const RefreshIndicator({
      Key key,
      @required Widget child,
      double displacement: 40.0,
      @required RefreshCallback onRefresh,
      Color color,
      Color backgroundColor,
      ScrollNotificationPredicate
          notificationPredicate: defaultScrollNotificationPredicate,
      String semanticsLabel,
      String semanticsValue
})

const RefreshIndicator({
  Key key,
  @required this.child,
  this.displacement = 40.0,
  @required this.onRefresh,
  this.color,
  this.backgroundColor,
  this.notificationPredicate = defaultScrollNotificationPredicate,
  this.semanticsLabel,
  this.semanticsValue,
}) : assert(child != null),
     assert(onRefresh != null),
     assert(notificationPredicate != null),
     super(key: key);

import 'dart:async';
import 'dart:math';

import 'dart:async';
import 'dart:math';
import 'package:flutter/material.dart';

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

class PullToRefresh extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Pull To Refresh',
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  var list;
  var random;

  var refreshKey = GlobalKey<RefreshIndicatorState>();

  @override
  void initState() {
    list = List.generate(3, (i) => "Item $i");
    super.initState();
    random = Random();
    refreshList();
  }

  Future<Null> refreshList() async {
    refreshKey.currentState?.show(atTop: false);
    await Future.delayed(Duration(seconds: 2));

    setState(() {
      list = List.generate(random.nextInt(10), (i) => "Item $i");
    });

    return null;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Pull to refresh"),
      ),
      body: RefreshIndicator(
        key: refreshKey,
        child: ListView.builder(
          itemCount: list?.length,
          itemBuilder: (context, i) => ListTile(
            title: Text(list[i]),
          ),
        ),
        onRefresh: refreshList,
      ),
    );
  }
}