PeopleActivity.kt

class PeopleActivity : AppCompatActivity() {

    private val people = listOf(
        Person(1, "Person 1", "Brazil"),
        Person(2, "Person 2", "USA"),
        Person(3, "Person 3", "Canada"),
        Person(4, "Person 4", "Russia"),
        Person(5, "Person 5", "Germany")
    )

    private lateinit var viewModel: PeopleViewModel
    private lateinit var adapter: PeopleAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_people)
        initComponents()
        observeChanges()
    }

    private fun initComponents() {
        viewModel = ViewModelProviders.of(this).get(PeopleViewModel::class.java)
        adapter = PeopleAdapter(people, AdapterListener())
        adapter.setHasStableIds(true)
        list.adapter = adapter
        button.setOnClickListener {
            val person = viewModel.getSelectedPerson().value
            if (person != null) {
                Toast.makeText(this, person.name, Toast.LENGTH_LONG).show()
            } else {
                Toast.makeText(this, "You need to select someone", Toast.LENGTH_LONG).show()
            }
        }
    }

    private fun observeChanges() {
        viewModel.getSelectedPerson().observe(this, Observer {
            adapter.updateSelectedPerson(it)
        })
    }

    private inner class AdapterListener : PeopleAdapter.Listener {
        override fun onItemClicked(person: Person) {
            viewModel.setSelectedPerson(person)
        }
    }
}