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<:eventhandler>(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。