Android SurfaceFlinger服务的消息循环过程源码分析

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【Android 显示模块】 2013-11-09 14:19
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EventThreadMessageQueueSurfaceFlingerSocketVSync

目录(?)[+]

SurfaceFlinger消息循环过程
DisplayHardware对象创建过程
EventThread线程启动过程
事件连接创建过程
事件连接Connection注册过程
事件连接Socket接收端注册过程

在Android
SurfaceFlinger服务启动过程源码分析中详细分析了SurfaceFlinger的整个启动过程，我们知道，在SurfaceFlinger线程执行过程中，循环调用waitForEvent()函数等待消息事件的到来

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bool SurfaceFlinger::threadLoop() { waitForEvent(); return true; }

bool SurfaceFlinger::threadLoop()
{
waitForEvent();
return true;
}

而waitForEvent函数则是调用SurfaceFlinger中的消息队列的waitMessage()函数来等待消息事件

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void SurfaceFlinger::waitForEvent() { mEventQueue.waitMessage(); }

void SurfaceFlinger::waitForEvent() {
mEventQueue.waitMessage();
}

通过跟踪代码发现，MessageQueue的waitMessage()函数则是调用Looper对象的pollOnce()函数来等待消息事件的

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void MessageQueue::waitMessage() {
do {
IPCThreadState::self()->flushCommands();
int32_t ret = mLooper->pollOnce(-1);
switch (ret) {
case ALOOPER_POLL_WAKE:
case ALOOPER_POLL_CALLBACK:
continue;
case ALOOPER_POLL_ERROR:
ALOGE("ALOOPER_POLL_ERROR");
case ALOOPER_POLL_TIMEOUT:
// timeout (should not happen)

continue;
default:
// should not happen

ALOGE("Looper::pollOnce() returned unknown status %d", ret);
continue;
}
} while (true);
}

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

该函数无限循环执行，关于Looper的pollOnce()函数在Android应用程序消息循环源码分析有详细的分析过程。本文就针对SurfaceFlinger的消息队列MessageQueue展开讨论，分析SurfaceFlinger的消息队列创建过程。由于在Android
SurfaceFlinger服务启动过程源码分析中对SurfaceFlinger的启动过程作了详细的介绍，这里只给出一些关键代码。

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void SurfaceFlinger::onFirstRef()
{
//消息队列初始化
mEventQueue.init(this);
//启动SurfaceFlinger线程

run("SurfaceFlinger", PRIORITY_URGENT_DISPLAY);
mReadyToRunBarrier.wait();
}

void SurfaceFlinger::onFirstRef()
{
//消息队列初始化
mEventQueue.init(this);
//启动SurfaceFlinger线程
run("SurfaceFlinger", PRIORITY_URGENT_DISPLAY);
mReadyToRunBarrier.wait();
}

SurfaceFlinger消息循环过程

在SurfaceFlinger内部定义了类型为MessageQueue的消息队列。

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void MessageQueue::init(const sp<SurfaceFlinger>& flinger) { mFlinger = flinger; mLooper = new Looper(true); mHandler = new Handler(*this); }

void MessageQueue::init(const sp<SurfaceFlinger>& flinger)
{
mFlinger = flinger;
mLooper = new Looper(true);
mHandler = new Handler(*this);
}

在初始化消息队列MessageQueue时，为该消息队列创建了一个Looper对象和一个Handler对象。SurfaceFlinger，MessageQueue，Looper，Handler对象之间的关系如下：



在SurfaceFlinger的消息队列MessageQueue的初始化函数中，该该消息队列创建了一个Handler对象，一个Looper对象，Android应用程序消息循环源码分析中分析了Handler，Looper类的相关知识，不过前面介绍的Handler对象是Java层的Handler对象，这里定义的Handler是C++层面的，Looper对象也是C++层的，不过在Android应用程序消息循环源码分析文中对Looper对象有详细的分析，这里就不在重复介绍，重点介绍C++层定义的Handler的实现过程及使用方法。既然SurfaceFlinger的消息队列拥有一个Looper对象，这就说明SurfaceFlinger线程是一个消息循环线程。那么SurfaceFlinger是如何处理消息的呢？

在前面的Android应用程序创建Surface过程源码分析中介绍了应用程序通过调用SurfaceFlinger为应用程序分配的Client对象的Binder远程代理对象的createSurface函数来创建一个Surface：

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sp<ISurface> Client::createSurface( ISurfaceComposerClient::surface_data_t* params, const String8& name, DisplayID display, uint32_t w, uint32_t h, PixelFormat format, uint32_t flags) { sp<MessageBase> msg = new MessageCreateSurface(mFlinger.get(),params, name, this, display, w, h, format, flags); mFlinger->postMessageSync(msg); return static_cast<MessageCreateSurface*>( msg.get() )->getResult(); }

sp<ISurface> Client::createSurface(
ISurfaceComposerClient::surface_data_t* params,
const String8& name,
DisplayID display, uint32_t w, uint32_t h, PixelFormat format,
uint32_t flags)
{
sp<MessageBase> msg = new MessageCreateSurface(mFlinger.get(),params, name, this, display, w, h, format, flags);
mFlinger->postMessageSync(msg);
return static_cast<MessageCreateSurface*>( msg.get() )->getResult();
}

函数首先将请求创建的Surface参数封装为MessageCreateSurface对象，然后调用SurfaceFlinger的postMessageSync函数往SurfaceFlinger的消息队列中发送一个同步消息，当消息处理完后，通过调用消息msg的getResult()函数来得到创建的Surface。

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status_t SurfaceFlinger::postMessageSync(const sp<MessageBase>& msg,
nsecs_t reltime, uint32_t flags) {
//往消息队列中发送一个消息
status_t res = mEventQueue.postMessage(msg, reltime);
//消息发送成功后，当前线程等待消息处理

if (res == NO_ERROR) {
msg->wait();
}
return res;
}

status_t SurfaceFlinger::postMessageSync(const sp<MessageBase>& msg,
nsecs_t reltime, uint32_t flags) {
//往消息队列中发送一个消息
status_t res = mEventQueue.postMessage(msg, reltime);
//消息发送成功后，当前线程等待消息处理
if (res == NO_ERROR) {
msg->wait();
}
return res;
}

其实这里发送的并不是消息对象，而是一个Handler对象，MessageCreateSurface的继承关系如下所示：



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status_t MessageQueue::postMessage(const sp<MessageBase>& messageHandler, nsecs_t relTime)
{
const Message dummyMessage;
//将messageHandler对象和dummyMessage消息对象发送到消息循环Looper对象中

if (relTime > 0) {
mLooper->sendMessageDelayed(relTime, messageHandler, dummyMessage);
} else {
mLooper->sendMessage(messageHandler, dummyMessage);
}
return NO_ERROR;
}

status_t MessageQueue::postMessage(const sp<MessageBase>& messageHandler, nsecs_t relTime)
{
const Message dummyMessage;
//将messageHandler对象和dummyMessage消息对象发送到消息循环Looper对象中
if (relTime > 0) {
mLooper->sendMessageDelayed(relTime, messageHandler, dummyMessage);
} else {
mLooper->sendMessage(messageHandler, dummyMessage);
}
return NO_ERROR;
}

关于消息循环Looper对象的消息发送函数sendMessage的调用流程在Android应用程序消息循环源码分析已经有详细的分析。这里再次贴上关于消息插入代码：

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void Looper::sendMessageAtTime(nsecs_t uptime, const sp<MessageHandler>& handler,
const Message& message) {
size_t i = 0;
{ // acquire lock
AutoMutex _l(mLock);
//获取消息队列中保存的消息个数
size_t messageCount = mMessageEnvelopes.size();
//按时间排序，查找当前消息应该插入的位置
while (i < messageCount && uptime >= mMessageEnvelopes.itemAt(i).uptime) {
i += 1;
}
//将Message消息及消息处理Handler封装为MessageEnvelope对象，并插入到消息队列mMessageEnvelopes中

MessageEnvelope messageEnvelope(uptime, handler, message);
mMessageEnvelopes.insertAt(messageEnvelope, i, 1);
if (mSendingMessage) {
return;
}
} // release lock
//唤醒消息循环线程以及时处理消息
if (i == 0) {
wake();
}
}

void Looper::sendMessageAtTime(nsecs_t uptime, const sp<MessageHandler>& handler,
const Message& message) {
size_t i = 0;
{ // acquire lock
AutoMutex _l(mLock);
//获取消息队列中保存的消息个数
size_t messageCount = mMessageEnvelopes.size();
//按时间排序，查找当前消息应该插入的位置
while (i < messageCount && uptime >= mMessageEnvelopes.itemAt(i).uptime) {
i += 1;
}
//将Message消息及消息处理Handler封装为MessageEnvelope对象，并插入到消息队列mMessageEnvelopes中
MessageEnvelope messageEnvelope(uptime, handler, message);
mMessageEnvelopes.insertAt(messageEnvelope, i, 1);
if (mSendingMessage) {
return;
}
} // release lock
//唤醒消息循环线程以及时处理消息
if (i == 0) {
wake();
}
}

到此消息发送就完成了，由于发送的是一个同步消息，因此消息发送线程此刻进入睡眠等待状态，而消息循环线程被唤醒起来处理消息，消息处理过程如下：

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//所有C++层的消息都封装为MessageEnvelope类型的变量并保存到mMessageEnvelopes链表中

while (mMessageEnvelopes.size() != 0) {
nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(0);
//处理当前时刻之前的所有消息
if (messageEnvelope.uptime <= now) {
{
//取出处理该消息的Hanlder
sp<MessageHandler> handler = messageEnvelope.handler;
//取出该消息描述符
Message message = messageEnvelope.message;
//从mMessageEnvelopes链表中移除该消息

mMessageEnvelopes.removeAt(0);
//表示当前消息循环线程正在处理消息，处于唤醒状态，因此消息发送线程无需唤醒消息循环线程

mSendingMessage = true;
mLock.unlock();
//调用该消息Handler对象的handleMessage函数来处理该消息

handler->handleMessage(message);
} // release handler

mLock.lock();
mSendingMessage = false;
result = ALOOPER_POLL_CALLBACK;
} else {
// The last message left at the head of the queue determines the next wakeup time.

mNextMessageUptime = messageEnvelope.uptime;
break;
}
}

//所有C++层的消息都封装为MessageEnvelope类型的变量并保存到mMessageEnvelopes链表中
while (mMessageEnvelopes.size() != 0) {
nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
const MessageEnvelope& messageEnvelope = mMessageEnvelopes.itemAt(0);
//处理当前时刻之前的所有消息
if (messageEnvelope.uptime <= now) {
{
//取出处理该消息的Hanlder
sp<MessageHandler> handler = messageEnvelope.handler;
//取出该消息描述符
Message message = messageEnvelope.message;
//从mMessageEnvelopes链表中移除该消息
mMessageEnvelopes.removeAt(0);
//表示当前消息循环线程正在处理消息，处于唤醒状态，因此消息发送线程无需唤醒消息循环线程
mSendingMessage = true;
mLock.unlock();
//调用该消息Handler对象的handleMessage函数来处理该消息
handler->handleMessage(message);
} // release handler
mLock.lock();
mSendingMessage = false;
result = ALOOPER_POLL_CALLBACK;
} else {
// The last message left at the head of the queue determines the next wakeup time.
mNextMessageUptime = messageEnvelope.uptime;
break;
}
}

消息处理过程就是调用该消息的Handler对象的handleMessage函数来完成，由于创建Surface时，往消息队列中发送的Handler对象类型为MessageCreateSurface，因此必定会调用该类的handleMessage函数来处理Surface创建消息。但该类并未实现

handleMessage函数，同时该类继承于MessageBase，由此可见其父类MessageBase必定实现了handleMessage函数：

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void MessageBase::handleMessage(const Message&) { this->handler(); barrier.open(); };

void MessageBase::handleMessage(const Message&) {
this->handler();
barrier.open();
};

该函数首先调用其子类的handler()函数处理消息，然后唤醒消息发送线程，表明发往消息队列中的消息已得到处理，消息发送线程可以往下执行了。由于MessageCreateSurface是MessageBase的子类，因此该类必定实现了handler()函数来处理Surface创建消息：

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class MessageCreateSurface : public MessageBase { sp<ISurface> result; SurfaceFlinger* flinger; ISurfaceComposerClient::surface_data_t* params; Client* client; const String8& name; DisplayID display; uint32_t w, h; PixelFormat format; uint32_t flags; public: MessageCreateSurface(SurfaceFlinger* flinger, ISurfaceComposerClient::surface_data_t* params, const String8& name, Client* client, DisplayID display, uint32_t w, uint32_t h, PixelFormat format, uint32_t flags) : flinger(flinger), params(params), client(client), name(name), display(display), w(w), h(h), format(format), flags(flags) { } sp<ISurface> getResult() const { return result; } virtual bool handler() { result = flinger->createSurface(params, name, client,display, w, h, format, flags); return true; } };

class MessageCreateSurface : public MessageBase {
sp<ISurface> result;
SurfaceFlinger* flinger;
ISurfaceComposerClient::surface_data_t* params;
Client* client;
const String8& name;
DisplayID display;
uint32_t w, h;
PixelFormat format;
uint32_t flags;
public:
MessageCreateSurface(SurfaceFlinger* flinger,
ISurfaceComposerClient::surface_data_t* params,
const String8& name, Client* client,
DisplayID display, uint32_t w, uint32_t h, PixelFormat format,
uint32_t flags)
: flinger(flinger), params(params), client(client), name(name),
display(display), w(w), h(h), format(format), flags(flags)
{
}
sp<ISurface> getResult() const { return result; }

virtual bool handler() {
result = flinger->createSurface(params, name, client,display, w, h, format, flags);
return true;
}
};

这里又调用SurfaceFlinger的createSurface函数来创建Surface，Surface的创建调用过程如下：



绕了一圈又回到SurfaceFlinger，为什么要这么做呢？因为在同一时刻可以有多个应用程序请求SurfaceFlinger为其创建Surface，通过消息队列可以实现请求排队，然后SurfaceFlinger依次为应用程序创建Surface。

DisplayHardware对象创建过程

SurfaceFlinger线程启动后，首先会调用readyToRun()函数来完成一些线程运行前的准备工作

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status_t SurfaceFlinger::readyToRun()
{
// initialize the main display

GraphicPlane& plane(graphicPlane(dpy));
//创建DisplayHardware对象

DisplayHardware* const hw = new DisplayHardware(this, dpy);
plane.setDisplayHardware(hw);
...
//创建EventThread线程对象

mEventThread = new EventThread(this);
mEventQueue.setEventThread(mEventThread);
hw.startSleepManagement();

mReadyToRunBarrier.open();
return NO_ERROR;
}

status_t SurfaceFlinger::readyToRun()
{
// initialize the main display
GraphicPlane& plane(graphicPlane(dpy));
//创建DisplayHardware对象
DisplayHardware* const hw = new DisplayHardware(this, dpy);
plane.setDisplayHardware(hw);
...
//创建EventThread线程对象
mEventThread = new EventThread(this);
mEventQueue.setEventThread(mEventThread);
hw.startSleepManagement();

mReadyToRunBarrier.open();
return NO_ERROR;
}

在该函数中创建了两个对象：DisplayHardware对象和EventThread对象，同时将EventThread对象设置到消息队列MessageQueue中。MessageQueue消息队列在前面已经介绍了，它是在SurfaceFlinger线程启动前被创建的。

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DisplayHardware::DisplayHardware( const sp<SurfaceFlinger>& flinger, uint32_t dpy) : DisplayHardwareBase(flinger, dpy), mFlinger(flinger), mFlags(0), mHwc(0) { init(dpy); }

DisplayHardware::DisplayHardware(
const sp<SurfaceFlinger>& flinger,
uint32_t dpy)
: DisplayHardwareBase(flinger, dpy),
mFlinger(flinger), mFlags(0), mHwc(0)
{
init(dpy);
}

构造DisplayHardware对象前首先会调用其父类DisplayHardwareBase的构造函数

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DisplayHardwareBase::DisplayHardwareBase(const sp<SurfaceFlinger>& flinger, uint32_t displayIndex) { mScreenAcquired = true; mDisplayEventThread = new DisplayEventThread(flinger); }

DisplayHardwareBase::DisplayHardwareBase(const sp<SurfaceFlinger>& flinger,
uint32_t displayIndex)
{
mScreenAcquired = true;
mDisplayEventThread = new DisplayEventThread(flinger);
}

这里创建了一个DisplayEventThread线程对象，关于DisplayEventThread线程的执行过程，在后面分析。在DisplayHardware构造函数中，调用的它的init函数来初始化DisplayHardware对象

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void DisplayHardware::init(uint32_t dpy)
{
mNativeWindow = new FramebufferNativeWindow();
framebuffer_device_t const * fbDev = mNativeWindow->getDevice();
if (!fbDev) {
ALOGE("Display subsystem failed to initialize. check logs. exiting...");
exit(0);
}
...
// initialize the H/W composer

mHwc = new HWComposer(mFlinger, *this, mRefreshPeriod);
if (mHwc->initCheck() == NO_ERROR) {
mHwc->setFrameBuffer(mDisplay, mSurface);
}
}

void DisplayHardware::init(uint32_t dpy)
{
mNativeWindow = new FramebufferNativeWindow();
framebuffer_device_t const * fbDev = mNativeWindow->getDevice();
if (!fbDev) {
ALOGE("Display subsystem failed to initialize. check logs. exiting...");
exit(0);
}
...
// initialize the H/W composer
mHwc = new HWComposer(mFlinger, *this, mRefreshPeriod);
if (mHwc->initCheck() == NO_ERROR) {
mHwc->setFrameBuffer(mDisplay, mSurface);
}
}

在这里仍然创建了两个对象，分别是FramebufferNativeWindow对象和HWComposer对象，同时将DisplayHardware对象保存到HWComposer的成员变量mEventHandler，接下来分析一下HWComposer对象的创建过程：

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HWComposer::HWComposer(
const sp<SurfaceFlinger>& flinger,
EventHandler& handler,
nsecs_t refreshPeriod)
: mFlinger(flinger),
mModule(0), mHwc(0), mList(0), mCapacity(0),
mNumOVLayers(0), mNumFBLayers(0),
mDpy(EGL_NO_DISPLAY), mSur(EGL_NO_SURFACE),
mEventHandler(handler),
mRefreshPeriod(refreshPeriod),
mVSyncCount(0), mDebugForceFakeVSync(false)
{
char value[PROPERTY_VALUE_MAX];
property_get("debug.sf.no_hw_vsync", value, "0");
//....
if (needVSyncThread) {
// we don't have VSYNC support, we need to fake it

mVSyncThread = new VSyncThread(*this);
}
}

HWComposer::HWComposer(
const sp<SurfaceFlinger>& flinger,
EventHandler& handler,
nsecs_t refreshPeriod)
: mFlinger(flinger),
mModule(0), mHwc(0), mList(0), mCapacity(0),
mNumOVLayers(0), mNumFBLayers(0),
mDpy(EGL_NO_DISPLAY), mSur(EGL_NO_SURFACE),
mEventHandler(handler),
mRefreshPeriod(refreshPeriod),
mVSyncCount(0), mDebugForceFakeVSync(false)
{
char value[PROPERTY_VALUE_MAX];
property_get("debug.sf.no_hw_vsync", value, "0");
//....
if (needVSyncThread) {
// we don't have VSYNC support, we need to fake it
mVSyncThread = new VSyncThread(*this);
}
}

这里首先加载HWComposer硬件抽象模块，HWComposer硬件模块用于产生VSync信号，如果加载失败，说明需要使用软件来模拟产生VSync信号。当变量needVSyncThread为true时，就创建一个VSyncThread线程来模拟产生VSync信号。到处总结一下DisplayHardware对象构造过程中所创建的对象：
1.DisplayHardware父类DisplayHardwareBase构造时创建了一个DisplayEventThread线程对象；
2.初始化DisplayHardware对象时创建了一个FramebufferNativeWindow对象；
3.初始化DisplayHardware对象时同时创建了一个HWComposer对象；
4.在HWComposer对象中创建了一个VSyncThread线程对象；

EventThread线程启动过程

EventThread对象构造过程：

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EventThread::EventThread(const sp<SurfaceFlinger>& flinger) : mFlinger(flinger), mHw(flinger->graphicPlane(0).editDisplayHardware()), mLastVSyncTimestamp(0), mVSyncTimestamp(0), mUseSoftwareVSync(false), mDeliveredEvents(0), mDebugVsyncEnabled(false) { }

EventThread::EventThread(const sp<SurfaceFlinger>& flinger)
: mFlinger(flinger),
mHw(flinger->graphicPlane(0).editDisplayHardware()),
mLastVSyncTimestamp(0),
mVSyncTimestamp(0),
mUseSoftwareVSync(false),
mDeliveredEvents(0),
mDebugVsyncEnabled(false)
{
}

由于EventThread继承RefBase类，因此在第一次强引用该对象时，会自动调用其onFirstRef()函数

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void EventThread::onFirstRef() {
mHw.setVSyncHandler(this);
run("EventThread", PRIORITY_URGENT_DISPLAY + PRIORITY_MORE_FAVORABLE);
}

void EventThread::onFirstRef() {
mHw.setVSyncHandler(this);
run("EventThread", PRIORITY_URGENT_DISPLAY + PRIORITY_MORE_FAVORABLE);
}

变量mHw在构造EventThread对象时被赋值为flinger->graphicPlane(0).editDisplayHardware()，在上一小节中创建的DisplayHardware对象通过plane.setDisplayHardware(hw)被保存到GraphicPlane的成员变量mHw中，函数graphicPlane(0)用于获取第0个显示屏GraphicPlane对象，然后调用GraphicPlane的editDisplayHardware()函数返回其成员变量mHw保存的对象指针，因此EventThread的成员变量mHw也保存了上一小节创建的DisplayHardware对象指针。mHw.setVSyncHandler(this)语句是将当前EventThread对象保存到DisplayHardware的成员变量mVSyncHandler中：

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void DisplayHardware::setVSyncHandler(const sp<VSyncHandler>& handler) { Mutex::Autolock _l(mLock); mVSyncHandler = handler; }

void DisplayHardware::setVSyncHandler(const sp<VSyncHandler>& handler) {
Mutex::Autolock _l(mLock);
mVSyncHandler = handler;
}

DisplayHardware的成员变量mVSyncHandler的类型为VSyncHandler，而EventThread是VSyncHandler的子类



然后启动EventThread线程。到此SurfaceFlinger所需的几个重要对象就基本创建完成了，下图显示了SurfaceFlinger启动过程创建了重要对象：

事件连接创建过程

SurfaceFlinger创建并启动完EventThread线程后，还会将该EventThread线程对象保存到SurfaceFlinger的消息队列mEventQueue中：

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mEventQueue.setEventThread(mEventThread);

mEventQueue.setEventThread(mEventThread);

setEventThread()函数的执行过程如下：

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void MessageQueue::setEventThread(const sp<EventThread>& eventThread)
{
mEventThread = eventThread;
mEvents = eventThread->createEventConnection();
mEventTube = mEvents->getDataChannel();
//将socket接收端添加到looper中监听，并且设置了事件处理回调函数cb_eventReceiver

mLooper->addFd(mEventTube->getFd(), 0, ALOOPER_EVENT_INPUT,MessageQueue::cb_eventReceiver, this);
}

void MessageQueue::setEventThread(const sp<EventThread>& eventThread)
{
mEventThread = eventThread;
mEvents = eventThread->createEventConnection();
mEventTube = mEvents->getDataChannel();
//将socket接收端添加到looper中监听，并且设置了事件处理回调函数cb_eventReceiver
mLooper->addFd(mEventTube->getFd(), 0, ALOOPER_EVENT_INPUT,MessageQueue::cb_eventReceiver, this);
}

函数首先将创建的EventThread对象保存到消息队列MessageQueue的成员变量mEventThread中，然后调用EventThread对象的createEventConnection()函数创建一个事件连接Connection，Connection类是基于Binder进程间通信框架设计的，因此该对象可以实现跨进程传输，函数调用，其在Binder框架下的类关系图如下：



EventThread类的createEventConnection()函数用于创建一个Connection对象:

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sp<EventThread::Connection> EventThread::createEventConnection() const { return new Connection(const_cast<EventThread*>(this)); }

sp<EventThread::Connection> EventThread::createEventConnection() const {
return new Connection(const_cast<EventThread*>(this));
}

参数this指向已创建的EventThread对象

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EventThread::Connection::Connection( const sp<EventThread>& eventThread) : count(-1), mEventThread(eventThread), mChannel(new BitTube()) { }

EventThread::Connection::Connection(
const sp<EventThread>& eventThread)
: count(-1), mEventThread(eventThread), mChannel(new BitTube())
{
}

Connection构造过程中，将EventThread对象保存到Connection的成员变量mEventThread中，同时创建一个BitTube对象，并保存到Connection的成员变量mChannel中。BitTube对象的构造过程如下:

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BitTube::BitTube(): mSendFd(-1), mReceiveFd(-1) { int sockets[2]; //创建一对socket if (socketpair(AF_UNIX, SOCK_SEQPACKET, 0, sockets) == 0) { int size = SOCKET_BUFFER_SIZE; setsockopt(sockets[0], SOL_SOCKET, SO_SNDBUF, &size, sizeof(size)); setsockopt(sockets[0], SOL_SOCKET, SO_RCVBUF, &size, sizeof(size)); setsockopt(sockets[1], SOL_SOCKET, SO_SNDBUF, &size, sizeof(size)); setsockopt(sockets[1], SOL_SOCKET, SO_RCVBUF, &size, sizeof(size)); fcntl(sockets[0], F_SETFL, O_NONBLOCK); fcntl(sockets[1], F_SETFL, O_NONBLOCK); mReceiveFd = sockets[0]; mSendFd = sockets[1]; } else { mReceiveFd = -errno; ALOGE("BitTube: pipe creation failed (%s)", strerror(-mReceiveFd)); } }

BitTube::BitTube(): mSendFd(-1), mReceiveFd(-1)
{
int sockets[2];
//创建一对socket
if (socketpair(AF_UNIX, SOCK_SEQPACKET, 0, sockets) == 0) {
int size = SOCKET_BUFFER_SIZE;
setsockopt(sockets[0], SOL_SOCKET, SO_SNDBUF, &size, sizeof(size));
setsockopt(sockets[0], SOL_SOCKET, SO_RCVBUF, &size, sizeof(size));
setsockopt(sockets[1], SOL_SOCKET, SO_SNDBUF, &size, sizeof(size));
setsockopt(sockets[1], SOL_SOCKET, SO_RCVBUF, &size, sizeof(size));
fcntl(sockets[0], F_SETFL, O_NONBLOCK);
fcntl(sockets[1], F_SETFL, O_NONBLOCK);
mReceiveFd = sockets[0];
mSendFd = sockets[1];
} else {
mReceiveFd = -errno;
ALOGE("BitTube: pipe creation failed (%s)", strerror(-mReceiveFd));
}
}

socketpair函数创建了彼此相连的两个套接字。



Connection只是对进程间使用Socket通信的封装类。



EventThread类的createEventConnection()就是简单地创建一个Connection对象，在构造Connection对象时，创建了彼此相连的两个套接字，EventThread线程在接收到VSync信号后，将通过Connection中的套接字发送端向SurfaceFlinger的消息循环中发送一个事件，而Connection中的套接字接收端将注册到SurfaceFlinger的消息循环Looper中，以接收EventThread线程分发过来的VSync事件，上图形象地描述了EventThread线程和SurfaceFlinger的消息循环线程之间的通信过程。

事件连接Connection注册过程

由于Connection继承了RefBase类同时实现了onFirstRef()函数，因此在第一次强引用Connection对象时，onFirstRef()函数自动被调用：

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void EventThread::Connection::onFirstRef() {
// NOTE: mEventThread doesn't hold a strong reference on us

mEventThread->registerDisplayEventConnection(this);
}

void EventThread::Connection::onFirstRef() {
// NOTE: mEventThread doesn't hold a strong reference on us
mEventThread->registerDisplayEventConnection(this);
}

该函数调用EventThread的registerDisplayEventConnection()函数将当前创建的Connection对象注册到EventThread线程对象中，以便EventThread线程可以将VSync信号事件分发给所有注册的Connection连接的另一端。

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status_t EventThread::registerDisplayEventConnection(
const sp<EventThread::Connection>& connection) {
Mutex::Autolock _l(mLock);
//将创建的Connection对象添加到EventThread的成员变量mDisplayEventConnections向量中

mDisplayEventConnections.add(connection);
//如果EventThread线程正处于睡眠等待，则唤醒EventThread线程

mCondition.broadcast();
return NO_ERROR;
}

status_t EventThread::registerDisplayEventConnection(
const sp<EventThread::Connection>& connection) {
Mutex::Autolock _l(mLock);
//将创建的Connection对象添加到EventThread的成员变量mDisplayEventConnections向量中
mDisplayEventConnections.add(connection);
//如果EventThread线程正处于睡眠等待，则唤醒EventThread线程
mCondition.broadcast();
return NO_ERROR;
}

到此EventThread线程就可以向已注册的Connection发送事件消息了，通过向Connection中的Socket发送端描述符写入数据，在Socket的接收端描述符中就可以接收到EventThread写入的数据。

事件连接Socket接收端注册过程

虽然此时创建了一对Socket用于EventThread线程和SurfaceFlinger的消息队列之间通信，但是消息队列MessageQueue并没有监听Socket的接收端描述符，因此即使EventThread线程通过Socket的发送端发送数据，消息队列MessageQueue仍然无法接收到EventThread线程发送的数据。这时需要将Socket的接收端文件描述符添加到MessageQueue的文件描述符监控池中：

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mEventTube = mEvents->getDataChannel();
//将socket接收端添加到SurfaceFlinger的消息队列中监控

mLooper->addFd(mEventTube->getFd(), 0, ALOOPER_EVENT_INPUT,MessageQueue::cb_eventReceiver, this);

mEventTube = mEvents->getDataChannel();
//将socket接收端添加到SurfaceFlinger的消息队列中监控
mLooper->addFd(mEventTube->getFd(), 0, ALOOPER_EVENT_INPUT,MessageQueue::cb_eventReceiver, this);

变量mEventTube指向创建的事件连接Connection对象中的BitTube对象

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sp<BitTube> EventThread::Connection::getDataChannel() const {
return mChannel;//mChannel(new BitTube())

}

sp<BitTube> EventThread::Connection::getDataChannel() const {
return mChannel;//mChannel(new BitTube())
}

函数getFd()用于返回创建的Sockets对的接收端描述符

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int BitTube::getFd() const { return mReceiveFd; }

int BitTube::getFd() const
{
return mReceiveFd;
}

mLooper->addFd(mEventTube->getFd(), 0, ALOOPER_EVENT_INPUT,MessageQueue::cb_eventReceiver, this)就是将Socket接收端文件描述符添加到消息循环Looper对象中，并且监听该文件描述符下的ALOOPER_EVENT_INPUT事件，当该Socket接收端接收到消息事件后，调用回调函数MessageQueue::cb_eventReceiver来处理该事件。关于向消息循环中添加监控句柄过程在Android应用程序消息循环源码分析中已经详细分析了，将Connection中的Socket接收端文件描述符注册到SurfaceFlinger的消息循环Looper中后，SurfaceFlinger的消息队列MessageQueue就可以接收到EventThread线程分发的事件消息了。