Android4.4深入浅出之SurfaceFlinger与Client通信框架（一）

时间： 2023-07-10 admin IT培训

SurfaceFlinger框架是基于Binder进程间通信机制搭建的，SF作为一个服务进程，用户程序想要跟它通信必然要经过Binder机制。首先说一下，用户要跟SF通信，那么SF必须出现在ServiceManager中，因为SF也是一个服务，所有的服务都由ServiceManager来进行统一管理。在系统启动的过程中，SF就在ServiceManager中注册好了，注册好之后，SF在后台中监视一些surface的变化从而做出处理。

而启动之后，用户程序想操作一些跟surface有关的动作，就必须和SF进行交互。而这种交互是基于Binder进程间通信机制的。下面是一张图简单说明了SF的基本框架：

大体结构是这样，有些地方有些小错误用图形不能很好的表达。用文字来说明一下：当应用程序需要请求SurfaceFlinger服务时，首先需要构造SurfaceComposerClient对象，通过SurfaceComposerClient对象就可以访问SurfaceFlinger服务了。基本的服务流程都是这么走的，我们看一下SurfaceComposerClient的构造函数：

SurfaceComposerClient::SurfaceComposerClient(): mStatus(NO_INIT), mComposer(Composer::getInstance())
{
}

可想而知没做啥事啊，SurfaceComposerClient 继承于RefBase类，在创建SurfaceComposerClient对象，系统第一次引用SurfaceComposerClient对象时，onFirstRef函数自动调用，我们看一下它的onFirstRef

void SurfaceComposerClient::onFirstRef() {sp<ISurfaceComposer> sm(ComposerService::getComposerService());if (sm != 0) {sp<ISurfaceComposerClient> conn = sm->createConnection();if (conn != 0) {mClient = conn;mStatus = NO_ERROR;}}
}

上面代码，第二行的作用是获得SurfaceFlinger这个服务，我们跟踪一下：

sp<ISurfaceComposer> ComposerService::getComposerService() {ComposerService& instance = ComposerService::getInstance();Mutex::Autolock _l(instance.mLock);if (instance.mComposerService == NULL) {ComposerService::getInstance().connectLocked();assert(instance.mComposerService != NULL);ALOGD("ComposerService reconnected");}return instance.mComposerService;
}

第二行得到了ComposerService的对象，是为了下一步访问它的成员变量mComposerService（ISurfaceComposer类的），跟踪到第五行的函数 connectLocked（）：

void ComposerService::connectLocked() {const String16 name("SurfaceFlinger");while (getService(name, &mComposerService) != NO_ERROR) {usleep(250000);}assert(mComposerService != NULL);                                                                                                         //省略了一些东西。。。}

这里看到第三行getService（），第一个参数是“SurfaceFlinger”，第二个参数是mComposerService，记住它的类型的强引用的ISurfaceComposer，对就是这句得到了一个SurfaceFlinger服务的代理对象，看一下这个的函数的实体：

template<typename INTERFACE>
status_t getService(const String16& name, sp<INTERFACE>* outService)
{const sp<IServiceManager> sm = defaultServiceManager();if (sm != NULL) {*outService = interface_cast<INTERFACE>(sm->getService(name));if ((*outService) != NULL) return NO_ERROR;}return NAME_NOT_FOUND;
}

到第四行这里跟之前启动过程中SurfaceFlinger的创建一模一样了，最后outService会被强转化成BpSurfaceComposer类，BpSurfaceComposer就是SurfaceFlinger在客户端这边的Binder代理，与之对应的是BnSurfaceComposer相当于SurfaceFlinger这边与BpSurfaceComposer进行交互的一方。获得了代理之后，我们再回到之前的onFirstRef函数：

void SurfaceComposerClient::onFirstRef() {sp<ISurfaceComposer> sm(ComposerService::getComposerService());if (sm != 0) {sp<ISurfaceComposerClient> conn = sm->createConnection();if (conn != 0) {mClient = conn;mStatus = NO_ERROR;}}
}

这时候走到if，sm有值了，所以往下走，看名字可以得出这是客户端在请求连接到SurfaceFlinger服务。我们跟踪一下createConnection：

virtual sp<ISurfaceComposerClient> createConnection(){uint32_t n;Parcel data, reply;data.writeInterfaceToken(ISurfaceComposer::getInterfaceDescriptor());remote()->transact(BnSurfaceComposer::CREATE_CONNECTION, data, &reply);return interface_cast<ISurfaceComposerClient>(reply.readStrongBinder());}

函数的大概意思是把一些连接信息写到数据包data里，然后通过transact这个函数传出去，remote（）之前有研究过他是IBinder类的，这里重点看transact这个函数，他是IBinder类的成员函数故它的实现必定在BpBinder类里，我们跳到这个函数的实体：

status_t BpBinder::transact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{// Once a binder has died, it will never come back to life.if (mAlive) {status_t status = IPCThreadState::self()->transact(mHandle, code, data, reply, flags);if (status == DEAD_OBJECT) mAlive = 0;return status;}return DEAD_OBJECT;
}

发现他又调用了transact这个函数，不同的是是在IPCThreadState类中的我们继续跟踪：

status_t IPCThreadState::transact(int32_t handle,uint32_t code, const Parcel& data,Parcel* reply, uint32_t flags)
{status_t err = data.errorCheck();flags |= TF_ACCEPT_FDS;IF_LOG_TRANSACTIONS() {TextOutput::Bundle _b(alog);alog << "BC_TRANSACTION thr " << (void*)pthread_self() << " / hand "<< handle << " / code " << TypeCode(code) << ": "<< indent << data << dedent << endl;}if (err == NO_ERROR) {LOG_ONEWAY(">>>> SEND from pid %d uid %d %s", getpid(), getuid(),(flags & TF_ONE_WAY) == 0 ? "READ REPLY" : "ONE WAY");err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);}if (err != NO_ERROR) {if (reply) reply->setError(err);return (mLastError = err);}if ((flags & TF_ONE_WAY) == 0) {#if 0if (code == 4) { // relayoutALOGI(">>>>>> CALLING transaction 4");} else {ALOGI(">>>>>> CALLING transaction %d", code);}#endifif (reply) {err = waitForResponse(reply);} else {Parcel fakeReply;err = waitForResponse(&fakeReply);}#if 0if (code == 4) { // relayoutALOGI("<<<<<< RETURNING transaction 4");} else {ALOGI("<<<<<< RETURNING transaction %d", code);}#endifIF_LOG_TRANSACTIONS() {TextOutput::Bundle _b(alog);alog << "BR_REPLY thr " << (void*)pthread_self() << " / hand "<< handle << ": ";if (reply) alog << indent << *reply << dedent << endl;else alog << "(none requested)" << endl;}} else {err = waitForResponse(NULL, NULL);}return err;
}

这里已经扯到Binder机制的基本原理了，简单说就是客户端的SF代理通过BpBinder跟/dev/binder设备进行交互，向binder里写东西，函数err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);就是实现了这个功能，写完后他又等待服务端（SurfaceFlinger）那端的回应，err = waitForResponse(reply);当然服务端那边同样有人跟他进行交互。到这里，如果服务端收到消息并且返回了一个消息给客户端，这说明客户端请求连接成功。

请求成功后conn就不为0了，回到onFirstRef，成功后把coon赋给mClient。

既然客户端有请求了，那么服务端肯定有东西会去留意这个消息，回到更以前，做出向服务端请求连接这件事其实是SurfaceFlinger在客户端这边的代理BpSurfaceComposer完成的，那么之前说过，与之对应的就是BnSurfaceComposer。我们看一下他类的定义：

class BnSurfaceComposer: public BnInterface<ISurfaceComposer> {
public:enum {// Note: BOOT_FINISHED must remain this value, it is called from// Java by ActivityManagerService.BOOT_FINISHED = IBinder::FIRST_CALL_TRANSACTION,CREATE_CONNECTION,CREATE_GRAPHIC_BUFFER_ALLOC,CREATE_DISPLAY_EVENT_CONNECTION,CREATE_DISPLAY,DESTROY_DISPLAY,GET_BUILT_IN_DISPLAY,SET_TRANSACTION_STATE,AUTHENTICATE_SURFACE,BLANK,UNBLANK,GET_DISPLAY_INFO,CONNECT_DISPLAY,CAPTURE_SCREEN,};virtual status_t onTransact(uint32_t code, const Parcel& data,Parcel* reply, uint32_t flags = 0);｝

发现他没有构造函数只有一个成员函数 onTransact。跟踪一下：

status_t BnSurfaceComposer::onTransact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{switch(code) {case CREATE_CONNECTION: {CHECK_INTERFACE(ISurfaceComposer, data, reply);sp<IBinder> b = createConnection()->asBinder();reply->writeStrongBinder(b);return NO_ERROR;}case CREATE_GRAPHIC_BUFFER_ALLOC: {CHECK_INTERFACE(ISurfaceComposer, data, reply);sp<IBinder> b = createGraphicBufferAlloc()->asBinder();reply->writeStrongBinder(b);return NO_ERROR;}case SET_TRANSACTION_STATE: {CHECK_INTERFACE(ISurfaceComposer, data, reply);size_t count = data.readInt32();ComposerState s;Vector<ComposerState> state;state.setCapacity(count);for (size_t i=0 ; i<count ; i++) {s.read(data);state.add(s);}count = data.readInt32();DisplayState d;Vector<DisplayState> displays;displays.setCapacity(count);for (size_t i=0 ; i<count ; i++) {d.read(data);displays.add(d);}uint32_t flags = data.readInt32();setTransactionState(state, displays, flags);return NO_ERROR;}case BOOT_FINISHED: {CHECK_INTERFACE(ISurfaceComposer, data, reply);bootFinished();return NO_ERROR;}case CAPTURE_SCREEN: {CHECK_INTERFACE(ISurfaceComposer, data, reply);sp<IBinder> display = data.readStrongBinder();sp<IGraphicBufferProducer> producer =interface_cast<IGraphicBufferProducer>(data.readStrongBinder());uint32_t reqWidth = data.readInt32();uint32_t reqHeight = data.readInt32();uint32_t minLayerZ = data.readInt32();uint32_t maxLayerZ = data.readInt32();status_t res = captureScreen(display, producer,reqWidth, reqHeight, minLayerZ, maxLayerZ);reply->writeInt32(res);return NO_ERROR;}case AUTHENTICATE_SURFACE: {CHECK_INTERFACE(ISurfaceComposer, data, reply);sp<IGraphicBufferProducer> bufferProducer =interface_cast<IGraphicBufferProducer>(data.readStrongBinder());int32_t result = authenticateSurfaceTexture(bufferProducer) ? 1 : 0;reply->writeInt32(result);return NO_ERROR;}case CREATE_DISPLAY_EVENT_CONNECTION: {CHECK_INTERFACE(ISurfaceComposer, data, reply);sp<IDisplayEventConnection> connection(createDisplayEventConnection());reply->writeStrongBinder(connection->asBinder());return NO_ERROR;}case CREATE_DISPLAY: {CHECK_INTERFACE(ISurfaceComposer, data, reply);String8 displayName = data.readString8();bool secure = bool(data.readInt32());sp<IBinder> display(createDisplay(displayName, secure));reply->writeStrongBinder(display);return NO_ERROR;}case DESTROY_DISPLAY: {CHECK_INTERFACE(ISurfaceComposer, data, reply);sp<IBinder> display = data.readStrongBinder();destroyDisplay(display);return NO_ERROR;}case GET_BUILT_IN_DISPLAY: {CHECK_INTERFACE(ISurfaceComposer, data, reply);int32_t id = data.readInt32();sp<IBinder> display(getBuiltInDisplay(id));reply->writeStrongBinder(display);return NO_ERROR;}case BLANK: {CHECK_INTERFACE(ISurfaceComposer, data, reply);sp<IBinder> display = data.readStrongBinder();blank(display);return NO_ERROR;}case UNBLANK: {CHECK_INTERFACE(ISurfaceComposer, data, reply);sp<IBinder> display = data.readStrongBinder();unblank(display);return NO_ERROR;}case GET_DISPLAY_INFO: {CHECK_INTERFACE(ISurfaceComposer, data, reply);DisplayInfo info;sp<IBinder> display = data.readStrongBinder();status_t result = getDisplayInfo(display, &info);memcpy(reply->writeInplace(sizeof(DisplayInfo)), &info, sizeof(DisplayInfo));reply->writeInt32(result);return NO_ERROR;}default: {return BBinder::onTransact(code, data, reply, flags);}}// should be unreachablereturn NO_ERROR;
}

代码虽然有点长，但是整体框架就是一个swith语句，看到第一个分支，马上就能看出来这是一个对请求连接信号的处理，这里大概能知道这个onTransact的功能就是判断收到的消息即参数code是哪种类型从而做出相应的处理。在server测，一开始创建SF服务的时候，在线城池创建的时候就已经开启线程去监听binder设备了我们来跟踪下：

从最开始SF服务启动的源文件开始：

int main(int argc, char** argv) {// When SF is launched in its own process, limit the number of// binder threads to 4.ProcessState::self()->setThreadPoolMaxThreadCount(4);// start the thread poolsp<ProcessState> ps(ProcessState::self());ps->startThreadPool();

是一个片段，最后一句就是开始线程池。跟踪其代码：

void ProcessState::startThreadPool()
{AutoMutex _l(mLock);if (!mThreadPoolStarted) {mThreadPoolStarted = true;spawnPooledThread(true);}
}

核心也在最后一句，

void ProcessState::spawnPooledThread(bool isMain)
{if (mThreadPoolStarted) {String8 name = makeBinderThreadName();ALOGV("Spawning new pooled thread, name=%s\n", name.string());sp<Thread> t = new PoolThread(isMain);t->run(name.string());}
}

到这里，可以看到实质的线程创建和开始运行。接下来我用一张图来表示他的函数调用的走向：

这里一旦检测到有连接请求消息就会跳到executeCommand的switch分支BR_TRANSACTION，去执行BBinder的transact，transact函数会调用onTransact函数，这个onTransact由其子类实现，在这里由BnSurfaceComposer完成，即调用的是BnSurfaceComposer::onTransact（），函数的具体内容在上面已贴出，就是处理不同的请求。

到此为止，客户端已经与SF进行连接，也就是交手过了，那么自然连接了，一定要做些事情比如客户端请求渲染一个surface，或者等等其他的。接下去的工作就是SurfaceFlinger的事，对客户端不同的请求而进行不同的处理，这才是SF核心工作所在。当然所有的通信机制都是基于binder机制，而求也有负责这个方面的客户端这边的binder代理BpSurfaceComposerClient和服务端这边的本地对象 BnSurfaceComposerClient，而这里有点不一样的是BnSurfaceComposerClient派生出Client所以事情都交与Client做了。