java.lang.IllegalStateException 在 Android Jetpack Compose 中使用状态时

Question

我有 ViewModel 和 Kotlin sealed class 来为 UI 提供不同的状态。另外，我使用 androidx.compose.runtime.State 对象通知 UI 状态变化。

如果 MyApi 请求发生错误，我将 UIState.Failure 放入 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.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)

ViemModel 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 - 它起作用了，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

如何使用 Compose State 处理密封 class UIState 以便它不会抛出 IllegalStateException？

Answer 1

https://kotlinlang.slack.com/archives/CJLTWPH7S/p1613581738163700.

中讨论了看起来有些相似的问题

我认为该讨论的一些相关部分（来自 Adam Powell）

As for the thread-safety aspects of snapshot state, what you've encountered is the result of snapshots being transactional.

When a snapshot is taken (and composition does this for you under the hood) the currently active snapshot is thread-local. Everything that happens in composition is part of this transaction, and that transaction hasn't committed yet.

So when you create a new mutableStateOf in composition and then pass it to another thread, as the GlobalScope.launch in the problem snippet does, you've essentially let a reference to snapshot state that doesn't exist yet escape from the transaction.

这里的具体情况略有不同，但我认为关键问题相同。可能不会完全这样做，但至少在这里它通过将 init 的内容移动到新的 getCategories() 方法，然后从 LaunchedEffect 块调用它。 FWIW 我在其他地方所做的事情（同时仍在 init 中调用）是在视图模型中使用 StateFlow，然后在 Compose 代码中调用 collectAsState()。

@Composable
fun CategoryScreen(
    viewModel: CategoryListViewModel,
    onCategoryClicked: (Category) -> Unit
) {
    LaunchedEffect(true) {
        viewModel.getCategories()
    }

    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
        )
    }
}

Answer 2

因此，在多次尝试解决此问题后，我找到了解决方案。在答案的帮助下，我知道快照是事务性的，并且运行在 ui 线程上 - 更改调度程序有帮助：

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

            } catch (e: Exception) {
                withContext(Dispatchers.Main) {
                    _categoryListState.value = UIState.Error
                }
            }
        }

java.lang.IllegalStateException 在 Android Jetpack Compose 中使用状态时

java.lang.IllegalStateException when using State in Android Jetpack Compose

android

kotlin

kotlin-coroutines

android-jetpack-compose