Android Kotlin Room获取LiveData时Observer回调返回null问题

我是移动开发新手，尝试在Fragment中从Room数据库获取LiveData，无报错但Observer回调返回null。数据库中存在数据，且可正常访问登录功能，恳请各位提供帮助。

相关代码 #

StepCount 实体类 #

@Entity(tableName = "stepcount", foreignKeys = [ForeignKey(entity = Users::class, parentColumns = ["userId"], childColumns = ["userId"], onDelete = CASCADE)])
data class StepCount (
    @PrimaryKey(autoGenerate = true)
    val id: Int = 0,

    val userId: Int,

    val date: String,

    val hour: Int,

    var steps: Int = 0
)

StepCountDao #

@Dao
interface StepCountDao {
    @Query("SELECT * FROM stepcount")
    fun getAllStepTable(): LiveData<List<StepCount>>
}

StepCountRepository #

class StepCountRepository(private val dao: StepCountDao) {
    fun getAllStepsTable() : LiveData<List<StepCount>> {
        return dao.getAllStepTable()
    }
}

StepCountViewModelFactory #

class StepCountViewModelFactory(
    private val repository: StepCountRepository,
            private val application: Application): ViewModelProvider.Factory {
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if(modelClass.isAssignableFrom(StepCountViewModel::class.java)) {
            return StepCountViewModel(repository, application) as T
        }
        throw IllegalArgumentException("Unknown View Model Class")
    }
}

StepCountViewModel #

class StepCountViewModel(private val repository: StepCountRepository, application: Application) : AndroidViewModel(application), Observable {
    private var _allStepsTable = MutableLiveData<List<StepCount>>()
    val allStepsTable : LiveData<List<StepCount>>
        get() = _allStepsTable

    fun getAllStepsTable() {
        viewModelScope.launch {
          _allStepsTable.postValue(repository.getAllStepsTable().value)
        }
    }
}

DayFragment #

class DayFragment : Fragment() {

    private lateinit var stepCountViewModel: StepCountViewModel

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val application = requireNotNull(this.activity).application
        val dao = UsersDatabase.getInstance(application).stepCountDao()
        val repository = StepCountRepository(dao)
        val factory = StepCountViewModelFactory(repository, application)

        stepCountViewModel = ViewModelProvider(this, factory).get(StepCountViewModel::class.java)

        val prefs = application.getSharedPreferences("prefs", Context.MODE_PRIVATE)
        val userId = prefs.getInt("userId", -1)
        Log.d("userId", userId.toString())

        stepCountViewModel.allStepsTable.observe(viewLifecycleOwner, Observer { allStepsTable ->
            if (allStepsTable !== null)
                Log.i("DAYFRAG_obs_all_steps", allStepsTable.toString())
        })
        stepCountViewModel.getAllStepsTable()
}

问题原因 #

你的StepCountViewModel存在关键错误：

• Room返回的LiveData<List<StepCount>>是异步加载的，初始调用.value时数据还未加载完成，因此拿到的是null
• 你直接将这个null通过postValue设置到自己的_allStepsTable中，且未观察Room返回的LiveData以接收后续加载完成的数据

修正方案 #

以下两种方案任选其一即可：

方案1：直接暴露Repository返回的LiveData（推荐） #

移除ViewModel中的MutableLiveData，直接返回Repository的LiveData，Fragment观察该LiveData后会自动接收数据更新：

class StepCountViewModel(private val repository: StepCountRepository, application: Application) : AndroidViewModel(application), Observable {
    // 直接暴露Repository的LiveData
    val allStepsTable: LiveData<List<StepCount>> = repository.getAllStepsTable()

    // 无需保留getAllStepsTable()方法
}

修改Fragment，删除调用getAllStepsTable()的代码：

class DayFragment : Fragment() {

    private lateinit var stepCountViewModel: StepCountViewModel

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // ... 初始化ViewModel的代码保持不变 ...

        stepCountViewModel.allStepsTable.observe(viewLifecycleOwner, Observer { allStepsTable ->
            if (allStepsTable !== null)
                Log.i("DAYFRAG_obs_all_steps", allStepsTable.toString())
        })
        // 删除该行：stepCountViewModel.getAllStepsTable()
}

方案2：观察Repository的LiveData并转发数据 #

若需在ViewModel中处理数据，可保留MutableLiveData，但需正确观察Repository的LiveData：

class StepCountViewModel(private val repository: StepCountRepository, application: Application) : AndroidViewModel(application), Observable {
    private var _allStepsTable = MutableLiveData<List<StepCount>>()
    val allStepsTable : LiveData<List<StepCount>>
        get() = _allStepsTable

    init {
        // ViewModel初始化时观察Repository的LiveData，转发数据
        repository.getAllStepsTable().observeForever { steps ->
            _allStepsTable.postValue(steps)
        }
    }

    // 无需保留getAllStepsTable()方法
}

同样，Fragment中删除调用getAllStepsTable()的代码。

额外建议 #

当前查询会返回所有用户的步数数据，而你已获取到当前登录用户的userId，建议修改Dao的查询逻辑，仅返回当前用户的数据，避免数据冗余：

@Dao
interface StepCountDao {
    @Query("SELECT * FROM stepcount WHERE userId = :userId")
    fun getStepTableByUserId(userId: Int): LiveData<List<StepCount>>
}

后续在Repository、ViewModel中传递userId参数，Fragment传入当前用户的userId即可。

内容的提问来源于stack exchange，提问作者michjea