FutureBuilder example

future_builder_example.dart

import 'package:flutter/material.dart';

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

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

class MyStatefulWidget extends StatefulWidget {
  MyStatefulWidget({Key key}) : super(key: key);

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

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  var increment = 0;
  Future<String> _myFuture = _buildAFuture();

  Widget build(BuildContext context) {
    return DefaultTextStyle(
      style: Theme.of(context).textTheme.headline2,
      textAlign: TextAlign.center,
      child: FutureBuilder<String>(
        future: _myFuture,
        builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
          List<Widget> children;
          if (_waitingForTheResult(snapshot)) {
            children = <Widget>[
              SizedBox(
                child: LinearProgressIndicator(),
                width: 200,
                height: 20,
              ),
              const Padding(
                padding: EdgeInsets.only(top: 16),
                child: Text('Waiting for the result...'),
              )
            ];
          } else if (snapshot.hasData) {
            children = <Widget>[
              Text('Result: ${snapshot.data}',  style: TextStyle(color: Colors.green)),
            ];
          } else if (snapshot.hasError) {
            children = <Widget>[
              Padding(
                padding: const EdgeInsets.only(top: 16),
                child: Text('Result: ${snapshot.error}',  style: TextStyle(color: Colors.red)),
              )
            ];
          }
          children.add(RaisedButton(
              child: Text("Try again"),
              onPressed: () {
                setState(() {
                  _calculation = _buildAFuture(increment);
                  increment++;
                  increment = increment;
                });
              }));
          return Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              crossAxisAlignment: CrossAxisAlignment.center,
              children: children,
            ),
          );
        },
      ),
    );
  }
}

_buildAFuture([int increment = 0]) => Future<String>.delayed(
    Duration(seconds: 2), () => increment % 2 == 0 ? throw 'Boom!' : 'Value');

_waitingForTheResult(AsyncSnapshot snapshot) => snapshot.connectionState == ConnectionState.waiting;