Android 显示一、 Vsync

Android 显示系统：Vsync机制

典型的显示系统中，一般包括CPU、GPU、Display三个部分，
CPU负责计算帧数据，
GPU对计算好的图形数据进行渲染，渲染好后放在buffer（图像缓冲区）里，
Display负责把buffer里的数据呈现到屏幕上

一、Vsync简介：

　　屏幕的刷新过程是每一行从左到右（行刷新，水平刷新，Horizontal Scanning），从上到下（屏幕刷新，垂直刷新，Vertical Scanning）。当整个屏幕刷新完毕，即一个垂直刷新周期完成，会有短暂的空白期，此时发出 VSync 信号。所以，VSync 中的 V 指的是垂直刷新中的垂直-Vertical。

　　Android系统每隔16ms发出VSYNC信号，触发对UI进行渲染，VSync是Vertical Synchronization(垂直同步)的缩写，是一种在PC上很早就广泛使用的技术，可以简单的把它认为是一种定时中断。而在Android 4.1(JB)中已经开始引入VSync机制，用来同步渲染，让AppUI和SurfaceFlinger可以按硬件产生的VSync节奏进行工作。

基础概念

屏幕刷新频率：一秒内屏幕刷新的次数（一秒内显示了多少帧的图像），单位Hz。刷新频率取决于硬件。
60HZ 1000/60 = 16.666 ms 刷新一次

逐行扫描：显示器对图像从左到右，从上到下逐行扫描，顺序显示到屏幕的一个个像素点。

帧率：GPU在一秒内绘制操作的帧数，单位为fps。帧率是动态变化的。当画面禁止时，此时GPU没有绘制操作，屏幕刷新的还是buffer中的数据，即GPU最后操作的帧数据。

画面撕裂：理想情况下帧率和刷新频率保存一致，即每绘制完成一帧，显示器显示一帧。但屏幕刷新频率是固定的，而CPU/GPU写数据是不可控的。即Display在显示的过程中，buffer里的数据被CPU/GPU修改，导致画面撕裂；

Android 4.1 之前，屏幕刷新遵循双缓存 + VSYNC机制。（注：此时VSync信号到来是不会强制CPU/GPU工作）。当VSYnc信号到来时，下一帧数据可能还没准备就绪，缓存没有交换，显示的仍是上一帧数据。即，发生了jank

二、三个方法改进显示系统

1.Vsync同步：

可见vsync信号没有提醒CPU/GPU工作的情况下，第二次vsync到来需要显示内容时，CPU和GPU还没有来得及准备好下一帧的数据，所以只能接着显示上一帧的数据，产生Jank!

　　CPU/GPU接收vsync信号提前准备下一帧要显示的内容，所以能够及时准备好每一帧的数据，保证画面的流程。

2.多级缓冲：

　　除了Vsync的机制，Android还使用了多级缓冲的手段以优化UI流程度，例如双缓冲(A+B)，在显示buffer A的数据时，CPU/GPU就开始在buffer B中准备下一帧数据：

　　但是不能保证每一帧CPU、GPU都运行状态良好，可能由于资源抢占等性能问题导致某一帧GPU掉链子，vsync信号到来时buffer B的数据还没准备好，而此时Display又在显示buffer A的数据，导致后面CPU/GPU没有新的buffer着手准备数据，空白时间无事可做，后面Jank频出：

　　因此用三级缓冲来提高系统对性能波动的容忍度：

虽然GPU在准备buffer B的数据耗时过长，第二帧Jank，但是新增1个buffer可以减少CPU和GPU在vsync同步间的空白间隙，此时CPU/GPU能够利用buffer C继续工作，所以后面就不会再产生Jank了，当然具体使用多少个buffer要根据实际硬件性能情况调整，最终目的就是解决Display的Jank产生。

3.Vsync虚拟化（Vsync App + Vsync SurfaceFlinger）：

　　虽然vsync使得CPU/GPU/Display同步了，但App UI和SurfaceFlinger的工作显然是一个流水线的模型。即对于一帧内容，先等App UI画完了，SurfaceFlinger再出场对其进行合并渲染后放入framebuffer，最后整到屏幕上。而现有的VSync模型是让大家一起开始干活，这样对于同一帧内容，第一个VSync信号时App UI的数据开始准备，第二个VSync信号时SurfaceFlinger工作，第三个VSync信号时用户看到Display内容，这样就两个VSync period(每个16ms)过去了，影响用户体验。
　　解决思路：SurfaceFlinger在App UI准备好数据后及时开工做合成。

　　Android 4.4(KitKat)引入了VSync的虚拟化，即把硬件的VSync信号先同步到一个本地VSync模型中，再从中一分为二，引出两条VSync时间与之有固定偏移的线程。示意图如下：

　　这样，大家工作既保持一定的节拍，又可以相互错开，一前一后保持着流水节奏。

　　注意其中两个Phase offset参数（即VSYNC_EVENT_PHASE_OFFSET_NS和SF_VSYNC_EVENT_PHASE_OFFSET_NS）是可调的。

　　处理流程：

　　类型DispSync表示了一个基于硬件VSync信号的同步模型，它会根据从HWComposer来的硬件VSync信号的采样来进行同步。其它两个EventThread分别用了两个不同的虚拟VSync信号源（用DispSyncSource表示，其中包含了与真实VSync信号的偏移值），这两个VSync信号源就是被虚拟出来分别用于控制App UI和SurfaceFlinger渲染。在EventThread的线程循环中，如果有需要就会向DispSync注册相应的listener。DispSyncThread就像乐队鼓手一样控制着大家的节奏。它在主循环中会先通过已经向DispSync注册的listener计算下一个要产生的虚拟VSync信号还要多久，等待相应时间后就会调用相应listener的callback函数。这样，对于那些注册了listener的监听者来说，就好像被真实的VSync信号控制着一样。至于EventControlThread是用来向真实的VSync硬件发命令。

三、Vsync框架

　1.硬件或者软件创建vsyncThread产生vsync。
　2.DispSyncThread处理vsync，把vsync虚拟化成vsync-app和vsync-sf。
　3.vsync-app/sf按需产生（如果App和SurfaceFlinger都没有更新请求，则休眠省电）：
　　APP端：APP需要更新界面时发出vsync请求给EventThread（设置connection.count>=0），DispSyncThread收到vsync信号后休眠offset，然后唤醒EventThread通知APP开始渲染。
　　SF端：sf请求EventThread-sf，EventThread-sf收到vsync后通知SF可以开始合成。

　　　　　　　　　　　　　　　　　　　　　　　　　　　　（vsync框架图）

　4.代码分析：

　 4.1 创建五个线程：SurfaceFlingerThread、DispSyncThread、EventThead-App、EventThead-SF、VsyncThread，都属于SurfaceFlinger进程：

　（1）启动SurfaceFlinger主线程：android-8.0.0_r4\frameworks\native\services\surfaceflinger\main_surfaceflinger.cpp

int main(int, char**) {startHidlServices();signal(SIGPIPE, SIG_IGN);// When SF is launched in its own process, limit the number of// binder threads to 4.ProcessState::self()->setThreadPoolMaxThreadCount(4);// start the thread poolsp<ProcessState> ps(ProcessState::self());ps->startThreadPool();// instantiate surfaceflingersp<SurfaceFlinger> flinger = new SurfaceFlinger(); //其中创建了 mPrimaryDispSync 成员变量setpriority(PRIO_PROCESS, 0, PRIORITY_URGENT_DISPLAY);set_sched_policy(0, SP_FOREGROUND);// Put most SurfaceFlinger threads in the system-background cpuset// Keeps us from unnecessarily using big cores// Do this after the binder thread pool initif (cpusets_enabled()) set_cpuset_policy(0, SP_SYSTEM);// initialize before clients can connectflinger->init(); //传入mPrimaryDispSync并创建EventThread（app/sf）、HWComposer// publish surface flingersp<IServiceManager> sm(defaultServiceManager());sm->addService(String16(SurfaceFlinger::getServiceName()), flinger, false);// publish GpuServicesp<GpuService> gpuservice = new GpuService();sm->addService(String16(GpuService::SERVICE_NAME), gpuservice, false);struct sched_param param = {0};param.sched_priority = 2;if (sched_setscheduler(0, SCHED_FIFO, &param) != 0) {ALOGE("Couldn't set SCHED_FIFO");}// run surface flinger in this thread (SF的主线程)flinger->run();return 0;
}

（2）new SurfaceFlinger() 时创建了成员变量 DispSync mPrimaryDispSync;　　android-8.0.0_r4\frameworks\native\services\surfaceflinger\SurfaceFlinger.h

　　其中 DispSync 的构造函数会启动DispSyncThread线程：

DispSync::DispSync(const char* name) :mName(name),mRefreshSkipCount(0),mThread(new DispSyncThread(name)), //创建了DispSyncThreadmIgnorePresentFences(!SurfaceFlinger::hasSyncFramework){mPresentTimeOffset = SurfaceFlinger::dispSyncPresentTimeOffset;mThread->run("DispSync", PRIORITY_URGENT_DISPLAY + PRIORITY_MORE_FAVORABLE);// set DispSync to SCHED_FIFO to minimize jitterstruct sched_param param = {0};param.sched_priority = 2;if (sched_setscheduler(mThread->getTid(), SCHED_FIFO, &param) != 0) {ALOGE("Couldn't set SCHED_FIFO for DispSyncThread");}reset();beginResync();if (kTraceDetailedInfo) {// If we're not getting present fences then the ZeroPhaseTracer// would prevent HW vsync event from ever being turned off.// Even if we're just ignoring the fences, the zero-phase tracing is// not needed because any time there is an event registered we will// turn on the HW vsync events.if (!mIgnorePresentFences && kEnableZeroPhaseTracer) {addEventListener("ZeroPhaseTracer", 0, new ZeroPhaseTracer());}}
}

（3）接着分析SurfaceFlinger对象，它是一个strong point,在引用时会调用其onFirstRef()方法：

void SurfaceFlinger::onFirstRef()
{//初始化消息队列，其中创建了loop和handlemEventQueue.init(this);
}

　　flinger->run()的实现： android-8.0.0_r4\frameworks\native\services\surfaceflinger\SurfaceFlinger_hwc1.cpp

void SurfaceFlinger::run() {do {waitForEvent(); //其中就是调用mEventQueue.waitMessage()} while (true);
}

　　waitMessage中等待AP和EventTHread给它发数据：

void MessageQueue::waitMessage() {do {IPCThreadState::self()->flushCommands();int32_t ret = mLooper->pollOnce(-1);switch (ret) {case Looper::POLL_WAKE:case Looper::POLL_CALLBACK:continue;case Looper::POLL_ERROR:ALOGE("Looper::POLL_ERROR");continue;case Looper::POLL_TIMEOUT:// timeout (should not happen)continue;default:// should not happenALOGE("Looper::pollOnce() returned unknown status %d", ret);continue;}} while (true);
}

　　SurfaceFlinger初始化最重要的函数是init():

void SurfaceFlinger::init() {ALOGI(  "SurfaceFlinger's main thread ready to run. ""Initializing graphics H/W...");ALOGI("Phase offest NS: %" PRId64 "", vsyncPhaseOffsetNs);{ // Autolock scopeMutex::Autolock _l(mStateLock);// initialize EGL for the default displaymEGLDisplay = eglGetDisplay(EGL_DEFAULT_DISPLAY);eglInitialize(mEGLDisplay, NULL, NULL);// start the EventThread (创建了两个EventThread，他们的名字和offset不同)sp<VSyncSource> vsyncSrc = new DispSyncSource(&mPrimaryDispSync,vsyncPhaseOffsetNs, true, "app");mEventThread = new EventThread(vsyncSrc, *this, false);sp<VSyncSource> sfVsyncSrc = new DispSyncSource(&mPrimaryDispSync,sfVsyncPhaseOffsetNs, true, "sf");mSFEventThread = new EventThread(sfVsyncSrc, *this, true);// 创建sf与EventThread之间的connectionmEventQueue.setEventThread(mSFEventThread);// set EventThread and SFEventThread to SCHED_FIFO to minimize jitterstruct sched_param param = {0};param.sched_priority = 2;if (sched_setscheduler(mSFEventThread->getTid(), SCHED_FIFO, &param) != 0) {ALOGE("Couldn't set SCHED_FIFO for SFEventThread");}if (sched_setscheduler(mEventThread->getTid(), SCHED_FIFO, &param) != 0) {ALOGE("Couldn't set SCHED_FIFO for EventThread");}// Get a RenderEngine for the given display / config (can't fail)mRenderEngine = RenderEngine::create(mEGLDisplay,HAL_PIXEL_FORMAT_RGBA_8888);}

　　其中 mEventQueue.setEventThread(mSFEventThread) 的实现：

//sufaceflinger/MessageQueue.cpp
void MessageQueue::setEventThread(const sp<EventThread>& eventThread)
{mEventThread = eventThread;//创建连接，从连接获得dataChannel，把它的Fd添加到Looper，//也就是把EventThread里的一个fd传给了SF线程，//以后EventThread与SF就可以通过这个fd通信，mEvents = eventTHread->createEnvetConnection();mEventTube = mEvents->getDataChannel();mLooper->addFd(mEventTube->getFd(), 0, Looper::EVENT_INPUT,MessageQueue::cb_eventReceiver, this); //这个cb_eventRecevier很重要，它负责处理EventThread发过来的信号
}

　（4）hwcomposer的构造函数：android-8.0.0_r4\frameworks\native\services\surfaceflinger\DisplayHardware\HWComposer_hwc1.cpp

HWComposer::HWComposer(const sp<SurfaceFlinger>& flinger,EventHandler& handler): mFlinger(flinger),mFbDev(0), mHwc(0), mNumDisplays(1),mCBContext(new cb_context),mEventHandler(handler),mDebugForceFakeVSync(false)
{for (size_t i =0 ; i<MAX_HWC_DISPLAYS ; i++) {mLists[i] = 0;}for (size_t i=0 ; i<HWC_NUM_PHYSICAL_DISPLAY_TYPES ; i++) {mLastHwVSync[i] = 0;mVSyncCounts[i] = 0;}char value[PROPERTY_VALUE_MAX];property_get("debug.sf.no_hw_vsync", value, "0");mDebugForceFakeVSync = atoi(value);bool needVSyncThread = true;// Note: some devices may insist that the FB HAL be opened before HWC.int fberr = loadFbHalModule();loadHwcModule();if (mFbDev && mHwc && hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1)) {// close FB HAL if we don't needed it.// FIXME: this is temporary until we're not forced to open FB HAL// before HWC.framebuffer_close(mFbDev);mFbDev = NULL;}// If we have no HWC, or a pre-1.1 HWC, an FB dev is mandatory.if ((!mHwc || !hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1))&& !mFbDev) {ALOGE("ERROR: failed to open framebuffer (%s), aborting",strerror(-fberr));abort();}// these display IDs are always reservedfor (size_t i=0 ; i<NUM_BUILTIN_DISPLAYS ; i++) {mAllocatedDisplayIDs.markBit(i);}if (mHwc) {ALOGI("Using %s version %u.%u", HWC_HARDWARE_COMPOSER,(hwcApiVersion(mHwc) >> 24) & 0xff,(hwcApiVersion(mHwc) >> 16) & 0xff);if (mHwc->registerProcs) {mCBContext->hwc = this;mCBContext->procs.invalidate = &hook_invalidate;mCBContext->procs.vsync = &hook_vsync;if (hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1))mCBContext->procs.hotplug = &hook_hotplug;elsemCBContext->procs.hotplug = NULL;memset(mCBContext->procs.zero, 0, sizeof(mCBContext->procs.zero));mHwc->registerProcs(mHwc, &mCBContext->procs);}// don't need a vsync thread if we have a hardware composerneedVSyncThread = false;// always turn vsync off when we starteventControl(HWC_DISPLAY_PRIMARY, HWC_EVENT_VSYNC, 0);// the number of displays we actually have depends on the// hw composer versionif (hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_3)) {// 1.3 adds support for virtual displaysmNumDisplays = MAX_HWC_DISPLAYS;} else if (hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1)) {// 1.1 adds support for multiple displaysmNumDisplays = NUM_BUILTIN_DISPLAYS;} else {mNumDisplays = 1;}}if (mFbDev) {ALOG_ASSERT(!(mHwc && hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1)),"should only have fbdev if no hwc or hwc is 1.0");DisplayData& disp(mDisplayData[HWC_DISPLAY_PRIMARY]);disp.connected = true;disp.format = mFbDev->format;DisplayConfig config = DisplayConfig();config.width = mFbDev->width;config.height = mFbDev->height;config.xdpi = mFbDev->xdpi;config.ydpi = mFbDev->ydpi;config.refresh = nsecs_t(1e9 / mFbDev->fps);disp.configs.push_back(config);disp.currentConfig = 0;} else if (mHwc) {// here we're guaranteed to have at least HWC 1.1for (size_t i =0 ; i<NUM_BUILTIN_DISPLAYS ; i++) {queryDisplayProperties(i);}}if (needVSyncThread) {// we don't have VSYNC support, we need to fake it// 如果不支持硬件Vsync则创建软件vysnc线程，它是一个sp<>,其onFirstRef()真正创建运行这个线程mVSyncThread = new VSyncThread(*this);}
}

　　加载并准备hw composer模块。Sets mHwc

void HWComposer::loadHwcModule()
{hw_module_t const* module;if (hw_get_module(HWC_HARDWARE_MODULE_ID, &module) != 0) {ALOGE("%s module not found", HWC_HARDWARE_MODULE_ID);return;}int err = hwc_open_1(module, &mHwc);if (err) {ALOGE("%s device failed to initialize (%s)",HWC_HARDWARE_COMPOSER, strerror(-err));return;}if (!hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_0) ||hwcHeaderVersion(mHwc) < MIN_HWC_HEADER_VERSION ||hwcHeaderVersion(mHwc) > HWC_HEADER_VERSION) {ALOGE("%s device version %#x unsupported, will not be used",HWC_HARDWARE_COMPOSER, mHwc->common.version);hwc_close_1(mHwc);mHwc = NULL;return;}
}

　4.2 SurfaceFLinger使用vsync过程

（1）App发数据给sf,sf发请求给EventThread-sf：AP是Producer，它通过listener->onFrameAvailable() ->进入消费者 -> mFrameAvailableListener(就是Layer对象) -> 进入SF线程-> mFlinger->signalLayerUpdate() -> mEventQueue.invalidate();

//surfaceflinger/MessageQueue.cpp
void MessageQueue::invalidate() {//mEvents是sp<IDisplayEventConnection>//也就是sf线程使用connection向EventThread线程请求下一个vsync信号mEvents->requestNextVsync();
}
--->
//surfaceFlinger/EventThread.cpp
void EventThread::Connection::requestNextVsync() {mEventThread->requestrNextVsync(this);
}
--->
void EventTHread::requestNextVysnc(const sp<EventTHread::Connection>& connection) {if(connection->count < 0){//若cnt小于0，则cnt=0，然后发出广播，来唤醒某个线程，//当connection的cnt >= 0怎么它需要从EventThread得到vsync//这个函数得代码在EventThread，但是它执行在SF线程//也就是说，SF线程使用EventThread的函数向EventThread发出广播来唤醒EventThread线程connection->count = 0;mCondition.boradcast();}    }

（2）EventThread-sf发请求给DispSyncThread，EventThread里面一定有一个threadLoop： android-8.0.0_r4\frameworks\native\services\surfaceflinger\EventThread.cpp

bool EventThread::threadLoop() {DisplayEventReceiver::Event event;Vector< sp<EventThread::Connection> > signalConnections;//1,EventThread向DispSyncThread发出vsync请求//2，等待vsync  signalConnections = waitForEvent(&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 event

　　　 　// 当EventThread收到Vsync，把它转交给SF线程status_t err = conn->postEvent(event);if (err == -EAGAIN || err == -EWOULDBLOCK) {// The destination doesn't accept events anymore, it's probably// full. For now, we just drop the events on the floor.// FIXME: Note that some events cannot be dropped and would have// to be re-sent later.// Right-now we don't have the ability to do this.ALOGW("EventThread: dropping event (%08x) for connection %p",event.header.type, conn.get());} else if (err < 0) {// handle any other error on the pipe as fatal. the only// reasonable thing to do is to clean-up this connection.// The most common error we'll get here is -EPIPE.removeDisplayEventConnection(signalConnections[i]);}}return true;
}

分析waitForEvent():

//this will return when
//1, a vsync event has benn recevied
//2,there was as least one connection interested in receiving it when we started waiting
Vector< sp<EventThread::Connection > >EventThread::waitForEvent(DisplayEventReceiver::Event* event)
{//find out connections waitting for eventssize_t count = mDisplayEventConnections.size();ofr(size_t i=0; i<count; i++) {if(connection->count >= 0) {//we need vsync events because at least //one connnection is waiting for itwaitForVSync = true;}......if(waitFOrVSync){enableVSyncLocked(); //如果上面的count >= 0，代表需要得到vsync信号，然后调用enableVSyncLocked()}if(waitForVSync){......} else {//EventThread之后就会休眠等待vsyncmCondition.wait(mLock);}}
}

　enableVSyncLocked中给DisplaySyncThread设置回调，在DisplaySyncThread收到vsync信号后就调用这个回调函数：

void EventThread::enableVSyncLocked() {if(!mVsyncEnabled) {mVsyncEnabled = true;mVSyncSource->setCallback(static_cast<VSyncSource::Callback*)(this));mVSYncSource->setVSyncEnabled(true);}
}

（3）H/S vsync唤醒DispSyncThread：

//surfaceFlinger/DIsplayHardware/HWComposer_hwc1.cpp
bool HWComposer::VSyncThread::threadLoop() {clock_nanosleep();//休眠 完成后，调用它，发出vsync信号//mEventHanlder就是SurfaceFlinger，mHwc.mEventHandler.onVSyncReceived(0, next_vsync);
}
--->
//surfaceflinger/SurafceFLinger.cpp
void SurfaceFLinger::onVSyncReceived(type, nescs_t timestamp){if(type == 0 && mPrimaryHWVsyncEnabled) {//DispSync::addResyncSample => updateModelLocked() => //mThread->updateModel => mCond.signal()来唤醒某个线程，//mCond.signal()在DispSync.cpp，属于DispSYncThread，//还是之前说的套路，swVsyncThread使用DispSync的函数唤醒DispSYncThreadneedsHwVsync = mPrimaryDIspSync.addResyncSample(timestamp);}if(needsHwVsync) {enableHardwareVsync();}
}

（4）DispSyncThread发信号给EventThread，EventThread发信号给SF线程。

//surfaceflinger/DispSync.cpp
class DispSyncThread : public Thread {virtual bool threadLoop() {//计算最近的eventThread的时间，EventThread是Listener，// =>computeListenerNextEventTimeLocked targetTime = computeNextEventTimeLocked(now);//休眠if(now < targetTime) {mCond.waitRelative(mMutex, targetrTime-now);}//之后被vsync信号唤醒，获取callbackcallbackInvacations = gatherCallbackInvocationsLocked(now);if(callbackInvocations.size() > 0) {//执行callback，导致EventThread的onVsyncEvent函数被调用fireCallbackInvocations(callbackInvovcations);}}
}

　　如果EventThread发现Connection的cnt >= 0，就会向DispSyncThread注册回调函数，最后会通过callback构造出Listener，而且EventThread运行时间是收到vsync之后加一个offset，fireCallbackInvocations()调用EventThread类的onVSyncEvent来唤醒EventThread线程：

//surfaceFlinger/EventThread.cpp
void EventThread::onVSyncEvent(nsecs_t timestamp) {Mutex::Autolock _l(mLock);mVSyncEvent[0].header.id = 0;mVsyncEvent[0].vsync.count++;//发出广播，唤醒EvenThread::threadLoop()向app或sf发出信号mCondition.broadcast();
}

　　EventThread::threadLoop()在waitForEvent里面休眠（mCondition.wait），收到广播后被唤醒，然后调用conn->postEvent(event)向SF线程或者AP发出信号，并通过connection的fd把数据写到SF线程，同样SF线程通过fd获得数据，然后调用SF线程的处理函数。

status_t EventThread::Connection::postEvent(const DisplayEventReceiver::Event& event) {ssize_t size = DisplayEventReceiver::sendEvents(&mChannel, &event, 1);return size < 0 ? status_t(size) : status_t(NO_ERROR);
}

　（5）sf线程对vsync的处理：在flinger->init()创建connection时，得到了一个fd，然后会检测fd。

　　　flinger->init() => MessageQueue::setEventThread：

mEventTube = mEvent->getDataChannel();
//检测fd，从fd得到数据，会调用MessageQueue::cb_eventRecevier函数
mLooper->addFd(mEventTube0>getFd(),....,MessageQueue::cb_eventRecevier, this);--->//surfaceflinger/MessageQueue.cpp
int MessageQueue::cb_eventRecevier(int fd, int event, void* data) {return queue->eventRecevier(fd, events);
}
--->
int MessageQueue::eventReceiver(int fd, int events) {mHanlder->displatchInvalidate();
}
--->
void MessageQueue::Handler::displayInvaliadate() {mQueue.mLooper->sendMessage(this, Message(MessageQueue::INVALIDATE));
}

SurfaceFlinger中接收并处理消息：

void MessageQueue::Handler::handleMessage(const Message& message) {switch(message.what) {...case INVALIDATE:mQueue.mFlinger->onMessageReceived(message.what);}
}--->// android-8.0.0_r4\frameworks\native\services\surfaceflinger\SurfaceFlinger_hwc1.cpp
void SurfaceFlinger::onMessageReceived(int32_t what) {ATRACE_CALL();switch (what) {case MessageQueue::INVALIDATE: {bool refreshNeeded = handleMessageTransaction(); //处理事务（设置flag，并未实际操作各个buffer）：//①在handleTransactionLocked()中遍历每个Layer,执行layer->doTransaction(0)//②处理Display事务(add/remove)//③Layer角度发生了变化//④处理sf本身事务：　Layer的增加或者删除refreshNeeded |= handleMessageInvalidate(); //处理各Layer的buffer更换，使原来的界面无效，并准备好新数据用来更新：//①accquire next buffer//②release previous buffer//③bindTextureImageLocked() ---> glEGLImageTargetTexture2DOES() refreshNeeded |= mRepaintEverything;if (refreshNeeded) {// Signal a refresh if a transaction modified the window state,// a new buffer was latched, or if HWC has requested a full// repaintsignalRefresh(); //发出Refresh信号，导致下面的 handleMessageRefresh()函数被调用}break;}case MessageQueue::REFRESH: {handleMessageRefresh();　　//①计算各Layer的可视区域//②合成显示：a.调用opengl把各个Layer的可视区域在一个内存上描绘出来//　　　　　　b.使用hw composer硬件合成

break; } } }

　　　　　　　　　　　　　　　　　　　　　　　　　　　　(Vsync时序图)