Window message PAINT (WM_PAINT)

WM_PAINT: An application gets the WM_PAINT message when the system or another application makes a request to paint a portion of an application's window. The message is sent when the UpdateWindow or RedrawWindow function is called, or by the DispatchMessage function when the application obtains a WM_PAINT message by using the GetMessage or PeekMessage function.

WM_PAINT 
hdc = (HDC) wParam; // the device context to draw in
Parameters
hdc: Handle to the device context to draw in. If this parameter is NULL, use the default device context. This parameter is used by some common controls to enable drawing in a device context other than the default device context. Other windows can safely ignore this parameter.

Source Code

#include <windows.h>

LRESULT CALLBACK WndProc(HWND hWnd,
                         UINT message, 
                         WPARAM wParam,
                         LPARAM lParam);

int APIENTRY WinMain(HINSTANCE hInstance,
                     HINSTANCE hPrevInstance,
                     LPSTR     lpCmdLine,
                     int       nCmdShow)
{
  WNDCLASS wc;
  MSG msg;
  HWND hWnd;
  ZeroMemory(&wc, sizeof(WNDCLASS));

  wc.style           = CS_HREDRAW | CS_VREDRAW;
  wc.lpfnWndProc     = (WNDPROC)WndProc;
  wc.hInstance       = hInstance;
  wc.hCursor         = LoadCursor(NULL, IDC_ARROW);
  wc.hbrBackground   = (HBRUSH)(COLOR_BACKGROUND-1);
  wc.lpszClassName   = (LPCTSTR)"wm_paintWin32Class";


  RegisterClass(&wc);

   hWnd = CreateWindow("wm_paintWin32Class", "WM_PAINT event demo", WS_OVERLAPPEDWINDOW,
      CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);

  ShowWindow(hWnd, nCmdShow);
  UpdateWindow(hWnd);
  while (GetMessage(&msg, NULL, 0, 0)) 
  {
    DispatchMessage(&msg);
  }
  return 0;
}

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

  switch (message) 
  {
    case WM_PAINT:
            hdc = BeginPaint(hWnd, &ps);
      SetTextColor(hdc, RGB(255,0,0));
            TextOut(hdc, 0, 0, "Hello, from WM_PAINT event!", 27); 
            EndPaint(hWnd, &ps); 
      break;
    case WM_DESTROY:
      PostQuitMessage(0);
      break;
    default:
      return DefWindowProc(hWnd, message, wParam, lParam);
   }
   return 0;
}

WM_PAINT

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Further readings

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