你不能在StatelessWidget中这样做。你必须将它转换为StatefulWidget，并覆盖State的dispose方法：

class Foo extends StatefulWidget {
  @override
  _FooState createState() => _FooState();
}

class _FooState extends State<Foo> {
  GestureRecognizer gestureRecognizer;

  @override
  void dispose() {
    gestureRecognizer?.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Container();
  }
}