Win32中的PollEvent（）

Question

I'm trying to mimic SFML's PollEvent(Event &event) function in Windows. It seems far more complicated that I imagined. Note that I already encapsulated the window procedure function in my class.

There could be many "window events" in my program - WindowMoved , WindowResized etc.

My first attempt was to have a private data member in the class, defined as WindowEvent *_lastWindowEvent . This variable will be set if PeekMessage() returns a non-zero value, just before DispatchMessage() is called. Then, winProc() will edit _lastWindowEvent , depending on the message it will receive. The drawback here is that I noticed that winProc() may be called with a MSG parameter regardless of DispatchMessage() , like with the WM_SETCURSOR message.

Then I thought about having instead a std::queue<WindowEvent> in my class, when winProc() continuously pushes WindowEvent s to it. The problem here is that sometimes the window procedure function keeps getting messages and won't return. This happens when I drag-move the window (then the WM_MOVING message is continuously called, along with other messages). The code after DispatchMessage() will not run until I release my mouse. This also happens when resizing the window.

Did I grasp anything wrong? How do you think such PollEvent function can be implemented?

Answer 1

Given that PollEvent is primarily for a game loop style design, you can probably poll for what you need while simultaneously servicing the Windows event loop:

class Window
{
    HWND _hwnd;           // Win32 handle to the window
    RECT _lastWindowSize; // last known window size
    POINT _lastMousePos;  // last known mouse position on window
    BYTE _lastKeyboardState[256]; // last known key state

    std::list<Event> _events; // unprocessed events

public:
    bool PollEvent(Event* pEvent);
};


bool Window::PollEvent(Event* pEvent)
{

   // return any previously queued events
   if (_events.size() > 0)
   {
        *pEvent = _events.pop_front();
        return true;
   }

   // process 1 windows event message
   if (PeekMessage(&msg, _hWnd, 0, 0, PM_REMOVE))
   {
       TranslateMessage(&msg);
       DispatchMessage(&msg);

       if (msg.msg == WM_QUIT)
       { 
           *pEvent = EXIT_EVENT; // special handling for WM_QUIT
           return true;
       }
   }

   // -----------------------------------------
   // poll keyboard state
   BYTE kbState[256];
   GetKeyboardState(kbState);
   bool isKeyboardEvent = false;
   if (memcmp(_lastKeyboardState, kbState, 256) != 0)
   {
        // not shown
        // compute diff of kbState and _lastKeyboardState
        // generate a keyboard event and add to the queue

        Event kbevt;
        kbevt.type = KeyEvent;
        kbevt.code = <computed code based on diff above>
        _events.push_back(kbevt);
   }
   memcpy(_lastKeyboardState, kbState, 256);
   // -----------------------------------------

   // -----------------------------------------
   // poll window size changes
   RECT rectWindowSize;
   int width, height, oldwidth, oldheight;
   GetClientRect(&rectWindowSize);
   width = rectWindowSize.right - rectWindowSize.left;
   height = rectWindowSize.bottom - rectWindowSize.top;
   oldwidth = _lastWindowSize.right - _lastWindowSize.left;
   oldheight = _lastWindowSize.bottom - _lastWindowSize.top;
   if ((width != oldwidth) || (height != oldheight))
   {
       Event sizeEvent;
       sizeEvent.type = SizeEvent;
       sizeEvent.width = width;
       sizeEvent.height = height;
       _events.push_back(kbevt);
   }
   _lastWindowSize = rectWindowSize;
   // -----------------------------------------

   // not shown - computing mouse position, joystick position, text stuff


   // if at least one event was queued - return it now
   if (_events.size() > 0)
   {
        *pEvent = _events.pop_front();
        return true;
   }

   return false;

}