unit Main;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    bLoop: Boolean;
    dwID: DWord;
    ThreadHandle: THandle;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

function PaintThread(Param: PBoolean): Integer;
var
  iWidth, iHeight: Integer;
  Pos: TPoint;
  DC: HDC;
  hOldBrush: hBrush;
  Count: Integer;
begin
  Result := 0;
  Count := 0;
  while Param^ do
  begin
    Inc(Count);
    iWidth := GetSystemMetrics(SM_CXSCREEN);
    iHeight := GetSystemMetrics(SM_CYSCREEN);
    Pos := Point(iWidth div 2 - 4, iHeight div 2 - 4);
    DC := GetWindowDC(GetDesktopWindow);
    hOldBrush := SelectObject(DC, CreateSolidBrush(RGB(250, 250, 0)));
    Ellipse(DC, Pos.X, Pos.Y, Pos.X + 8, Pos.Y + 8);
    DeleteObject(SelectObject(DC, hOldBrush));
    ReleaseDC(GetDesktopWindow, DC);
    Sleep(0);
    if Count shr 4 = 0 then
      Sleep(1);
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  bLoop := True;
  ThreadHandle := BeginThread(nil, 0, @PaintThread, @bLoop, 0, dwID);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  InterlockedExchange(Integer(Pointer(@bLoop)^), 0);
  WaitForSingleObject(ThreadHandle, INFINITE);
  CloseHandle(ThreadHandle);
  Close;
end;

end.