在 Android Jetpack Compose 中使用 State 时出现 java.lang.IllegalStateException

I have ViewModel使用 Kotlin 密封类为 UI 提供不同的状态。另外，我用androidx.compose.runtime.State通知 UI 有关状态更改的对象。

如果出现错误MyApi请求发生时，我把UIState.Failure to MutableState对象然后我得到IllegalStateException:

 java.lang.IllegalStateException: Reading a state that was created after the snapshot was taken or in a snapshot that has not yet been applied
        at androidx.compose.runtime.snapshots.SnapshotKt.readError(Snapshot.kt:1524)
        at androidx.compose.runtime.snapshots.SnapshotKt.current(Snapshot.kt:1764)
        at androidx.compose.runtime.SnapshotMutableStateImpl.setValue(SnapshotState.kt:797)
        at com.vladuken.compose.ui.category.CategoryListViewModel$1.invokeSuspend(CategoryListViewModel.kt:39)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:104)
        at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:571)
        at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:738)
        at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:678)
        at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:665)

Viem模型代码：

@HiltViewModel
class CategoryListViewModel @Inject constructor(
    private val api: MyApi
) : ViewModel() {

    sealed class UIState {
        object Loading : UIState()
        data class Success(val categoryList: List<Category>) : UIState()
        object Error : UIState()
    }

    val categoryListState: State<UIState>
        get() = _categoryListState
    private val _categoryListState =
        mutableStateOf<UIState>(UIState.Loading)

    init {
        viewModelScope.launch(Dispatchers.IO) {
            try {
                val categories = api
                    .getCategory().schemas
                    .map { it.toDomain() }
                _categoryListState.value = UIState.Success(categories)

            } catch (e: Exception) {
                //this does not work
                _categoryListState.value = UIState.Error
            }
        }
    }

}

我尝试延迟设置 UIState.Error - 它有效，但我认为这不是正常的解决方案：

viewModelScope.launch(Dispatchers.IO) {
            try {
                val categories = api
                    .getCategory().schemas
                    .map { it.toDomain() }
                _categoryListState.value = UIState.Success(categories)

            } catch (e: Exception) {
                //This works 
                delay(10)
                _categoryListState.value = UIState.Error
            }
        }

我观察 Composable 函数中的 State 对象如下：

@Composable
fun CategoryScreen(
    viewModel: CategoryListViewModel,
    onCategoryClicked: (Category) -> Unit
) {
    when (val uiState = viewModel.categoryListState.value) {
        is CategoryListViewModel.UIState.Error -> CategoryError()
        is CategoryListViewModel.UIState.Loading -> CategoryLoading()
        is CategoryListViewModel.UIState.Success -> CategoryList(
            categories = uiState.categoryList,
            onCategoryClicked
        )
    }
}

撰写版本：1.0.0-beta03

如何处理密封类UIState与撰写State这样它就不会抛出 IllegalStateException？

解决这个问题的三种方法是

• 1. 调用可组合项中启动的效果块中的方法
• 2. 或者在使用 withContext(Dispatchers.Main) 设置 mutableState 的值时将 Context 设置为 Dispatchers.Main
• 3. 或者将 viewModel 中的可变状态更改为 mutableState 流，并使用 composable 中的collectAsState() 将其收集为状态。