Spy - Um spy é um test double usado para inspecionar a saída produzida pelo sistema em teste.

main.dart

// Interface do LayoutManager
abstract class LayoutManager {
  void paintDarkMode();
  void paintLightMode();
}

// Implementação Dummy do LayoutManager
class LayoutManagerDummy implements LayoutManager {
  @override
  void paintDarkMode() {
    // Não faz nada
  }

  @override
  void paintLightMode() {
    // Não faz nada
  }
}

abstract class SettingsStorage {
  void setDarkModeEnabled(bool enabled);
}

class SettingsController {
  final LayoutManager layoutManager;
  final SettingsStorage settingsStorage;

  SettingsController({
    required this.layoutManager,
    required this.settingsStorage,
  });

  void setDarkModeEnabled(bool enabled) {
    if (enabled) {
      layoutManager.paintDarkMode();
    } else {
      layoutManager.paintLightMode();
    }

    settingsStorage.setDarkModeEnabled(enabled);
  }
}

class SettingsStorageSpy implements SettingsStorage {
  bool? darkModeEnabled;

  @override
  void setDarkModeEnabled(bool enabled) {
    darkModeEnabled = enabled;
  }
}

void main() {
  final layoutManager = LayoutManagerDummy();
  final settingsStorageSpy = SettingsStorageSpy();
  final settingsController = SettingsController(
    layoutManager: layoutManager,
    settingsStorage: settingsStorageSpy,
  );

  settingsController.setDarkModeEnabled(true);

  print(settingsStorageSpy.darkModeEnabled);
}