Proper way of using DI (injectable-getIt) for testing in flutter? - unit-testing

Question

Is it fine to get instances from getIt initialised in lib(dev) code or should I use another
way, or another DI setup for tests?
Please refer the code below:
void main() {
TravellerProfileViewModel travellerProfileViewModel;
UserService mockUserService;
setUpAll(() {
// CoreInjector is initialised in lib, and gives the getIt instance
CoreInjector.initialize();
//register mockUserService in place of actual user service in the CoreInjector.getIt
mockUserService = registerAndGetMock<UserService>(MockUserService());
// CoreInjector gives TravellerProfileViewModel, injected with UserService
travellerProfileViewModel =
CoreInjector.getIt.get<TravellerProfileViewModel>();
});
}
T registerAndGetMock<T>(T mockAble) {
_removeRegistrationIfExists<T>();
CoreInjector.getIt.registerFactory<T>(() => mockAble);
return mockAble;
}
CoreInjector Code
class CoreInjector {
static GetIt _getIt;
static GetIt get getIt => _getIt;
static void initialize() {
_getIt ??= _configureInjection();
}
}
#InjectableInit(preferRelativeImports: false)
GetIt _configureInjection() =>
$initGetIt(GetIt.asNewInstance());
The TravellerProfileViewModel has a dependency on a Service, which is injected under the hood through the same CoreInjector.
Also is it fine to have constructor injections, or should I make my constructor params optional? What is the better way, among the two below?
#injectable
class TravellerProfileViewModel extends BaseViewModel {
final UserService _userService;
//injected by DI under the hood
TravellerProfileViewModel(this._userService);
}
Injectable generated code:
GetIt $initGetIt(
GetIt get, {
String environment,
EnvironmentFilter environmentFilter,
}) {
final gh = GetItHelper(get, environment, environmentFilter);
gh.factory<UserService>(() => UserService());
gh.factory<TravellerProfileViewModel>(
() => TravellerProfileViewModel(get<UserService>()));
return get;
}
or
#injectable
class TravellerProfileViewModel extends BaseViewModel {
UserService _userService;
//injected by DI under the hood, also gives liberty to pass optional params in case of
testing
TravellerProfileViewModel({UserService userService}){
_userService = userService?? CoreInjector.getIt.get<UserService>();
}
}
Injectable generated code:
GetIt $initGetIt(
GetIt get, {
String environment,
EnvironmentFilter environmentFilter,
}) {
final gh = GetItHelper(get, environment, environmentFilter);
gh.factory<UserService>(() => UserService());
gh.factory<TravellerProfileViewModel>(
() => TravellerProfileViewModel(userService: get<UserService>()));
return get;
}