition = mHadClientComposition ||getBE().mHwc->hasClientComposition(displayDevice->getHwcDisplayId());}mVsyncModulator.onRefreshed(mHadClientComposition);mLayersWithQueuedFrames.clear();
}