program Project1;

uses
  Windows,
  Messages;

{供 WM_LBUTTONDOWN 消息调用的自定义过程}
procedure OnLButtonDown(h: HWND);
var
  BrushHandle: HBRUSH;
  Rect: TRect;
begin
  GetClientRect(h, Rect);                        {获取客户区矩形}
  BrushHandle := CreateSolidBrush(RGB(0,0,0)); {建立黑色笔刷}
  FillRect(GetDC(h), Rect, BrushHandle);         {填充客户区}
  DeleteObject(BrushHandle);                     {删除笔刷}
end;

{供 WM_LBUTTONUP 消息调用的自定义过程}
procedure OnLButtonUp(h: HWND);
var
  Rect: TRect;
begin
  GetClientRect(h, Rect);         {获取客户区矩形}
  InvalidateRect(h, @Rect, True); {使客户区无效, 从而强制重绘}
end;

{供 WM_MOUSEMOVE 消息调用的自定义过程}
procedure OnMouseMove(h: HWND; lParam: Integer);
var
  pt: TPoint;
  buf: array[0..255] of Char;
begin
  pt.X := LoWord(lParam); {lParam 中的低两位是 x 坐标}
  pt.Y := HiWord(lParam); {lParam 中的高两位是 y 坐标}
  wvsprintf(buf, '%d, %d', @pt); {格式化到缓冲区}
  SetWindowText(h, buf);         {显示在标题}
end;

function WndProc(wnd: HWND; msg: UINT; wParam: Integer; lParam: Integer): Integer; stdcall;
begin
  Result := 0;
  case msg of
    WM_LBUTTONDOWN: OnLButtonDown(wnd); {鼠标左键按下的消息}
    WM_LBUTTONUP  : OnLButtonUp(wnd);   {鼠标左键抬起的消息}
    WM_MOUSEMOVE  : OnMouseMove(wnd, lParam);   {鼠标移动的消息, 坐标位置在 lParam 中}
    WM_DESTROY    : PostQuitMessage(0);
  else
    Result := DefWindowProc(wnd, msg, wParam, lParam);
  end;
end;

function RegMyWndClass: Boolean;
var
  cls: TWndClass;
begin
  cls.style         := CS_HREDRAW or CS_VREDRAW;
  cls.lpfnWndProc   := @WndProc;
  cls.cbClsExtra    := 0;
  cls.cbWndExtra    := 0;
  cls.hInstance     := HInstance;
  cls.hIcon         := 0;
  cls.hCursor       := LoadCursor(0, IDC_ARROW);
  cls.hbrBackground := HBRUSH(COLOR_WINDOW + 1 );
  cls.lpszMenuName  := nil;
  cls.lpszClassName := 'MyWnd';
  Result := RegisterClass(cls) <> 0;
end;

{程序入口}
const
  tit = 'New Form';
  ws = WS_OVERLAPPEDWINDOW;
  x = 100; y = 100; w = 300; h = 180;
var
  hWnd: THandle;
  Msg : TMsg;
begin
  RegMyWndClass;
  hWnd := CreateWindow('MyWnd', tit, ws, x, y, w, h, 0, 0, HInstance, nil);
  ShowWindow(hWnd, SW_SHOWNORMAL);
  UpdateWindow(hWnd);

  while(GetMessage(Msg, 0, 0, 0)) do
  begin
    TranslateMessage(Msg);
    DispatchMessage(Msg);
  end;
end.