flutter组件抽离思考

源代码:

import 'package:flutter/material.dart';

main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title:'app标题' ,
      home: HYHomePage(),
    );
  }
}

class HYHomePage extends StatefulWidget {
  @override
  _HYHomePageState createState() => _HYHomePageState();
}

class _HYHomePageState extends State<HYHomePage> {
  ScrollController _controller;
  bool _isShowTop=false;
  @override
  void initState() {
    // TODO: implement initState
    _controller=ScrollController();
    _controller.addListener(() {
      final tempOffSet=_controller.offset>=1000;
      if(tempOffSet!=_isShowTop){
        setState(() {
          _isShowTop=tempOffSet;
        });
      }
    });
    super.initState();
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("标题"),
      ),
      body: ListView.builder(
        itemExtent: 50,
        itemCount: 100,
        controller: _controller,
        itemBuilder: (ctx,index){
           return ListTile(title: Text("第$index条,"),);
        }
        ),
      floatingActionButton: !_isShowTop ? null : FloatingActionButton(
        child: Icon(Icons.arrow_upward),
        onPressed: () {
          _controller.animateTo(0, duration: Duration(milliseconds: 1000), curve: Curves.ease);
        },
      ),
    );
  }
}

//思考：如果把BodyDemo抽离出来,此处是拿不到_controller的，应该怎么办？
//class BodyDemo extends StatelessWidget {
//  @override
//  Widget build(BuildContext context) {
//        return ListView.builder(
//        itemExtent: 50,
//        itemCount: 100,
//        controller: _controller,
//        itemBuilder: (ctx,index){
//           return ListTile(title: Text("第$index条,"),);
//        }
//     );
//  }
//}

//class BodyDemo extends StatefulWidget {
//  @override
//  _BodyDemoState createState() => _BodyDemoState();
//}
//
//class _BodyDemoState extends State<BodyDemo> {
//
//  @override
//  Widget build(BuildContext context) {
//    return ListView.builder(
//        itemExtent: 50,
//        itemCount: 100,
//        controller: this._controller,
//        itemBuilder: (ctx,index){
//           return ListTile(title: Text("第$index条"),);
//        }
//        );
//  }
//}

思考：怎么拿到_controller?