what is the best of creating a generic object that...

# announcements

[amadeu01]

what is the best of creating a generic object that has an argument in its constructor.
For instance if I just need to create a new object that the class does not have arguments in its contructor I would be able to make

T::class.java.newInstance() // Where T is a generic type

But, if I want to make an new object that need to receive an argument.

The usage of this would be in a recycler view where I have different view holders but each one has a constructor with only one argument. like

inner class ShortTextViewHolder(
        private val viewBinding: ItemRowFormFieldTextBinding
    ) : RecyclerView.ViewHolder(viewBinding.root), FieldViewHolder {

        override fun configureWith(field: FormField) = viewBinding.run {
            with(field) {
                textFieldName.text = label
                textFieldDescription.text = description
            }
        }
    }

So basically, I would like to simplify this

override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
        val view = ItemRowFormFieldTextBinding.inflate(parent.inflater)
        if (viewType == ITEM_TYPE_SHORT_TEXT) {
            return ShortTextViewHolder(view)
        }
        return LongTextViewHolder(view)
    }

By calling a Factory like factory<LongTextViewHolder>(view) or factory(view, myPossibleTypes[viewType])
where myPossibleTypes would be a list of the types I can instantiate

[araqnid]

you need to go through getConstructor(..) and call newInstance(..) on the Constructor returned rather than simply calling newInstance on the class (which is a shortcut for .getConstructor().newInstance())

[Thiago]

What do you need is the old reflection concept to create instances using they constructor.

data class ViewHolder(val name: String)

inline fun <reified T> check() {
    val instance = T::class.constructors.first().apply {
        isAccessible = true
    }.call("Amadeu")
    println(">>>>> My name is: $instance")
}

fun main() {
    check<ViewHolder>()
}

Warning: I don't recomend to use reflection in the Android context, because can create bugs not easy to find.

[Thiago]

Don't forget to scape classes in the Proguard, Dexguard, R8 files and kotlin-reflect dependency

[amadeu01]

I managed to do something like

inline fun <reified Field: FieldHolder, View: ViewBinding> myFactory(view: View, init: (View) -> Field): Field {
    return init(view)
}

myFactory<ShortTextViewHolder, ItemRowFormFieldTextBinding>(view, ::ShortTextViewHolder)
// or
val field = myFactory(view, ::ShortTextViewHolder)

Which is basically calling the constructor.
Is this ::MyClass using reflection ?

[araqnid]

I don’t think that uses reflection, as its making a reference to the constructor all at compile time, so there’s no need, it’s essentially just a {view -> ShortTextViewHolder(view) } lambda