Optimized Paint procedures with subclassing

Optimized Paint procedures with subclassing

The Paint event doesn’t provide with information about which region of the form must be actually repainted, and therefore forces the programmer to repaint the entire client area, which in some cases can be a time-consuming operation.

You can determine the smallest rectangle that needs to be updated by subclassing the form and trapping the WM_PAINT message before it reaches the original window procedure in the VB runtime. If you do so, you can invoke the GetUpdateRect API function to retrieve the rectangle that must be updated:

' REQUIRES THE MSGHOOK.DLL COMPONENT' you can omit the following constant definition, ' because it is defined in the MsgHook type libraryPrivate Const WM_PAINT = &HFPrivate Type RECT    Left As Long    Top As Long    Right As Long    Bottom As LongEnd TypeDeclare Function GetUpdateRect Lib "user32" (ByVal hWnd As Long, lpRect As RECT, _    ByVal bErase As Long) As LongDim WithEvents FormHook As MsgHookPrivate Sub Form_Load()    ' start subclassing of the form    Set FormHook = New MsgHook    FormHook.StartSubclass hWndEnd SubPrivate Sub FormHook_BeforeMessage(uMsg As Long, wParam As Long, lParam As Long, _    retValue As Long, Cancel As Boolean)    If uMsg = WM_PAINT Then        ' Windows is asking the window to repaint itself        Dim lpRect As RECT        GetUpdateRect Me.hWnd, lpRect, False        ' now lpRect holds the size and position (in pixels)        ' of the rectangle that must be updated        ' ...        ' ... (write your repaint procedure here) ...        ' ...    End SelectEnd Sub


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