const
  MYMESSAGE = WM_USER + 42;

type
  TTimerTestClass = class
  private
    FWnd: HWnd;
    FOnTimer: TNotifyEvent;
    FTimer: THandle;
    procedure ClearMessageQueue;
    procedure DoOnTimer(var Msg: TMessage);
  public
    constructor Create;
    destructor Destroy; override;
    procedure StartTimer;
    procedure StopTimer;
    property OnTimer: TNotifyEvent read FOnTimer write FOnTimer;
  end;

implementation

procedure TTimerTestClass.ClearMessageQueue;
var
  Msg: TMsg;
begin
  while PeekMessage(Msg, FWnd, MYMESSAGE, MYMESSAGE, PM_REMOVE) do;
end;

constructor TTimerTestClass.Create;
begin
  FWnd := AllocateHWnd(DoOnTimer);
end;

destructor TTimerTestClass.Destroy;
begin
  StopTimer;
  DeallocateHWnd(FWnd);
  inherited;
end;

procedure TTimerTestClass.DoOnTimer(var Msg: TMessage);
begin
  ClearMessageQueue;
  if Assigned(FOnTimer) then
    FOnTimer(self);
end;

procedure WaitOrTimer(lpParameter: Pointer; TimerOrWaitFired: Boolean); stdcall;
var
  Test: TTimerTestClass;
begin
  Test := TTimerTestClass(lpParameter);
  if Assigned(Test) then
    PostMessage(Test.FWnd, MYMESSAGE, 0, 0);
end;

procedure TTimerTestClass.StartTimer;
begin
  if not CreateTimerQueueTimer(FTimer, 0, WaitOrTimer, self, 0, 50,
    WT_EXECUTELONGFUNCTION) then
    FTimer := 0;
end;

procedure TTimerTestClass.StopTimer;
begin
  if FTimer <> 0 then
    begin
      DeleteTimerQueueTimer(0, FTimer, 0);
      FTimer := 0;
    end;
end;