AwesomeFragment.kt

class AwesomeFragment : Fragment() {
    private var callback: CallbackListener? = null
    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_awesome, container, false)
    }
    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        if(activity is CallbackListener) {
          callback = activity
        }
    }
    override fun onViewCreated(view: View, savedInstance: Bundle?) {
        super.onViewCreated(view, savedInstance)
        val button = view.findViewById<Button>(R.id.my_button)
        button.setOnClickListener { callback?.let { it.onCallback() } }
    }
}