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

SurfaceFlinger有着自己的消息隊列MessageQueue，用來處理顯示相關的消息，比如Vsync消息。

相關文件：

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

類MessageQueue和Handler定義在MessageQueue.h文件中，代碼如下：

class MessageQueue {
    // 類Handler定義也在這裏
    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線程循環體，主要用於取消息
• mHander消息處理類
• mEventThread、mEvents、mEventTube主要與Vsync消息的分發有關

SurfaceFlinger類中有一個MessageQueue類型的成員變量，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三個成員變量

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);
}

• 設置mEventThread、mEvents、mEventTube，並且添加文件句柄到mLooper

消息的監聽主要在SurfaceFlinger的run函數裏

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

• 死循環一直監聽消息
• 調用waitForEvent進行等待消息事件

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

• 調用MessageQueue的waitMessage等待消息

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方法來監聽消息