I got some issues with Kotlin when translating my android project from java to Kotlin. Say i have interface I and interface O which extends interface I.
interface I{
}
interface O: I{
}
And generic class A which have generic parameter V that extends interfaceI, and generic class B which extends class A:
abstract class A<V: I> {
}
class B : A<O>() {
}
When i'm trying to create such property:
val returnB: A<I>
get() = b
I'm getting compiler error 'required A, found B'. In Java this will work without any issues. How can i access this using Kotlin ?
I need to use this approach for Basic classes in my application.
BaseViewModel which have generic parameter for Navigator class:
abstract class BaseViewModel<N>(application: Application, val repositoryProvider:
RepositoryProvider) : AndroidViewModel(application) {
var navigator: N? = null
fun onDestroyView() {
navigator = null
}
open fun onViewAttached() {
}
}
BaseActivity class:
abstract class BaseActivity<T : ViewDataBinding, V : BaseViewModel<BaseNavigator>> : AppCompatActivity(),
BaseFragment.Callback, BaseNavigator {
// .......
private var mViewModel: V? = null
/**
* Override for set view model
* @return view model instance
*/
abstract val viewModel: V
// .......
}
BaseNavigator interface uses for VM - View communication:
interface BaseNavigator {
fun invokeIntent(intent: Intent?, b: Bundle?, c: Class<*>?,
forResult: Boolean, requestCode: Int)
fun replaceFragment(fragment: Fragment, addToBackStack: Boolean)
fun showDialogFragment(fragment: DialogFragment?, tag: String?)
fun showToast(message: String?)
}
Here example code where i'm extending these classes:
AuthViewModel:
class AuthViewModel(context: Application, repositoryProvider: RepositoryProvider) :
BaseViewModel<AuthNavigator>(context,repositoryProvider) {
// ....
}
AuthNavigator:
interface AuthNavigator : BaseNavigator {
fun requestGoogleAuth(code: Int)
fun requestFacebookAuth(callback: FacebookCallback<LoginResult>)
}
And AuthActivity class where error was appeared:
class AuthActivity : BaseActivity<ActivityAuthBinding, BaseViewModel<BaseNavagator>>(),
GoogleApiClient.OnConnectionFailedListener, AuthNavigator {
@Inject
lateinit var mViewModel: AuthViewModel
override val viewModel: BaseViewModel<BaseNavigator>
get() = mViewModel // Required:BaseViewModel<BaseNavigator> Found: AuthViewModel
}
I'm also tried to change generic parameter in AuthActivity from BaseViewModel to AuthViewModel, but compiler throws error 'required BaseViewModel'.
And i tried to change
override val viewModel: BaseViewModel<BaseNavigator>
get() = mViewModel
to
override val viewModel: AuthViewModel
get() = mViewModel
but in this case compiler throws error 'Property type is 'AuthViewModel', which is not a subtype type of overridden'.
update: That works when i add out property to BaseViewModel:
BaseViewModel<out N : BaseNavigator>
But in this case i can only create
private var navigator: N? = null
which i need to be public so i can set it in the Activity class. Can i create public setter for this property? When i'm trying to create setter an error occurs:
private var navigator: N? = null
fun setNavigator(n: N) { // error: Type parameter N is declared as 'out' but occurs in 'in' position in type N
navigator = n
}
It looks like you are expecting the type parameter to behave covariantly. Kotlin uses declaration-site variance. If you do not specify the variance, generic type parameters are invariant.
In other words, right now there is no relationship between A<I> and A<O>. But if you declare
abstract class A<out V : I>
then A<O> is a subtype of A<I>.
(There is also <in> for contravariance, which works the other way around. See https://kotlinlang.org/docs/reference/generics.html for more details.)