Kotlin: How can an inner class get access to a variable declared as a parameter in an outer class?

Question

I have a DetailAdapter class with a parameter categoryId of type String. I need access to this variable in the inner class DetailHolder. I get the following error:

Unresolved reference: categoryId

How can I solve this?

class DetailAdapter(lifecycleOwner: LifecycleOwner, categoryId: String) : FirebaseRecyclerAdapter<Detail, DetailAdapter.DetailHolder>(DetailAdapter.buildOptions(lifecycleOwner, categoryId)) {

companion object {
    private fun buildQuery(categoryId: String) = FirebaseDatabase.getInstance()
            .reference
            .child("").child("details").child(categoryId)
            .limitToLast(50)

    private fun buildOptions(lifecycleOwner: LifecycleOwner, categoryId: String) = FirebaseRecyclerOptions.Builder<Detail>()
            .setQuery(buildQuery(categoryId), Detail::class.java)
            .setLifecycleOwner(lifecycleOwner)
            .build()
}

class DetailHolder(val customView: View, var detail: Detail? = null) : RecyclerView.ViewHolder(customView) {
    private val TAG = DetailHolder::class.java.simpleName

    fun bind(detail: Detail) {
        with(detail) {
            customView.textView_name?.text = detail.detailName
            customView.textView_description?.text = detail.detailDescription

            val detailId = detail.detailId

            customView.setOnClickListener {
                    // do something
            }

            customView.setOnLongClickListener(
                    {
                        showDeleteDetailDialog(it, categoryId, detailId)
                        true
                    }
            )
        }
    }

Answer 1

DetailHolder in your code is a nested class, not an inner one (its equivalent in Java would be a static nested class).

To define an inner class you need to use the inner keyword:

inner class DetailHolder( ...

This way DetailHolder will hold an implicit reference to its enclosing class (DetailAdapter) and you'll be able to access its properties as well.

Check out the documentation on Nested and Inner Classes.