Android basic brush up - AtoF & FtoA communication

readme.md

activity.kt

class DummyActivity : AppCompatActivity(R.layout.activity_dummy) {

    private val btn by lazy {
        findViewById<Button>(R.id.btn)
    }

    private val tv by lazy {
        findViewById<TextView>(R.id.tv)
    }
    var aToFragCallback: ActivityToFragCallBack<String>? = null

    override fun onResume() {
        super.onResume()
        var count = 0
        btn.setOnClickListener {
            aToFragCallback?.invoke("Hello from activity, count: ${++count}")
        }
    }

    val fToActCallback: FragToActivityCallBack<String> = {
        tv.text = it
    }
}

fragment.kt

class DummyFrag : Fragment(R.layout.frag_dummy) {

    private lateinit var inflatedView: View

    private val btn by lazy {
        inflatedView.findViewById<Button>(R.id.btn)
    }

    private val tv by lazy {
        inflatedView.findViewById<TextView>(R.id.tv)
    }

    private var fToActCallback: FragToActivityCallBack<String>? = null

    override fun onAttach(context: Context) {
        super.onAttach(context)
        //  f to a
        fToActCallback = (context as? DummyActivity)?.fToActCallback
        //  a to f
        (context as? DummyActivity)?.aToFragCallback = {
            tv.text = it
        }
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        inflatedView = view
        var count = 0
        btn.setOnClickListener {
            fToActCallback?.invoke("Hello world from fragment, count: ${++count}")
        }
    }
}