Thread: What is causing WM_PAINT to be sent?

Hello,

On my machine, running the following causes "Hello, Windows 98!" to be painted immediately, even with the call of UpdateWindow() in WinMain commented out. According to Charles Petzold in "Programming Windows", the first WM_PAINT message would normally be sent as a result of this call of UpdateWindow().

My question is thus: if it is not UpdateWindow() that is causing WM_PAINT to be sent, what is causing it be sent? I presume Windows is at some point deciding that the client area is invalid and thus sending the WM_PAINT message. But when does that happen? During or immediately after execution of ShowWindow(), or at some later stage?

Code:

#include <windows.h>

LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM) ;

int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance,
                    PSTR szCmdLine, int iCmdShow)
{
     static TCHAR szAppName[] = TEXT ("HelloWin") ;
     HWND         hwnd ;
     MSG          msg ;
     WNDCLASS     wndclass ;

     wndclass.style         = CS_HREDRAW | CS_VREDRAW ;
     wndclass.lpfnWndProc   = WndProc ;
     wndclass.cbClsExtra    = 0 ;
     wndclass.cbWndExtra    = 0 ;
     wndclass.hInstance     = hInstance ;
     wndclass.hIcon         = LoadIcon (NULL, IDI_APPLICATION) ;
     wndclass.hCursor       = LoadCursor (NULL, IDC_ARROW) ;
     wndclass.hbrBackground = (HBRUSH) GetStockObject (WHITE_BRUSH) ;
     wndclass.lpszMenuName  = NULL ;
     wndclass.lpszClassName = szAppName ;

     if (!RegisterClass (&wndclass))
     {
          return 0 ;
     }

      hwnd = CreateWindow (szAppName,
                  TEXT ("Application"),
                  WS_OVERLAPPEDWINDOW,
                  CW_USEDEFAULT,
                  CW_USEDEFAULT,
                  CW_USEDEFAULT,
                  CW_USEDEFAULT,
                  NULL,
                  NULL,
                  hInstance,
                  NULL) ;

     ShowWindow (hwnd, iCmdShow) ;
     //UpdateWindow (hwnd) ;

     while (GetMessage (&msg, NULL, 0, 0))
     {
          TranslateMessage (&msg) ;
          DispatchMessage (&msg) ;
     }
     return msg.wParam ;
}

LRESULT CALLBACK WndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
     HDC         hdc ;
     PAINTSTRUCT ps ;
     RECT        rect ;

     switch (message)
     {
     case WM_PAINT:
          hdc = BeginPaint (hwnd, &ps) ;

          GetClientRect (hwnd, &rect) ;

          DrawText (hdc, TEXT ("Hello, Windows 98!"), -1, &rect,
                    DT_SINGLELINE | DT_CENTER | DT_VCENTER) ;

          EndPaint (hwnd, &ps) ;
          return 0 ;

     }
     return DefWindowProc (hwnd, message, wParam, lParam) ;
}