ViewModel 如何在配置更改中幸存下来

Question

我正在尝试在我的应用中使用 ViewModel。我想到的问题是视图模型如何在配置更改中幸存下来。我读过很多博客文章说“

它将创建一个 HolderFragment 以添加到您的活动或您的 片段，它是不可见的，当配置改变时，活动 已销毁，但持有者片段还活着

这是有道理的。但我尝试对此进行更多探索，发现在 support library 27.1.0+ 中，他们已删除了带有 Description 的 HolderFragment

弃用 ViewModelStores.of() 和它所依赖的 HolderFragment 因为它们不再需要了 link for android.googlesource。

现在的问题是他们现在如何做同样的事情？

Answer 1

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，事情有点复杂。 FragmentManagerImpl 现在有一个名为 mNonConfig 的字段：

private FragmentManagerViewModel mNonConfig;

其中存储了 Fragment 的 UUID 和 ViewModelStore 的哈希图。

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);
        }
    }

Answer 2

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of，它如下所示：

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);
}

所以，重要的部分是这个方法 - activity.getViewModelStore()，因为它为您的所有 ViewModel 对象返回一个包装器对象（HashMap 持有者），如果它可以在配置更改中幸存下来，那么您的所有 ViewModel对象：

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 是在配置更改后仍然存在的对象，因此用于存储ViewModelStore。这就是为什么在旋转后返回相同的 ViewModelStore 对象 - 它存储在独立于配置更改的 NonConfigurationInstances 中：

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，仅当由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();
                }
            }
        }
    });
...   

类似的技巧用于将ViewModel 存储为Fragment。