LiveStreamActivity.kt

class LiveStreamActivity : AppCompatActivity(R.layout.activity_main) {
    private val adapter = MessagesListAdapter()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        loadMockVideoStream()
        messagesList.adapter = adapter

        val viewModel: LiveStreamViewModel by viewModels()
        viewModel.viewState.observe(this, Observer {
            when (it) {
                is State.Messages -> updateMessagesList(it.messages)
                is State.NewMessage -> updateMessagesList(adapter.currentList + it.message)
                is State.Error -> showToast(it.message)
            }
        })

        sendMessageButton.setOnClickListener {
            viewModel.sendButtonClicked(messageInput.text.toString())
            messageInput.setText("")
            messageInput.clearFocus()
            messageInput.hideKeyboard()
        }
    }
    ...
}