SliverAppBar滚动时flexibleSpace中的图标未隐藏

import 'package:flutter/material.dart';

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  ScrollController _scrollController;
  bool _showIcons = true;

  @override
  void initState() {
    super.initState();
    _scrollController = ScrollController();
    _scrollController.addListener(() {
      setState(() {
        _showIcons = _scrollController.position.pixels < 200;
      });
    });
  }

  @override
  void dispose() {
    _scrollController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        controller: _scrollController,
        slivers: [
          SliverAppBar(
            expandedHeight: 200,
            flexibleSpace: FlexibleSpaceBar(
              background: Container(
                color: Colors.blue,
              ),
              title: _showIcons ? Icon(Icons.arrow_upward) : null,
            ),
          ),
          // Other sliver widgets
        ],
      ),
    );
  }
}