LiveData是否在更改可见性时不更新？

Question

MainActivity

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    activityMainBinding = DataBindingUtil.setContentView(this, 
 R.layout.activity_main)
    mainViewModel = ViewModelProviders.of(
        this@MainActivity,
        mainViewModelFactory
    )[MainViewModel::class.java]
    activityMainBinding.viewmodel = mainViewModel
    activityMainBinding.lifecycleOwner =   this

        mainViewModel.fetchBusinessList()
mainViewModel.businessList.observe(this, Observer { items ->
        run {
          mainViewModel.isLoading.value = false
        }
    })

MainViewModel

class MainViewModel(val repository: MainActivityRepo) : ViewModel() {

val isLoading: MutableLiveData<Boolean> = MutableLiveData()
lateinit var businessList: LiveData<List<Content>>

fun fetchBusinessList() {
    isLoading.value = true
    businessList = repository.getBusinessList()
}

布局

    <androidx.core.widget.ContentLoadingProgressBar
        android:id="@+id/clp_fetching"
        style="@style/Widget.AppCompat.ProgressBar"
        android:layout_width="40dp"
        android:layout_height="40dp"
        android:visibility="@{viewmodel.isLoading?View.VISIBLE:View.GONE}"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

我尝试过setValue，postValue并还添加了LifeCycleOwner，这不会影响到false的可见性。

Answer 1

这里是一个观察结果

您已将isLoading声明为'val'，这意味着它是只读的，无法更新。

val isLoading：MutableLiveData = MutableLiveData（）

将其更改为

var isLoading：MutableLiveData = MutableLiveData（）

术语是var声明了一个可变变量，而val声明了一个只读或一次分配的变量-因此这两种都称为变量。

关于ViewModel用户的更多观察结果

您正在尝试从活动中更新视图模型中的属性，但不应这样做。

查看模型

class MainViewModel(val repository: MainActivityRepo) : ViewModel() {

val isLoading: MutableLiveData<Boolean> = MutableLiveData()
lateinit var businessList: LiveData<List<Content>>

fun fetchBusinessList() {
    isLoading.value = true
    businessList = repository.getBusinessList()
}

// This is where you the content of the list would be received
fun onListUpdated(contentList : List<Content>){
    businessList.value = contentList;
    isLoading.value = false;
 }

活动

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
    activityMainBinding = DataBindingUtil.setContentView(this, 
 R.layout.activity_main)
    mainViewModel = ViewModelProviders.of(
        this@MainActivity,
        mainViewModelFactory
    )[MainViewModel::class.java]
    activityMainBinding.viewmodel = mainViewModel
    activityMainBinding.lifecycleOwner =   this

        mainViewModel.fetchBusinessList()
mainViewModel.businessList.observe(this, Observer { items ->
        // Update UI Here
    })