2.kt

class HistoryViewModel : ViewModel() {

    private val mAppDatabase : AppDatabase
    private val mHistoryDAO: HistoryDAO
    
    private val mDate = MutableLiveData<String>()

    init {
        mAppDatabse = AppDatabase.getAppDatabase()
        mHistoryDAO = mAppDatabase.historyDAO()
    }    

    val date : LiveData<String>
        get() = mDate

    fun loadHistory() : LiveData<History> {
        return Transformations.switchMap(mDate) { date ->
            mHistoryDAO.loadHistory(date)
        }
    }

    fun setDate(date: String){
        mDate.value = date
    }
}