3.kt

class HistoryFragment : BaseFragment() {
    
    private lateinit var mHistoryViewModel : HistoryViewModel

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        
        super.onViewCreated(view, savedInstanceState)

        mHistoryViewModel = ViewModelProvider(this).get(HistoryViewModel::class.java)

        subscribeObservers()
    }

    fun subscribeObservers(){
        mHistoryViewModel.loadHistory().observe(viewLifecycleOwner, Observer {
            // your code
        })
    }

    fun showDatePickerDialog(){
        // your code for selecting date
        // val date : String = 선택한 date
        
        mHistoryViewModel.setDate(date) 
        // 이 구간에서 mDate로 선언된 Livedata value가 변경되어 loadHistory()도 Callback으로 같이 호출된다.
    }
}