Hi, shouldn’t this be allowed? _*screenshot_1*_ us...

# android

[Raed Ghazal]

Hi, shouldn’t this be allowed? _*screenshot_1*_
using the same generic type works _*screenshot_2*_ just fine, but using a child generic type doesn’t and throws an error

anyone got an idea of how to fix it? or another way of perform something like this?

[Michaeljon Hayden]

Variance is weird and i can never remember which is which but have you tried using in or out with T in abstract class ViewHolder<T>?

[Raed Ghazal]

hmm, yeah I always find it confusing, let me try

[ephemient]

abstract class ViewHolder<out T>

[ephemient]

when interface F<T>, F<A> and F<A.B> have no relation
when interface F<out T>, F<A.B> : F<A>
when interface F<in T>, F<A> : F<A.B>

[Raed Ghazal]

@ephemient well, the issue is that I need to use it as in

[Raed Ghazal]

but even in doesn’t work

[ephemient]

then returning a subclass there is unsafe

[Raed Ghazal]

hmm, right, but is there a clean way to do something like this 😅 ?

[Michaeljon Hayden]

You might need to mark the ViewHolder return type in foo as well.

[Michaeljon Hayden]

fun foo(): ViewHolder<in A> {}

[ephemient]

the types you want say that

val vh: ViewHolder<A> = foo()
vh.bind(something that is A but not A.B)

should work but if it requires a A.B then it can't

[Raed Ghazal]

@Michaeljon Hayden here is the issue, this is a simple representation just to show you

foo() might return different type of ViewHolders, depending on some value, each one inherits ViewHolder with a different generic type from A

[Raed Ghazal]

Screen Shot 2022-05-26 at 19.02.13.png

[Raed Ghazal]

something like this

[Raed Ghazal]

alright, as it seems unsafe and impossible to do it safely, I fixed it in another way without generics and just did normal if(item is A.B) bind(item)

[Michaeljon Hayden]

Sorry, i missed something, Does foo(): ViewHolder<in A> not work?

[Raed Ghazal]

yes it doesn’t

[Raed Ghazal]

here is what I eventually did

[Michaeljon Hayden]

sealed class A {
    class B: A()
    class C: A()
}

abstract class VH<T> {
    abstract fun bind(t: T)
}

class ABViewHolder: VH<A.B>() {
    override fun bind(t: B) {

    }
}

class ACViewHolder: VH<A.C>() {
    override fun bind(t: C) {

    }
}

fun foo(f: A): VH<out A> {
    return when(f) {
        is B -> ABViewHolder()
        is C-> ACViewHolder()
        else -> {}
    }
}

[Michaeljon Hayden]

This works but if you're using a when statement you'll need to cover the A case

[Raed Ghazal]

yeah this won’t work either because the function that will call foo().bind(param) won’t know which parameter to pass and it won’t be safe as @ephemient said

[Michaeljon Hayden]

If you're passing in the type to the foo function to get the correct view holder AND that type has all the info you need to do your binding you could do something like this to get around the type safety issue.

sealed class Alpha {
    object B: Alpha()
    object C: Alpha()
}

abstract class VH<A: Alpha> {
    abstract val type: A
    abstract fun bind()
}

class ABViewHolder(
    override val type: B
): VH<Alpha.B>() {
    override fun bind() {
        TODO("Not yet implemented")
    }
}

class ACViewHolder(
    override val type: C
): VH<Alpha.C>() {
    override fun bind() {
        TODO("Not yet implemented")
    }
}

fun foo(type: Alpha): VH<out Alpha> {
    return when(type) {
        is B -> ABViewHolder(type)
        is C-> ACViewHolder(type)
    }
}

[Raed Ghazal]

well, yeah that might also solve the problem, thanks for suggesting!