这是与 LiveData 相关的错误吗

Question

我已经分离出显示此问题的代码。

我在片段中有一个 recyclerview。我还有 LiveData 和 MutableLiveData。

在 ViewModel.kt 代码中有以下两行，并且工作正常：

private val mMLD = MutableLiveData<OneLine>()
var mLD : LiveData<OneLine> = getMLD()

如果我倒序，即

var mLD : LiveData<OneLine> = getMLD()
private val mMLD = MutableLiveData<OneLine>()

程序崩溃。

这是一个错误吗？

我正在使用 Android Studio Chipmunk 2021.2.1

这是我的代码：

class Adapter(private var mList: List<OneLine>) : RecyclerView.Adapter<Adapter.ViewHolder>() {
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        val view = LayoutInflater.from(parent.context).inflate(R.layout.one_line, parent, false)
        return ViewHolder(view)
    }
    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        val DisplayViewModel = mList.get(position)
            holder.textView.text = DisplayViewModel.text
    }
    override fun getItemCount(): Int {
        return mList.size
    }
    class ViewHolder(ItemView: View) : RecyclerView.ViewHolder(ItemView) {
        val textView: TextView = itemView.findViewById(R.id.textView)
    }
}

class Fragment : Fragment() {
    private lateinit var adapter : Adapter
    private lateinit var viewModel: ViewModel
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
    }
    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        val view: View = inflater.inflate(R.layout.fragment, container, false)

        viewModel = ViewModelProvider(requireActivity()).get(ViewModel::class.java)
        viewModel.Initialize()
        enableRecyclerView(view)
        viewModel.mLD.observe(viewLifecycleOwner, Observer {
            adapter.notifyDataSetChanged()
        })
        return view
    }

    fun enableRecyclerView(view:View){
        var mRecyclerView: RecyclerView? = null

        mRecyclerView = view.findViewById(R.id.recycler_view)
        val linearLayoutManager = LinearLayoutManager(getContext(), LinearLayoutManager.VERTICAL, false)
        mRecyclerView.layoutManager = linearLayoutManager
        adapter = Adapter(viewModel.rvData)   // This will pass the ArrayList to our Adapter
        mRecyclerView.adapter = adapter
   }
}

class MainActivity : AppCompatActivity()  {
    private lateinit var operator_display: ViewModel
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }
}

data class OneLine(val text: String) {
}

class ViewModel : ViewModel(){
    var mLD : LiveData<OneLine> = getMLD()             // observer watches this variable
    private val mMLD = MutableLiveData<OneLine>()
    private fun getMLD():MutableLiveData<OneLine>{return mMLD}
    var rvData = ArrayList<OneLine>()
    fun Initialize(){
        rvData.add(OneLine("Item A"))
        rvData.add(OneLine("Item B"))
        rvData.add(OneLine("Item C"))
    }
}

Answer 1

private val _thing = "hi"
val wow = getThing() // _thing already assigned
fun getThing():String { return _thing }

fun main() {
    println(wow)
}

> hi

val wow = getThing() // _thing not assigned yet
private val _thing = "hi"
fun getThing():String { return _thing } // called when assigning wow, _thing not assigned yet

fun main() {
    println(wow)
}

> null

当您通过函数初始化属性时，您可以运行遇到这样的问题，其中声明顺序很重要，或者编译器无法判断变量肯定在某处初始化。所以在这里，您有效地跳到了一个在变量被赋值之前引用该变量的函数

由于尚未分配，因此处于默认状态，即 null。另请注意，即使函数的 return 类型为 non-null String，它仍为 null - 此时 null 检查系统无法帮助您。

你没有 post 你的实际错误，但猜测它是运行进入那个 null 并且当你尝试调用 [=14= 时崩溃 NullPointerException ] 在上面，或者 null 检查器看到你将它分配给不应该为 null 的东西，并且它会“等待第二个伙伴”

这是与 LiveData 相关的错误吗

Is this a bug relating to LiveData

android

kotlin

android-livedata

mutablelivedata