Flutter分配通用未来＆lt; t＆gt;从函数回调

Question

为什么不通用未来＆lt; t＆gt;分配工作？我遇到了这个错误：类型'Future＆lt; t＆gt;？'的值无法分配给类型“未来＆lt; t＆gt;？”的变量。

typedef SimpleFutureFunction<T> = Widget Function(void Function(Future<T>? newFuture) onFuture);

class SimpleFuture<T> extends StatefulWidget {
  const SimpleFuture({Key? key, required this.simple, this.future}) : super(key: key);

  final SimpleFutureFunction simple;
  final Future<T>? future;

  @override
  State<SimpleFuture> createState() => _SimpleFutureState();
}

class _SimpleFutureState<T> extends State<SimpleFuture<T>> {
  Future<T>? _future;

  @override
  void initState() {
    super.initState();
    _future = widget.future;
  }

  @override
  Widget build(BuildContext context) {
    return widget.simple.call(<T>(Future<T>? newFuture) {
      setState(() {
        _future = newFuture;
      });
    });
  }
}