Exemplo indicador de progresso

main.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Exemplo de indicador de progresso'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Future<List<int>> result;
  void _updateList() {
    setState(() {
      result =
          Future.delayed(Duration(milliseconds: 800), () => [1, 2, 3, 4, 5]);
    });
  }

  @override
  void initState() {
    super.initState();
    result = Future.delayed(Duration(milliseconds: 800), () => [1, 2, 3, 4, 5]);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: FutureLoadingProgress(
        future: result,
        onFinishedBuilder: (ctx, data) => ListView.builder(
          itemCount: data.length,
          itemBuilder: (BuildContext ctxt, int index) {
            return new Padding(
                padding: EdgeInsets.all(20),
                child: Text('Elemento ${data[index]}'));
          },
        ),
        onErrorBuilder: (ctx, error) => Text('Erro na obtenção dos dados: $error')
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _updateList,
        tooltip: 'Refresh',
        child: Icon(Icons.refresh),
      ),
    );
  }
}

class FutureLoadingProgress extends StatelessWidget {
  final Future<dynamic> future;
  final Widget Function(BuildContext ctx, dynamic data) onFinishedBuilder;
  final Widget Function(BuildContext ctx, dynamic error) onErrorBuilder;

  FutureLoadingProgress({
    @required this.future,
    @required this.onFinishedBuilder,
    @required this.onErrorBuilder,
  });

  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
        future: future,
        builder: (ctx, asyncSnapshot) {
          if (asyncSnapshot.connectionState == ConnectionState.waiting) {
            return Center(child: CircularProgressIndicator());
          } else if (asyncSnapshot.error != null) {
            return onErrorBuilder(ctx, asyncSnapshot.error);
          } else {
            return onFinishedBuilder(ctx, asyncSnapshot.data);
          }
        });
  }
}