Android SurfaceFlinger服務（四） ----- 訊息機制MessageQueue

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SurfaceFlinger有著自己的訊息佇列MessageQueue，用來處理顯示相關的訊息，比如Vsync訊息。

相關檔案：

• frameworks/native/services/surfaceflinger/MessageQueue.h
• frameworks/native/services/surfaceflinger/MessageQueue.cpp

類MessageQueue和Handler定義在MessageQueue.h檔案中，代碼如下：

class MessageQueue {    // 類Handler定義也在這裡    class Handler : public MessageHandler {          enum {            eventMaskInvalidate     = 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<Handler> mHandler;    static int cb_eventReceiver(int fd, int events, void* data);    int eventReceiver(int fd, int events);   public:    enum {        INVALIDATE  = 0,        REFRESH     = 1,        TRANSACTION = 2    };    MessageQueue();    ~MessageQueue();    void init(const sp<SurfaceFlinger>& flinger);    void setEventThread(const sp<EventThread>& events);    void waitMessage();    status_t postMessage(const sp<MessageBase>& message, nsecs_t reltime=0);    // sends INVALIDATE message at next VSYNC    void invalidate();    // sends REFRESH message at next VSYNC    void refresh();    // sends TRANSACTION message immediately    void invalidateTransactionNow();};

• mLooper線程迴圈體，主要用於取訊息
• mHander訊息處理類
• mEventThread、mEvents、mEventTube主要與Vsync訊息的分發有關

SurfaceFlinger類中有一個MessageQueue類型的成員變數，定義如下：

    // these are thread safe    mutable MessageQueue mEventQueue;

• 因些在SurfaceFlinger被構造的時候就構造一個mEventQueue。

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

• 調用MessageQueue的init函數進行初始化

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

• 初始化mFlinger、mLooper、mHandler三個成員變數

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

• 在SurfaceFlinger的init函數中調用MessageQueue的setEventThread函數

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);}

• 設定mEventThread、mEvents、mEventTube，並且添加檔案控制代碼到mLooper

訊息的監聽主要在SurfaceFlinger的run函數裡

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

• 死迴圈一直監聽訊息
• 調用waitForEvent進行等待訊息事件

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

• 調用MessageQueue的waitMessage等待訊息

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主要通過調用mLooper的pollOnce方法來監聽訊息

Android SurfaceFlinger服務（四） ----- 訊息機制MessageQueue