[英]How Best To Implement ViewModel( in AndroidX) So Data Survives Configuration Changes
[英]How ViewModel survives configuration change
我正在尝试在我的应用程序中使用 ViewModel。 我想到的问题是 View Model 如何在配置更改中幸存下来。 我读了很多博客文章说“
它将创建一个HolderFragment以添加到您的活动或片段中,它是不可见的,当配置更改时,活动被破坏,但持有者片段仍然存在
这是有道理的。但我试图对此进行更多探索,发现在支持库 27.1.0+中,他们已经删除了带有描述的 HolderFragment 说
弃用 ViewModelStores.of() 和它所依赖的 HolderFragment,因为它们不再需要android.googlesource 的链接。
现在的问题是他们现在如何做同样的事情?
使用ViewModelProviders.of()
方法创建的 ViewModel 存储在ViewModelStore
hashmap 中,所以真正的问题是ViewModelStore
是如何存储的。
对于活动,这个逻辑很简单。 ViewModelStore
使用onRetainNonConfigurationInstance
方法存储:
@Override
@Nullable
public final Object onRetainNonConfigurationInstance() {
Object custom = onRetainCustomNonConfigurationInstance();
ViewModelStore viewModelStore = mViewModelStore;
if (viewModelStore == null) {
// No one called getViewModelStore(), so see if there was an existing
// ViewModelStore from our last NonConfigurationInstance
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
viewModelStore = nc.viewModelStore;
}
}
if (viewModelStore == null && custom == null) {
return null;
}
NonConfigurationInstances nci = new NonConfigurationInstances();
nci.custom = custom;
nci.viewModelStore = viewModelStore;
return nci;
}
对于片段,事情要复杂一些。 FragmentManagerImpl
现在有一个名为mNonConfig
的字段:
private FragmentManagerViewModel mNonConfig;
它存储 Fragment 的 UUID 和ViewModelStore
的 hashmap 。
这个mNonConfig
字段在FragmentManagerImpl#attachController
方法中初始化:
public void attachController(@NonNull FragmentHostCallback host,
@NonNull FragmentContainer container, @Nullable final Fragment parent) {
if (mHost != null) throw new IllegalStateException("Already attached");
mHost = host;
mContainer = container;
mParent = parent;
if (mParent != null) {
// Since the callback depends on us being the primary navigation fragment,
// update our callback now that we have a parent so that we have the correct
// state by default
updateOnBackPressedCallbackEnabled();
}
// Set up the OnBackPressedCallback
if (host instanceof OnBackPressedDispatcherOwner) {
OnBackPressedDispatcherOwner dispatcherOwner = ((OnBackPressedDispatcherOwner) host);
mOnBackPressedDispatcher = dispatcherOwner.getOnBackPressedDispatcher();
LifecycleOwner owner = parent != null ? parent : dispatcherOwner;
mOnBackPressedDispatcher.addCallback(owner, mOnBackPressedCallback);
}
// Get the FragmentManagerViewModel
if (parent != null) {
mNonConfig = parent.mFragmentManager.getChildNonConfig(parent);
} else if (host instanceof ViewModelStoreOwner) {
ViewModelStore viewModelStore = ((ViewModelStoreOwner) host).getViewModelStore();
mNonConfig = FragmentManagerViewModel.getInstance(viewModelStore);
} else {
mNonConfig = new FragmentManagerViewModel(false);
}
}
基本上,为了在Activity
中检索ViewModel
,应该调用ViewModelProviders.of(this).get(SomeViewModel.class)
。 现在,如果我们查看它of
如下所示:
public static ViewModelProvider of(@NonNull FragmentActivity activity,
@Nullable Factory factory) {
Application application = checkApplication(activity);
if (factory == null) {
factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
}
return new ViewModelProvider(activity.getViewModelStore(), factory);
}
因此,重要的部分是这个方法 - activity.getViewModelStore()
因为它为所有ViewModel
对象返回一个包装器 object ( HashMap
持有者),如果它可以在配置更改中幸存下来,那么所有ViewModel
对象也可以:
public ViewModelStore getViewModelStore() {
if (getApplication() == null) {
throw new IllegalStateException("Your activity is not yet attached to the "
+ "Application instance. You can't request ViewModel before onCreate call.");
}
if (mViewModelStore == null) {
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
// Restore the ViewModelStore from NonConfigurationInstances
mViewModelStore = nc.viewModelStore;
}
if (mViewModelStore == null) {
mViewModelStore = new ViewModelStore();
}
}
return mViewModelStore;
}
mViewModelStore
将从NonConfigurationInstances
恢复或从头开始创建。 几乎, NonConfigurationInstances
是 object ,它在配置更改后仍然存在,因此用于存储ViewModelStore
。 这就是为什么旋转后返回相同的ViewModelStore
object - 它存储在与配置更改无关的NonConfigurationInstances
:
如果您查看onRetainNonConfigurationInstance()
,您实际上会发现您的ViewModelStore
保存在那里:
public final Object onRetainNonConfigurationInstance() {
...
NonConfigurationInstances nci = new NonConfigurationInstances();
nci.custom = custom;
nci.viewModelStore = viewModelStore;
return nci;
}
此外,仅当出于非配置更改原因调用onDestroy
时才会清除它:
...
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_DESTROY) {
if (!isChangingConfigurations()) {
getViewModelStore().clear();
}
}
}
});
...
类似的技巧用于存储Fragment
的ViewModel
。
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