Flutter 滚动改变透明度

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'dart:ui';


class HomeNavigationBar extends StatefulWidget {
  HomeNavigationBar({ this.sctrollController }) : super();
  ScrollController sctrollController = new ScrollController();

  @override
  _HomeNavigationBar createState() {
    return _HomeNavigationBar();
  }
}

class _HomeNavigationBar extends State<HomeNavigationBar> {
  final double _statusBarHeight = MediaQueryData.fromWindow(window).padding.top;
  // 滚动视图
  ScrollController _sctrollController = new ScrollController();
  double _opacity = 0;

  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    setState(() {
      _sctrollController = widget.sctrollController;
      _sctrollController.addListener(() {
        double scrollProportion = _sctrollController.offset / (400 - _statusBarHeight - 44);
        if (scrollProportion > 1) {
          scrollProportion = 1;
        }else if (scrollProportion < 0) {
          scrollProportion = 0;
        }
        setState(() {
          _opacity = scrollProportion;
        });
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    final size = MediaQuery.of(context).size;
    final width = size.width;

    return new Opacity(
      opacity: _opacity,
      child: Container(
         width,
        height: _statusBarHeight + 44,
        color: Colors.white,
      ),
    );
  }
}