Flutter顶部滑动渐变

顶部滑动渐变   和  顶部导航 
```python
import 'package:flutter/material.dart';
import 'package:flutter_swiper/flutter_swiper.dart';
import 'package:flutter_screenutil/flutter_screenutil.dart';
const APPBAR_SCORLL_OFFSET =100;
class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State {
  List _imageUrls = [
    'https://qnwww2.autoimg.cn/newsdfs/g26/M09/39/1A/autohomecar__ChsEnF3LyfyAPLEOAAPPcEtAcsc635.jpg',
    'https://img2.autoimg.cn/admdfs/g1/M04/0C/D7/ChsEmV236AWAeVxxAADNmEFdNug314.jpg',
    'https://qnwww2.autoimg.cn/newsdfs/g25/M03/94/0C/autohomecar__ChsEel3Lcj6Ab6fcAAXx3D2fNSg287.jpg',
    'https://qnwww2.autoimg.cn/newsdfs/g1/M09/4F/91/autohomecar__ChsEj13LyA6AVsDHAAexjI07sWo824.jpg'
  ];
  // 定义透明度的变量
  double appBarAlpha = 0;
  
  _onScroll(offset) {
    double alpha = offset/APPBAR_SCORLL_OFFSET;
    if(alpha<0){
      alpha = 0;
    }else if(alpha > 1){
      alpha = 1;
    }
    
    setState(() {
      appBarAlpha = alpha;
    });
    
  }

  @override
  Widget build(BuildContext context) {
    ScreenUtil.instance = ScreenUtil(width: 750,height: 1334)..init(context);
    return Scaffold(
        body: Stack(
      children: [
        MediaQuery.removePadding(
            removeTop: true,
            context: context,
            child: NotificationListener(
              onNotification: (scrollNotification) {
                if (scrollNotification is ScrollUpdateNotification &&
                    scrollNotification.depth == 0) {
                  // 滚动且列表滚动的时候
                  _onScroll(scrollNotification.metrics.pixels);
                }
              },
              child: ListView(
                children: [
                  Container(
                    /**
                     * 在需要的页面使用 屏幕适配
                     */
                    width: ScreenUtil().setWidth(750),
                    height: ScreenUtil().setHeight(400),
                    child: Swiper(
                      itemCount: _imageUrls.length,
                      autoplay: true,
                      itemBuilder: (BuildContext context, int index) {
                        return Image.network(
                          _imageUrls[index],
                          fit: BoxFit.cover,
                        );
                      },
                      pagination: SwiperPagination(),
                    ),
                  ),
                  Container(
                    height: 800.0,
                    child: ListTile(
                      title: Text('哈哈'),
                    ),
                  )
                ],
              ),
            )),
            Opacity(
              opacity:appBarAlpha,
              child: Container(
                height: 60,
                decoration: BoxDecoration(
                  color: Colors.white
                ),
                child: Center(
                  child: Padding(
                    padding: EdgeInsets.only(top:20),
                    child: Text('首页'),
                  ),
                ),
              ),
            )
      ],
    ));
  }
}