TVectorLayer32.OnDraw

property OnDraw: TNotifyEvent; //read and write

This event is called every time the Paths property is updated.

Before drawing Paths onto a layer's Image, they'll need to offset relative to the layer's position.

And apart from drawing, this event is also the perfect time to update hit-testing (see UpdateHitTestMask).

procedure TForm1.DrawPolygonLayer(Sender: TObject);
var
  tmpPaths: TPathsD;
begin
  with Sender as TVectorLayer32 do
  begin
	tmpPaths := OffsetPath(Paths, -Left, -Top);
	DrawPolygon(Image, tmpPaths, frEvenOdd, $8000FFFF);
	DrawLine(Image, tmpPaths, DpiAware(1.5), clNavy32, esPolygon);
	UpdateHitTestMask(tmpPaths, frEvenOdd);
  end;
end;

See Also

TLayer32.Image, UpdateHitTestMask