Android 房间查询永远不会 return 为空？

Question

class LottoRepository(private val dao: LottoDao) {

    suspend fun getLotto(gameNum: Int): LiveData<Lotto> {

        if (dao.getLotto(gameNum) != null) { // This line shows me a tooltip that says it's always true

        }

    }
}

@Dao
interface LottoDao {

    @Query("SELECT * FROM lotto_numbers_table WHERE game_num = :gameNum")
    fun getLotto(gameNum: Int): LiveData<Lotto>

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    fun insertLottos(vararg lottos: Lotto)
}

@Entity(tableName = "lotto_numbers_table")
data class Lotto (
    @PrimaryKey @Json(name = "drwNo") @ColumnInfo(name = "game_num")
    val gameNum: Int,
    @Json(name = "drwNoDate") val date: String,
    @Json(name = "drwtNo1") val num1: Int,
    @Json(name = "drwtNo2") val num2: Int,
    @Json(name = "drwtNo3") val num3: Int,
    @Json(name = "drwtNo4") val num4: Int,
    @Json(name = "drwtNo5") val num5: Int,
    @Json(name = "drwtNo6") val num6: Int,
    @Json(name = "bnusNo") val bonusNum: Int,
    @Json(name = "totSellamnt") val totalSale: Long,
    @Json(name = "firstWinamnt") val firstWinnerReward: Long,
    @Json(name = "firstPrzwnerCo") val firstWinnerCount: Int,
    @Json(name = "firstAccumamnt") val firstWinnerTotalReward: Long
)

这些是我的代码，我想做的是检查与 gameNum 匹配的行是否存在。但是当我检查 null 值时，Android Studio 给我一条消息，说它总是正确的。如果数据库中没有匹配的行，我认为它应该为空。

留言是"Condition 'dao.getLotto(gameNum) != null' is always 'true'"

Answer 1

Livedata的值(dao.getLotto(gameNum).value)可能为null，但Livedata的实例不会为null。它类似于 List<Lotto>，列表中可能没有任何 Lotto 实例，但 List 实例不为空 [考虑到列表已创建]

Android 房间查询永远不会 return 为空？

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