Flutter Testing - Given-When-Then Example

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Home(user: User()),
    );
  }
}

class Home extends StatelessWidget {
  final User user;

  const Home({Key key, this.user}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter Test Demo'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            FlatButton(
              child: Text('Tap me!'),
              onPressed: () => _onButtonPressed(context),
            ),
          ],
        ),
      ),
    );
  }

  void _onButtonPressed(BuildContext context) {
    if (user.isLoggedIn) {
      showDialog(
        context: context,
        builder: (context) => AlertDialog(title: Text('I\'m logged in!')),
      );
    } else {
      showDialog(
        context: context,
        builder: (context) => AlertDialog(title: Text('I\'m logged out...')),
      );
    }
  }
}

class User {
  bool get isLoggedIn => true;
}

pubspec.yaml

name: flutter_testing
description: A new Flutter project.
publish_to: 'none'
version: 1.0.0+1

environment:
  sdk: ">=2.7.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter
  cupertino_icons: ^1.0.0

dev_dependencies:
  flutter_test:
    sdk: flutter
  mockito: ^4.1.3

flutter:
  uses-material-design: true

widget_test.dart

import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:flutter_testing/main.dart';
import 'package:mockito/mockito.dart';

void main() {
  testWidgets(
      'GIVEN user is logged in '
      'WHEN user taps on a button '
      'THEN dialog is shown', (tester) async {
    
    // given
    final user = UserMock();
    when(user.isLoggedIn).thenReturn(true);

    // when
    await tester.pumpWidget(MaterialApp(home: Home(user: user)));
    await tester.tap(find.byType(FlatButton));
    await tester.pump();

    // then
    expect(find.text('I\'m logged in!'), findsOneWidget);
  });

  testWidgets(
      'GIVEN user is logged in '
      'WHEN user taps on a button '
      'THEN dialog is shown', (tester) async {
    
    // given
    final user = UserMock();
    when(user.isLoggedIn).thenReturn(false);

    // when
    await tester.pumpWidget(MaterialApp(home: Home(user: user)));
    await tester.tap(find.byType(FlatButton));
    await tester.pump();

    // then
    expect(find.text('I\'m logged out...'), findsOneWidget);
  });
}

class UserMock extends Mock implements User {}

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