Kotlin DAO 訪問時返回 null

Question

我是 Android 開發的新手。 當我嘗試從房間訪問 id 時。 我得到 null。我做錯了什么嗎？

語言

@Entity(tableName = "language_table6")
data class Language(
    val isChecked: Boolean = false,
    val language: String,
    val isFavourite: Int = 0,
    @PrimaryKey(autoGenerate = true) val id: Int = 0

)

語言道

@Query("SELECT id FROM language_table6 WHERE language = :language")
     fun getEnglishId(language: String): LiveData<Long>

最喜歡的視圖模型

class FavouriteViewModel @Inject constructor(
    val languageDao: LanguageDao
) : ViewModel() {
val favouriteLanguage = languageDao.getFavouriteLanguageByName().asLiveData()
// Try to access get the id in language model
val englishObject: LiveData<Long> = languageDao.getEnglishId("English")

fun onFavouriteLanguage(languageId: Int) = CoroutineScope(Dispatchers.IO).launch {
    languageDao.resetFavouriteLanguage()
    languageDao.setFavouriteLanguage(languageId)
}

}

收藏片段

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

        val binding = FavouriteBottomSheetBinding.bind(view)
        val favouriteAdapter = FavouriteAdapter(this)
        
        // Try to try to get the value from viewModel  
        val getEnglishId: LiveData<Long> = viewModel.englishObject

        d("readEnglishId", getEnglishId.value.toString())

Answer 1

當您調用getEnglishId.value時，數據庫讀取尚未完成，LiveData 包含初始值null 。 您需要觀察片段中的 LiveData。

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    val binding = FavouriteBottomSheetBinding.bind(view)
    val favouriteAdapter = FavouriteAdapter(this)
        
    viewModel.englishObject.observe(viewLifecycleOwner) { id ->
        // Here you can use the live data value (id)
    }
}