Flutter - Android - extensions to mock more easy on testing with mockito

example.dart

// The abstractions
abstract class Result<T> {
}

class Success<T> extends Result<T> {
  final T body;

  Success(this.body);
}

class Failure<T> extends Result<T> {
  final String cause;

  Failure({this.cause});
}

class Session {
   int userId;
}

class UserRepository {
    Future<Result<Session>> login(String email, String password) { 
      // Do the Http call here and get the user id...
    }
}


// The Mockito test extensions
extension ResultMock<T> on PostExpectation<Future<Result<T>>> {
  // One for success
  void thenSucceed(T body) {
    var result = Result.success(body);
    thenAnswer((realInvocation) => Future.value(result));
  }

  // One for failures
  void thenFail([String message = 'mock-error']) {
    var result = Failure(message);
    thenAnswer((realInvocation) => Future.value(result));
  }
}

example.kt

// Abstractions
sealed class Result<T>
class Success<T>(val body: T) : Result<T>()
class Failure<T>(val cause: String) : Result<T>()


// Mockito extensions 
fun <T> OngoingStubbing<Flowable<Result<T>>>.thenReturn(data: T): OngoingStubbing<Flowable<Result<T>>> {
    return thenReturn(Flowable.just(Success(data)))
}