state.kt

    var state: MutableMap<String, Any?> = HashMap()

    private var foo: Int by state
    private var bar: String by state


    override fun onSaveInstanceState(): Parcelable {

        // ...
            
		putSerializable("state", state as Serializable)

        // ...
    }

    override fun onRestoreInstanceState(state: Parcelable?) {
		// ...
		with(this.state) {
			clear()
			putAll(state.getSerializable("state") as MutableMap<String, Any?>)
		}
		// ...

    }