DART

display alert dialog automatically in flutter when app opens

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

  void main() {
    runApp(new MyApp());
  }

  class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
          title: 'Trial',
          home: Scaffold(
              appBar: AppBar(title: Text('List scroll')), body: new MyHome()));
    }
  }

  class MyHome extends StatelessWidget { // Wrapper Widget
    @override
    Widget build(BuildContext context) {
      Future.delayed(Duration.zero, () => showAlert(context));
      return Container(
        child: Text("Hello world"),
      );
    }

    void showAlert(BuildContext context) {
      showDialog(
          context: context,
          builder: (context) => AlertDialog(
                content: Text("hi"),
              ));
    }
  }

flutter display alert dialog after server error

Provider.of<NewsProvider>(context, listen: false)
.fetchAndSetNewsList()
.catchError((error) => showDialog(context: context, builder: (context) {
   return new SimpleDialog(
       children: <Widget>[
            new Center(child: new Container(child: new Text('foo')))
       ]);
}))