Room 在使用实时数据时返回 Null 值但 returns 未使用 Livedata 包装时的正确值

Question

我正在使用以下 DAO

@Dao
interface GroceryListDao {

@Insert
fun insert(list: GroceryList)

@Update
fun update(list: GroceryList)

@Query("Delete from grocery_list_table")
fun clear()

@Query ("Select * from grocery_list_table")
fun getAllItems(): LiveData<List<GroceryList>>

@Query ("Select * from grocery_list_table where itemId = :item")
fun getItem(item: Long): GroceryList?

@Query ("Select * from grocery_list_table where item_status = :status")
fun getItemsBasedOnStatus(status: Int): List<GroceryList>


}

我的数据库有 3 列 groceryId(Long - autogenerated)、groceryName(String) 和 groceryStatus(Int - 1/0)。

当我在不使用 LiveData 的情况下使用 getItemsBasedOnStatus(status: Int) 时，我能够检索数据。但是当它被 LiveData 包裹时，我得到了 null。

另一个问题是当我从数据库中获取项目列表时没有用 LiveData 包装并分配给 ViewModel 中的 MutableLiveData，然后分配的 MutableLiveData 显示空值。我看到很多关于这个的问题，但没有答案。

为我的 viewModel 和 Fragment 添加代码

视图模型

class GroceryListViewModel(
val database: GroceryListDao,
application: Application
) : AndroidViewModel(application) {

private var viewModelJob = Job()
private val uiScope = CoroutineScope(Dispatchers.Main + viewModelJob)

var grocerylistItems = database.getAllItems()
var groceryItem = MutableLiveData<GroceryList>()
var groceryitems = MutableLiveData<List<GroceryList>>()

init {
    getAllItemsFromDatabase()
}

fun insertIntoDatabase(item: GroceryList) {
    uiScope.launch {
        insert(item)
    }
}

private suspend fun insert(item: GroceryList) {
    withContext(Dispatchers.IO) {
        database.insert(item)
    }
}

fun updateGrocerylist(itemId: Long, status: Int) {
    uiScope.launch {
        groceryItem.value = getItem(itemId)
        groceryItem.value?.itemStatus = status
        groceryItem.value?.let { update(it) }

    }
}

private suspend fun update(item: GroceryList) {
    withContext(Dispatchers.IO) {
        database.update(item)
    }
}


private suspend fun getItem(itemId: Long): GroceryList? {
    return withContext(Dispatchers.IO) {
        var item = database.getItem(itemId)
        item
    }
}

fun getAllItemsFromDatabase() {
    uiScope.launch {
        getAllItems()
    }
}

private suspend fun getAllItems() {
    withContext(Dispatchers.IO) {
        grocerylistItems = database.getAllItems()


    }
}

fun getPendingItemsFromDatabase(status: Int) {
    uiScope.launch {
        getPendingItems(status)
    }
}

private suspend fun getPendingItems(status: Int) {
    withContext(Dispatchers.IO) {
        val items = database.getItemsBasedOnStatus(status)
        groceryitems.postValue(items)
        Log.i("Grocery List", "Pending Items:" + items.size)

    }
}

fun getDoneItemsFromDatabase(status: Int) {
    uiScope.launch {
        getDoneItems(status)
    }
}

private suspend fun getDoneItems(status: Int) {
    withContext(Dispatchers.IO) {
        val items = database.getItemsBasedOnStatus(status)
        groceryitems.postValue(items)
        Log.i("Grocery List", "Done Items:" + items.size)
    }
}

fun clearAllItemsFromDatabase() {
    uiScope.launch {
        clearItems()
    }
}

private suspend fun clearItems() {
    withContext(Dispatchers.IO) {
        database.clear()
        getAllItemsFromDatabase()
    }
}

override fun onCleared() {
    super.onCleared()
    viewModelJob.cancel()
}

}

片段

class GroceryLIstFragment : Fragment() {

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    // Inflate the layout for this fragment
    val binding = FragmentGroceryLIstBinding.inflate(inflater,container,false)

    val application = requireNotNull(this.activity).application
    val dataSource = GroceryDatabase.getInstance(application)?.groceryListDatabaseDao

    val viewModelFactory = dataSource?.let { GroceryListViewModelFactory(it, application) }

    val viewModel = viewModelFactory?.let {
        ViewModelProvider(
            this,
            it
        ).get(GroceryListViewModel::class.java)
    }

    viewModel?.grocerylistItems?.observe(this , Observer {
        binding.grocerylistView.removeAllViews() // is it correct ?
        for (item in it){
            Log.i("Grocery List","Grocery Id=" + item.itemId+" ,Grocery Name=" + item.itemName +", Grocery status="+item.itemStatus)
            addItemToScrollbar(item, binding, viewModel)
        }
    })


    binding.addGrocery.setOnClickListener {

        val imm = context?.getSystemService(Context.INPUT_METHOD_SERVICE) as InputMethodManager
        imm.hideSoftInputFromWindow(view?.windowToken, 0)

        val item = binding.groceryitemField.text.toString()

        if (!item.isNullOrBlank()) {
            val newItem = GroceryList(itemName = item)
            viewModel?.insertIntoDatabase(newItem)
            if (viewModel != null) {
                addItemToScrollbar(newItem, binding,viewModel)
            }
            binding.groceryitemField.text.clear()
        }
    }

    binding.doneCheckbox.setOnClickListener {
        if (binding.doneCheckbox.isChecked)
            viewModel?.getDoneItemsFromDatabase(1)
        else
            viewModel?.getAllItemsFromDatabase()

    }

    binding.pendingCheckbox.setOnClickListener {
        if (binding.pendingCheckbox.isChecked) {
            viewModel?.getPendingItemsFromDatabase(0)
        }
        else
            viewModel?.getAllItemsFromDatabase()

    }

    binding.clearGrocery.setOnClickListener {
        viewModel?.clearAllItemsFromDatabase()
        binding.grocerylistView.removeAllViews()
    }

    return binding.root
}

private fun addItemToScrollbar(
    item: GroceryList,
    binding: FragmentGroceryLIstBinding,
    viewModel: GroceryListViewModel
) {
    val itemBox = AppCompatCheckBox(context)
    itemBox.text = item.itemName
    itemBox.isChecked = item.itemStatus == 1
    binding.grocerylistView.addView(itemBox)
    itemBox.setOnClickListener {
        val itemstatus: Int = if (itemBox.isChecked)
            1
        else {
            0
        }
        viewModel?.updateGrocerylist(item.itemId,itemstatus)

    }

}

}

如有任何帮助，我们将不胜感激。

Answer 1

这很可能是与相同的问题（阅读链接的答案）。由于 LiveData 的异步工作方式，当您调用它时它将 return null。 LiveData 旨在与观察者结合使用，一旦观察对象发生变化就会被触发。

Observer 可以像这样

    database.getItemsBasedOnStatus(status).observe(viewLifecycleOwner, Observer  { groceryList->
    // Do cool grocery stuff here

    })

如果你想在没有 viewLifecycleOwner 的 ViewModel 中检索数据，则可以使用 "observeForever()"，但你必须明确删除 Observer，请参阅此。

同样的问题和答案也在这个

Room 在使用实时数据时返回 Null 值但 returns 未使用 Livedata 包装时的正确值

Room returning Null value while using Live Data but returns Proper value when its not wrapped with Livedata

android

android-room

android-livedata