MainActivity.kt

class MainActivity : AppCompatActivity() {

    lateinit var instanceA : ClassA
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        instanceA = ClassA()
    }
    override fun onResume() {
        super.onResume()
        instanceA.registerListenerAChange()
    }
    override fun onStop() {
        super.onStop()
        instanceA.unRegisterListenerAChange()
    }
    override fun onDestroy() {
        super.onDestroy()
        instanceA.releaseAll()
    }