Android Surfaceflinger Service (iv)-----messaging mechanism MessageQueue

Surfaceflinger has its own message queue MessageQueue, which is used to handle display-related messages, such as VSync messages.

Related documents:

• Frameworks/native/services/surfaceflinger/messagequeue.h
• Frameworks/native/services/surfaceflinger/messagequeue.cpp

The classes MessageQueue and handler are defined in the MessageQueue.h file with the following code:

Class MessageQueue {//Class Handler definition is also here class Handler:public MessageHandler {enum {Eventmaski Nvalidate = 0x1, Eventmaskrefresh = 0x2, eventmasktransaction =        0x4};                   messagequeue& Mqueue;    int32_t Meventmask;  Public:handler (messagequeue& queue): Mqueue (Queue), Meventmask (0) {} virtual void Handlemessage (const        message& message);        void Dispatchrefresh ();        void Dispatchinvalidate ();    void Dispatchtransaction ();    };    Friend class Handler;    Sp<surfaceflinger> Mflinger;    Sp<looper> Mlooper;    Sp<eventthread> Meventthread;    Sp<idisplayeventconnection> mevents;    Sp<bittube> Meventtube;    Sp

• Mlooper thread loop body, mainly used for fetching messages
• Mhander Message Processing class
• Meventthread, mevents, meventtube mainly related to the distribution of VSYNC messages

The Surfaceflinger class has a member variable of type MessageQueue, which is defined as follows:

    // these are thread safe    mutable MessageQueue mEventQueue;

• A meventqueue is constructed when the Surfaceflinger is constructed.

void SurfaceFlinger::onFirstRef(){    mEventQueue.init(this);}

• Call the init function of MessageQueue to initialize

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

• Initialize Mflinger, Mlooper, mhandler three member variables

void SurfaceFlinger::init() {        ......        mSFEventThread = new EventThread(sfVsyncSrc);    mEventQueue.setEventThread(mSFEventThread);        ......    }

• Call MessageQueue's Seteventthread function in Surfaceflinger's init function

void MessageQueue::setEventThread(const sp<EventThread>& eventThread){       mEventThread = eventThread;    mEvents = eventThread->createEventConnection();    mEventTube = mEvents->getDataChannel();    mLooper->addFd(mEventTube->getFd(), 0, Looper::EVENT_INPUT,            MessageQueue::cb_eventReceiver, this);}

• Set Meventthread, Mevents, Meventtube, and add file handles to Mlooper

The message is monitored primarily in the surfaceflinger run function.

void SurfaceFlinger::run() {    do {        waitForEvent();    } while (true);}

• The dead loop has been listening to messages
• Call Waitforevent to wait for a message event

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

• Call MessageQueue's WaitMessage wait message

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

• WaitMessage listens to messages primarily by calling Mlooper's Pollonce method

Android Surfaceflinger Service (iv)-----messaging mechanism MessageQueue