func (w *Window) OnKeyDown(fn func(Event, interface{}) bool, data interface{}) {
	if fn == nil {
		w.onKeyDown = nil
	} else {
		w.onKeyDown = &keyDownCb{data: data, fn: fn}
	}
}
func (w *Window) OnKeyDown(fn func(Event, interface{}) bool, data interface{}) {
	if fn == nil {
		w.onKeyDown = nil
	} else {
		w.onKeyDown = &keyDownCb{data: data, fn: fn}
	}
}
func ( w * Window ) OnKeyDown ( fn func ( Event , interface { } ) bool , data interface { } ) { if fn == nil { w . onKeyDown = nil } else { w . onKeyDown = & keyDownCb { data : data , fn : fn } } }
// OnKeyDown sets the callback that is called when a user presses a key
// while the Window is active
OnKeyDown sets the callback that is called when a user presses a key while the Window is active