为什么推荐使用ViewModel
架构组件,如果它不处理 process death
?
例如,如果我用 onSaveInstanceState
保存状态状态持续到 configuration change
和 process death
, 而 ViewModel
只能生存configuration change
, 为了生存 process death
需要从 ViewModel
获取状态并使用相同的onSaveInstanceState
机制。
我想念什么?
最佳答案
Why it is recommend to use ViewModel architecture component, if it not handle process death?
What do I miss?
你是对的,直到 2020 年 1 月发生,他们终于发布了
viewmodel-savedstate:1.0.0
然后使AndroidX Activity、Fragment 和Navigation 最新库版本提供SavedStateViewModelFactory
默认 ViewModelProvider.Factory
.现在您可以获得
SavedStateHandle
进入您的ViewModel
这可以帮助在进程死亡期间持久化/恢复内容。class MyViewModel(private val savedStateHandle: SavedStateHandle): ViewModel() {
val someState: MutableLiveData<String> = savedStateHandle.getLiveData("someState")
}
或者class MyAndroidViewModel(application: Application, private val savedStateHandle: SavedStateHandle): AndroidViewModel(application) {
val someState: MutableLiveData<String> = savedStateHandle.getLiveData("someState")
}
这些将通过它们的默认构造函数作为 SavedStateViewModelFactory
工作。通过反射创建它们。如果您需要
SavedStateHandle
以及附加参数,而不是使用默认的 ViewModelProvider.Factory
,您可以改为从 AbstractSavedStateViewModelFactory
扩展这会给你一个SavedStateHandle
它实际上可以在进程死亡过程中自动工作并持续/恢复。val viewModel = ViewModelProvider(this, myAbstractSavedStateViewModelFactoryImpl).get(MyViewModel::class.java)
或者用KTX的东西private val viewModel by viewModels { myAbstractSavedStateViewModelFactoryImpl }
您还可以将 ViewModels 限定为 Jetpack Navigation 的 NavGraph,这有点棘手但可行:inline fun <reified T : ViewModel> SavedStateRegistryOwner.createAbstractSavedStateViewModelFactory(
arguments: Bundle,
crossinline creator: (SavedStateHandle) -> T
): ViewModelProvider.Factory {
return object : AbstractSavedStateViewModelFactory(this, arguments) {
@Suppress("UNCHECKED_CAST")
override fun <T : ViewModel?> create(
key: String, modelClass: Class<T>, handle: SavedStateHandle
): T = creator(handle) as T
}
}
inline fun <reified T : ViewModel> Fragment.navGraphSavedStateViewModels(
@IdRes navGraphId: Int,
crossinline creator: (SavedStateHandle) -> T
): Lazy<T> {
// Wrapped in lazy to not search the NavController each time we want the backStackEntry
val backStackEntry by lazy { findNavController().getBackStackEntry(navGraphId) }
return createViewModelLazy(T::class, storeProducer = {
backStackEntry.viewModelStore
}, factoryProducer = {
backStackEntry.createAbstractSavedStateViewModelFactory(
arguments = backStackEntry.arguments ?: Bundle(), creator = creator
)
})
}
然后class MyFragment: Fragment() {
private val mySharedViewModel by navGraphSavedStateViewModels(R.id.registration_graph) { handle ->
MySharedViewModel(handle)
}
}
关于android - ViewModel 和进程死亡,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/62342411/