Android 显示系统：Vsync机制
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。
二、黄油计划：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 pool sp<ProcessState> ps(ProcessState::self()); ps->startThreadPool(); // instantiate surfaceflinger sp<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 init if (cpusets_enabled()) set_cpuset_policy(0, SP_SYSTEM); // initialize before clients can connect flinger->init(); //传入mPrimaryDispSync并创建EventThread（app/sf）、HWComposer // publish surface flinger sp<IServiceManager> sm(defaultServiceManager()); sm->addService(String16(SurfaceFlinger::getServiceName()), flinger, false); // publish GpuService sp<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, ¶m) != 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DispSyncThread mIgnorePresentFences(!SurfaceFlinger::hasSyncFramework){ mPresentTimeOffset = SurfaceFlinger::dispSyncPresentTimeOffset; mThread->run("DispSync", PRIORITY_URGENT_DISPLAY + PRIORITY_MORE_FAVORABLE); // set DispSync to SCHED_FIFO to minimize jitter struct sched_param param = {0}; param.sched_priority = 2; if (sched_setscheduler(mThread->getTid(), SCHED_FIFO, ¶m) != 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handle mEventQueue.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 happen ALOGE("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 scope Mutex::Autolock _l(mStateLock); // initialize EGL for the default display mEGLDisplay = 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connection mEventQueue.setEventThread(mSFEventThread); // set EventThread and SFEventThread to SCHED_FIFO to minimize jitter struct sched_param param = {0}; param.sched_priority = 2; if (sched_setscheduler(mSFEventThread->getTid(), SCHED_FIFO, ¶m) != 0) { ALOGE("Couldn't set SCHED_FIFO for SFEventThread"); } if (sched_setscheduler(mEventThread->getTid(), SCHED_FIFO, ¶m) != 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 reserved for (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; else mCBContext->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 composer needVSyncThread = false; // always turn vsync off when we start eventControl(HWC_DISPLAY_PRIMARY, HWC_EVENT_VSYNC, 0); // the number of displays we actually have depends on the // hw composer version if (hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_3)) { // 1.3 adds support for virtual displays mNumDisplays = MAX_HWC_DISPLAYS; } else if (hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1)) { // 1.1 adds support for multiple displays mNumDisplays = 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 1.1 for (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 events size_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 it waitForVSync = 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vsync mCondition.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DispSYncThread needsHwVsync = 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callback callbackInvacations = 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 // repaint signalRefresh(); //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)
posted on 2019-09-01 21:14 sheldon_blogs 阅读(19205) 评论(2) 编辑 收藏 举报
