什么原因会引发displayeventreceiver一直在运行

什么原因会引发displayeventreceiver一直在运行

们先看收到VSync事件是如何处理的，再看底层是怎样向上层通知VSync事件。

1.SurfaceFlinger如何处理Vsync事件
在SurfaceFlinger定义了一个消息队列的成员变量：
mutableMessageQueue mEventQueue;
MessageQueue是一个消息处理类，专门处理如下三种消息：
INVALIDATE， REFRESH， TRANSACTION。
这三个消息都与UI更新相关。SurfaceFlinger在自己的主线程中等待上述消息的到来，以便进行相应的处理，代码如下（后面我们会看到，VSync事件会触发上述消息）：
[cpp] view plain copy
bool SurfaceFlinger::threadLoop() {
waitForEvent();
return true;
}

其中waitForEvent又调用的MessageQueue的waitMessage函数：
[cpp] view plain copy
void SurfaceFlinger::waitForEvent() {
mEventQueue.waitMessage();
}

所以SurfaceFlinger依赖MessageQueue截获上述消息，那又是通过怎样的流程向MessageQueue发送上述消息呢？换句话说，MessageQueue又是如何接收到上述三个消息的呢？
而这一切又是与EventThread类相关。通过查看MessageQueue的定义，可以看到，它定义了一个EventThread的成员变量，并通过setEventThread初始化，代码如下：

[cpp] view plain copy
void MessageQueue::setEventThread(const sp& eventThread)
{
mEventThread = eventThread;
mEvents = eventThread->createEventConnection();
mEventTube = mEvents->getDataChannel();
mLooper->addFd(mEventTube->getFd(), 0, ALOOPER_EVENT_INPUT,
MessageQueue::cb_eventReceiver, this);
}

通过此接口，不仅获取了一个EventThread实例的引用，同时，通过EventThread创建一个IDisplayEventConnection对象，它代表了接收显示事件（主要是VSync事件以及HotPlug事件）的一个远程连接接收端，并通过Looper对象监听发生在此对象上的显示事件。在EventThread的线程循环中，等待VSync事件的发生，代码如下：
[cpp] view plain copy
bool EventThread::threadLoop() {
DisplayEventReceiver::Event event;
Vector< sp > signalConnections;
signalConnections = waitForEvent(&event);

// dispatch events to listeners...
const size_t count = signalConnections.size();
for (size_t i=0 ; i const sp& conn(signalConnections[i]);
// now see if we still need to report this event
status_t err = conn->postEvent(event);
…
}
return true;
}

在waitForEvent中，检测VSync事件，Connection::postEvent（DisplayEventReceiver::sendEvents）则将检测到的VSync事件发送给监听者。前面提到，通过Looper对象监听发生的VSync事件，它会调用回调函数MessageQueue::cb_eventReceiver，它实际会调用MessageQueue::eventReceiver，代码如下：

[cpp] view plain copy
int MessageQueue::eventReceiver(int fd, int events) {
ssize_t n;
DisplayEventReceiver::Event buffer[8];
while ((n = DisplayEventReceiver::getEvents(mEventTube, buffer, 8)) > 0) {
for (int i=0 ; i if (buffer[i].header.type == DisplayEventReceiver::DISPLAY_EVENT_VSYNC) {
#if INVALIDATE_ON_VSYNC
mHandler->dispatchInvalidate();
#else
mHandler->dispatchRefresh();
#endif
break;
}
}
}
return 1;
}

对于VSync事件的处理，会调用MessageQueue::Handler::dispatchInvalidate()或MessageQueue::Handler::dispatchRefresh()，代码如下：

[cpp] view plain copy
void MessageQueue::Handler::dispatchRefresh() {
if ((android_atomic_or(eventMaskRefresh, &mEventMask) & eventMaskRefresh) == 0) {
mQueue.mLooper->sendMessage(this, Message(MessageQueue::REFRESH));
}
}

void MessageQueue::Handler::dispatchInvalidate() {
if ((android_atomic_or(eventMaskInvalidate, &mEventMask) & eventMaskInvalidate) == 0) {
mQueue.mLooper->sendMessage(this, Message(MessageQueue::INVALIDATE));
}
}

最终在如下消息处理函数中，对VSync事件进行处理：

[cpp] view plain copy
void MessageQueue::Handler::handleMessage(const Message& message) {
switch (message.what) {
case INVALIDATE:
android_atomic_and(~eventMaskInvalidate, &mEventMask);
mQueue.mFlinger->onMessageReceived(message.what);
break;
case REFRESH:
android_atomic_and(~eventMaskRefresh, &mEventMask);
mQueue.mFlinger->onMessageReceived(message.what);
break;
case TRANSACTION:
android_atomic_and(~eventMaskTransaction, &mEventMask);
mQueue.mFlinger->onMessageReceived(message.what);
break;
}
}

如上代码所示，实际上会执行SurfaceFlinger类里定义的回调函数，在那里面对开头提到的三种消息进行真正的处理，即对VSync事件的响应最终转变成对上述三种消息进行处理。

2.VSync事件产生流程分析
上述分析了，从等待接收VSync事件到最后处理VSync事件的过程。下面我们要看下VSync事件是如何产生的，即是如何通知SurfaceFlinger::waitEvent返回的。众所周知，VSync事件一般由硬件周期性产生，如果没有相应的硬件产生VSync事件，则通过软件模拟。下面主要看下从硬件产生VSync事件到触发MessageQueue处理VSync事件的过程。
首先，看下HWComposer的定义：

[cpp] view plain copy
class HWComposer
{
public:
class EventHandler {
friend class HWComposer;
virtual void onVSyncReceived(int disp, nsecs_t timestamp) = 0;
virtual void onHotplugReceived(int disp, bool connected) = 0;
protected:
virtual ~EventHandler() {}
};

enum {
MAX_DISPLAYS = HWC_NUM_DISPLAY_TYPES + 1
};

HWComposer(
const sp& flinger,
EventHandler& handler);
…
}

可以看到，HWComposer::EventHandler定义了通知VSync事件的回调接口。而SurfaceFlinger是实现了上述接口。所以问题变成，当底层硬件发生VSync事件时，会回调一个EventHandler对象的onVSyncReceived函数。
实际上，在HWComposer的构造函数中，注册了底层hwcomposer硬件发送VSync事件的回调函数，首先，在SurfaceFlinger的readyToRun函数中，新建了一个HWComposer对象：

[cpp] view plain copy
// Initialize the H/W composer object. There may or may not be an
// actual hardware composer underneath.
mHwc = new HWComposer(this,
*static_cast(this));

接着在HWComposer的构造函数中，注册了hwcomposer回报VSync事件时调用的回调函数：

[cpp] view plain copy
HWComposer::HWComposer(
const sp& flinger,
EventHandler& handler)
: mFlinger(flinger),
mFbDev(0), mHwc(0), mNumDisplays(1),
mCBContext(new cb_context),
mEventHandler(handler),
mVSyncCount(0), 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);

// the number of displays we actually have depends on the
// hw composer version
if (hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_EXP)) {
// 1.?? adds support for virtual displays
mNumDisplays = MAX_DISPLAYS;
} else if (hwcHasApiVersion(mHwc, HWC_DEVICE_API_VERSION_1_1)) {
// 1.1 adds support for multiple displays
mNumDisplays = NUM_PHYSICAL_DISPLAYS;
} else {
mNumDisplays = 1;
}
}

…

if (needVSyncThread) {
// we don't have VSYNC support, we need to fake it
mVSyncThread = new VSyncThread(*this);
}
}

支持一下感觉挺不错的