Построение формы на чистом API

program PlainAPI;

uses
  Windows,
  Messages;

{$R *.res}

function PlainWinProc (hWnd: THandle; nMsg: UINT;
  wParam, lParam: Cardinal): Cardinal; export; stdcall;
var
  hdc: THandle;
  ps: TPaintStruct;
begin
  Result := 0;
  case nMsg of
    wm_lButtonDown:
      MessageBox (hWnd, 'Mouse Clicked',
        'Plain API', MB_OK);
    wm_Paint:
    begin
      hdc := BeginPaint (hWnd, ps);
      Ellipse (hdc, 100, 100, 300, 300);
      EndPaint (hWnd, ps);
    end;
    wm_Destroy:
      PostQuitMessage (0);
    else
      Result := DefWindowProc (hWnd, nMsg, wParam, lParam);
  end;
end;

procedure WinMain;
var
  hWnd: THandle;
  Msg: TMsg;
  WndClassEx: TWndClassEx;
begin
  // initialize the window class structure
  WndClassEx.cbSize := sizeOf (TWndClassEx);
  WndClassEx.lpszClassName := 'PlainWindow';
  WndClassEx.style := cs_VRedraw or cs_HRedraw;
  WndClassEx.hInstance := HInstance;
  WndClassEx.lpfnWndProc := @PlainWinProc;
  WndClassEx.cbClsExtra := 0;
  WndClassEx.cbWndExtra := 0;
  WndClassEx.hIcon := LoadIcon (hInstance,
    MakeIntResource ('MAINICON'));
  WndClassEx.hIconSm  := LoadIcon (hInstance,
    MakeIntResource ('MAINICON'));
  WndClassEx.hCursor := LoadCursor (0, idc_Arrow);;
  WndClassEx.hbrBackground := GetStockObject (white_Brush);
  WndClassEx.lpszMenuName := nil;
  // register the class
  if RegisterClassEx (WndClassEx) = 0 then
    MessageBox (0, 'Invalid class registration',
      'Plain API', MB_OK)
  else
  begin
    hWnd := CreateWindowEx (
      ws_Ex_OverlappedWindow, // extended styles
      WndClassEx.lpszClassName, // class name
      'Plain API Demo', // title
      ws_OverlappedWindow, // styles
      cw_UseDefault, 0, // position
      cw_UseDefault, 0, // size
      0, // parent window
      0, // menu
      HInstance, // instance handle
      nil); // initial parameters
    if hWnd = 0 then
      MessageBox (0, 'Window not created',
        'Plain API', MB_OK)
    else
    begin
      ShowWindow (hWnd, sw_ShowNormal);
      while GetMessage (Msg, 0, 0, 0) do
      begin
        TranslateMessage (Msg);
        DispatchMessage (Msg);
      end;
    end;
  end;
end;

begin
  WinMain;
end.