class FirstViewModel(oldCounter: Int) : ViewModel() {
    val counter: LiveData<Int>
        get() = _counter

    private var _counter = MutableLiveData<Int>()

    init {
        _counter.value = oldCounter
    }

    fun add() {
        val count = _counter.value ?: 0
        _counter.value = count + 1
    }

    fun clear() {
        _counter.value = 0
    }
}

Activity：

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_first)
        // ViewModel
        viewModel = ViewModelProvider(this).get(FirstViewModel::class.java)
        // UI
        val startBtn: AppCompatButton = findViewById(R.id.first_start_btn)
        contentTv = findViewById(R.id.first_content_tv)

        startBtn.setOnClickListener {
            viewModel.add()
        }

        viewModel.counter.observe(this) {
            contentTv.text = it.toString()
        }
    }