Example why do we need service locators

service_locator.dart

const serviceLocator = <String, dynamic>{};

/// single place when we are setting up our dependancies
void setupServiceLocator() {
  // could change service implementation here and it will be used in whole program
  serviceLocator['CatFactService'] = CatFactServiceImpl();
}

void main() {
  // setting up service locator at the start of the app
  setupServiceLocator();
  // getting instance of service from service locator
  final CatFactService service = serviceLocator['CatFactService'];
  // passing service to the function, that depends on it.
  // It could be any implementation, coz they all implements the interface, not implementation.
  printDataFromService(service);
}
/// method that relies on service interface
void printDataFromService(CatFactService service) async {
  /// using service, not knowing what implementation are
  final fact = await service.getCatFact();
  final fact2 = await service.getCatFact();
  print(fact);
  print(fact2);
}

/// interface with list of methods that every service should have
abstract class CatFactService {
  Future<String> getCatFact();
}

/// get from API
class CatFactServiceImpl implements CatFactService {
  @override
  Future<String> getCatFact() => Future<String>.value('Data from api');
}

/// mocked data
class CatFactServiceMocked implements CatFactService {
  @override
  Future<String> getCatFact() => Future<String>.value('mocked data');
}

/// from second api
class CatFactServiceFromAnotherServed implements CatFactService {
  @override
  Future<String> getCatFact() => Future<String>.value('Data from another api');
}