user.dart

class UserDetails {
  UserDetails(this.id);

  final int id;
}

class User extends StatefulWidget {
  const User({Key key, this.id, this.builder, this.child}) : super(key: key);

  final int id;
  final ValueWidgetBuilder<AsyncSnapshot<UserDetails>> builder;
  final Widget child;
  @override
  _UserState createState() => _UserState();
}

class _UserState extends State<User> {
  Future<UserDetails> future;

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

  @override
  void didUpdateWidget(User oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (widget.id != oldWidget.id) {
      future = fetchUser(widget.id);
    }
  }

  Future<UserDetails> fetchUser(int id) => Future.value(UserDetails(id));

  @override
  Widget build(BuildContext context) {
    return FutureBuilder<UserDetails>(
      future: future,
      builder: (context, snapshot) {
        return widget.builder(context, snapshot, widget.child);
      },
    );
  }
}