pull down refresh before

pull_down_refresh .dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: PullDownRefresh(),
    );
  }
}

class PullDownRefresh extends StatefulWidget {
  @override
  _PullDownRefreshState createState() => _PullDownRefreshState();
}

class _PullDownRefreshState extends State<PullDownRefresh> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: createBody(),
    );
  }

  int itemCount = 30;
  int itemBefore = 0;
  double beforeHeight = 0.0;
  Key key = GlobalKey();
  Widget createBody() {
    return NotificationListener<ScrollNotification>(
      onNotification: (value) {
        if (value is OverscrollNotification &&
            value.metrics.axisDirection == AxisDirection.down) {
          if (beforeHeight == 30.0) return true;
          setState(() {
            beforeHeight = 30.0;
            Future.delayed(Duration(seconds: 3), () {
              setState(() {
                beforeHeight = 0.0;
                itemBefore += 10;
              });
            });
          });
        }
        return true;
      },
      child: CustomScrollView(
        slivers: <Widget>[
          SliverToBoxAdapter(
            child: Container(
              alignment: Alignment.center,
              child: Text('加载前天...'),
              height: beforeHeight,
            ),
          ),
          SliverList(
            delegate: SliverChildBuilderDelegate(
              (c, index) {
                return Text('我是前天的数据$index');
              },
              childCount: itemBefore,
            ),
          ),
          SliverList(
            key: key,
            delegate: SliverChildBuilderDelegate((c, index) {
              return Text('center$index');
            }, childCount: itemCount),
          ),
        ],
        center: key,
      ),
    );
  }
}