Mocking with noSuchMethod - by Simon Lightfoot :: #HumpdayQandA on 9th April 2025 :: https://www.youtube.com/watch?v=LddUVHziMaM

humpday_2025-04-09-mock.dart

// MIT License
//
// Copyright (c) 2025 Simon Lightfoot
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//
import 'package:flutter/material.dart';

void main() {
  runApp(
    App(backend: Backend()),
  );
}

class AppConfig {
  AppConfig(this._config);

  final Map<String, dynamic> _config;

  String get welcomeMessage => _config['welcome_message'];
}

class Api {
  Api();

  Future<Map<String, dynamic>> fetchAppConfig() async {
    // TODO: go to server and fetch content
    return {
      'welcome_message': 'Welcome',
    };
  }
}

class Backend {
  Backend({Api? api}) {
    this.api = api ?? Api();
  }

  late final Api api;

  AppConfig? _config;

  AppConfig get config => _config!;

  Future<void> loadApp() async {
    _config = AppConfig(await api.fetchAppConfig());
    await Future.delayed(const Duration(milliseconds: 2000));
  }

  void dispose() {
    // Cleanup backend
  }
}

class App extends StatefulWidget {
  const App({
    super.key,
    this.backend,
  });

  final Backend? backend;

  static AppConfig appConfigOf(BuildContext context) {
    return context.findAncestorStateOfType<_AppState>()!.backend.config;
  }

  @override
  State<App> createState() => _AppState();
}

class _AppState extends State<App> {
  late Backend backend;
  late Future<void> _appLoader;

  @override
  void initState() {
    super.initState();
    backend = widget.backend ?? Backend();
    _appLoader = backend.loadApp();
  }

  @override
  void dispose() {
    backend.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark(),
      home: FutureBuilder(
        future: _appLoader,
        builder: (BuildContext context, AsyncSnapshot<dynamic> snapshot) {
          return snapshot.connectionState != ConnectionState.done //
              ? Splash()
              : Home();
        },
      ),
    );
  }
}

class Splash extends StatelessWidget {
  const Splash({super.key});

  @override
  Widget build(BuildContext context) {
    final theme = Theme.of(context);
    return Material(
      color: theme.hintColor,
      child: Placeholder(
        color: theme.highlightColor,
      ),
    );
  }
}

class Home extends StatelessWidget {
  const Home({super.key});

  @override
  Widget build(BuildContext context) {
    return Material(
      child: Center(
        child: Text(
          App.appConfigOf(context).welcomeMessage,
          style: TextStyle(
            fontSize: 32.0,
          ),
        ),
      ),
    );
  }
}

humpday_2025-04-09-mock_test.dart

// MIT License
//
// Copyright (c) 2025 Simon Lightfoot
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//
import 'package:flutter_test/flutter_test.dart';
import 'package:flutter_test_app2/humpday_2025-04-09-mock.dart';

void main() {
  testWidgets(
    'Testing welcome message of app',
    (tester) async {
      await tester.pumpWidget(App());
      await tester.pump(const Duration(seconds: 3));
      expect(find.text('Welcome'), findsOneWidget);
    },
  );
  testWidgets(
    'Testing alternative welcome message of app',
    (tester) async {
      final api = MockApi();
      await tester.pumpWidget(
        App(backend: Backend(api: api)),
      );
      await tester.pump(const Duration(seconds: 3));
      expect(
        find.text('Welcome'),
        findsExactly(0),
        reason: 'Welcome message exists when it shouldn\'t',
      );
      expect(
        find.text('Alternative'),
        findsOneWidget,
        reason: 'Alternative welcome text missing',
      );
    },
  );
}

class MockApi implements Api {
  MockApi();

  @override
  dynamic noSuchMethod(Invocation invocation) {
    if (invocation.isMethod && invocation.memberName == #fetchAppConfig) {
      return Future.value(<String, dynamic>{
        'welcome_message': 'Alternative',
      });
    }
    return super.noSuchMethod(invocation);
  }
}