class MySuperCoolActivity : Activity() {
    private var importantNumber: Int = 3
    private var importantNullableNumber: Int? = null
    private var importantFlag: Boolean = false
    ///and many other super important properties


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        if (savedInstanceState != null) {
            importantNumber = savedInstanceState.getInt("important property key")
            importantFlag = savedInstanceState.getBoolean("important flag key")
            if (savedInstanceState.containsKey("important nullable property key")) {
                importantNullableNumber = savedInstanceState.getInt("important nullable property key")            
            }
            //....
        }
        //some important logic...
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        outState.putInt("important property key", importantNumber)
        outState.putBoolean("important flag key", importantFlag)
        importantNullableNumber?.let {
            outState.putInt("important nullable property key", it)
        }
        //....
    }

    //some more code..
}

class MySuperCoolActivity : Activity(), Stateful by state() {
    private var importantNumber: Int by statefulProperty(3)
    private var importantNullableNumber: Int? by statefulNullableProperty<Int>(null)
    private var importantFlag: Boolean by statefulProperty(false)
    ///and many more super important properties


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        restore(savedInstanceState)
        //some important logic...
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        save(outState)
    }

    //some more code..
}