Android为了增强界面流畅度引入了VSync机制。VSync信号一般由硬件产生，为了兼容一些老的机器，也可以软件产生。因些，就可以分为硬件VSync和软件VSync。VSync的生成逻辑主要在HWComposer里面。相关文件如下：

• frameworks/native/services/surfaceflinger/DisplayHardware/HWComposer.cpp
• frameworks/native/services/surfaceflinger/DisplayHardware/HWComposer.h

HWComposer在SurfaceFlinger的init函数里创建：

void SurfaceFlinger::init() {    ALOGI(  "SurfaceFlinger's main thread ready to run. "            "Initializing graphics H/W...");    ......    // Initialize the H/W composer object.  There may or may not be an    // actual hardware composer underneath.    mHwc = new HWComposer(this,            *static_cast
    
     (this));        ......    }
    

• SurfaceFlinger成员变量保存了HWComposer对象指针
• 创建HWComposer时，使用SurfaceFlinger作为参数传入

HWComposer::HWComposer(        const sp
    
     & flinger,        EventHandler& handler)    : mFlinger(flinger),      mFbDev(0), mHwc(0), mNumDisplays(1),      mCBContext(new cb_context),      mEventHandler(handler),      mDebugForceFakeVSync(false){    ......    bool needVSyncThread = true;    // Note: some devices may insist that the FB HAL be opened before HWC.    int fberr = loadFbHalModule();    loadHwcModule();    .......    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);        ......    }    ......    if (needVSyncThread) {        // we don't have VSYNC support, we need to fake it        mVSyncThread = new VSyncThread(*this);    }}
    

• 在构造函数里把SurfaceFlinger对象指针赋值给成员变量mFlinger和mEventHandler
• 首先尝试加载HWC模块，如果加载成功就使用硬件vsync，加载失败就使用软件模拟vsync

在加载HWC硬件模块时，会注册vsync回调函数HWComposer::hook_vsync。当硬件vsync到来时会调用些函数。代码如下：

void HWComposer::hook_vsync(const struct hwc_procs* procs, int disp,        int64_t timestamp) {    cb_context* ctx = reinterpret_cast
    
     (             const_cast
     
      (procs));    ctx->hwc->vsync(disp, timestamp);}
     
    

• 调用hwc对象的vsync函数

void HWComposer::vsync(int disp, int64_t timestamp) {     if (uint32_t(disp) < HWC_NUM_PHYSICAL_DISPLAY_TYPES) {                                                                                                                                           {            Mutex::Autolock _l(mLock);                                                                                                                                                                   // There have been reports of HWCs that signal several vsync events            // with the same timestamp when turning the display off and on. This            // is a bug in the HWC implementation, but filter the extra events            // out here so they don't cause havoc downstream.            if (timestamp == mLastHwVSync[disp]) {                ALOGW("Ignoring duplicate VSYNC event from HWC (t=%" PRId64 ")",                                                                                                                                     timestamp);                            return;            }            mLastHwVSync[disp] = timestamp;                                                                                                                                                          }        char tag[16];        snprintf(tag, sizeof(tag), "HW_VSYNC_%1u", disp);        ATRACE_INT(tag, ++mVSyncCounts[disp] & 1);                                                                                                                                                   mEventHandler.onVSyncReceived(disp, timestamp);                                                                                                                                          }  }

• 最终调用mEventHandler的onVSyncReceived函数，也既SurfaceFlinger的onVSyncReceived

软件vsync在VSyncThread线程中实现，看看线程循环函数：

bool HWComposer::VSyncThread::threadLoop() {    { // scope for lock        Mutex::Autolock _l(mLock);             while (!mEnabled) {            mCondition.wait(mLock);            }    }      const nsecs_t period = mRefreshPeriod;    const nsecs_t now = systemTime(CLOCK_MONOTONIC);    nsecs_t next_vsync = mNextFakeVSync;    nsecs_t sleep = next_vsync - now;     if (sleep < 0) {        // we missed, find where the next vsync should be        sleep = (period - ((now - next_vsync) % period));         next_vsync = now + sleep;      }      mNextFakeVSync = next_vsync + period;                                                                                                                                                        struct timespec spec;    spec.tv_sec  = next_vsync / 1000000000;    spec.tv_nsec = next_vsync % 1000000000;                                                                                                                                                      int err;    do {        err = clock_nanosleep(CLOCK_MONOTONIC, TIMER_ABSTIME, &spec, NULL);                                                                                                                      } while (err<0 && errno == EINTR);                                                                                                                                                           if (err == 0) {        mHwc.mEventHandler.onVSyncReceived(0, next_vsync);                                                                                                                                       }      return true;}

• 以固定的时间间隔发送vsync信号
• 调用mEventHandler的onVSyncReceived函数，最终调用SurfaceFlinger的onVSyncReceived函数。
• 这样不管是硬件vsync还是软件vsync都达成了统一。当VSync信号产生时调用SurfaceFlinger的回调函数onVSyncReceived。