display rectangle on a win32 child window if cursor is on it and erase if cursor leaves child window

Question

I need to display some shapes (5 rectangles to be precise) on my caption less child window when ever my cursor is on the window, and erase them when cursor leaves the window; i.e enters into the parent window region.

I am tracking the mouse movement to the child window through NCHITTEST, and the rectangles pop up perfectly. But i cant get them to disappear when my cursor leaves the child window, they just remain there on the client area until WM_PAINT is called on to the window.

Can anybody tell me how to achieve this functionality? I need to use the NCHITTEST case as rest of my functionality depends on it. I have tried tracking mouse_move and lbuttondown events, but these events aren't being captured along with nchittest.

Answer 1

Look at the TrackMouseEvent() function.

This needs to be called when the mouse enters the window (WM_MOUSEMOVE if it's not already being tracked) and will notify your window when the mouse leaves (WM_MOUSELEAVE).

Here's some sample VB6 code but the should be easily convertable to any other language.

Select Case Msg
    Case WM_MOUSEMOVE
      If Not MouseInWindow Then
        Dim ET As TRACKMOUSEEVENTTYPE
        'Set up the mouse leave notification
        ET.cbSize = Len(ET)
        ET.hwndTrack = Me.hWnd
        ET.dwFlags = TME_HOVER Or TME_LEAVE
        ET.dwHoverTime = 0
        TrackMouseEvent ET

        MouseInWindow = True
        'The mouse has just entered
        Redraw
      End If

    Case WM_MOUSELEAVE
      If MouseInWindow Then
        MouseInWindow = False
        'The mouse has just left
        Redraw
      End If
End Select