ViewModel 如何在配置更改后存活下来

Question

我正在尝试在我的应用程序中使用 ViewModel。我想到的问题是视图模型如何在配置更改后幸存下来。我读了很多博客文章说“

It will create a HolderFragment to add to your activity or your fragment, it's invisible, when the configuration changed, activity destroyed, but holder fragment is still alive

这使得 sense.But 我试图对此进行更多探索，发现在 支持库 27.1.0+ 中，他们删除了 HolderFragment，其描述为

Deprecate ViewModelStores.of() and the HolderFragment it relies on as they are no longer needed link for android.googlesource.

现在的问题是他们现在是如何做同样的事情的？

Answer 1

用 ViewModelProviders.of() 方法创建的 ViewModels 存储在 ViewModelStore hashmap 中，所以真正的问题是如何存储 ViewModelStore。

对于活动来说，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;

其中存储了片段的 UUID 和 ViewModelStore.

的散列图

此 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，要在 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);
}

因此，重要的部分是此方法 - activity.getViewModelStore() 因为它 returns 是所有 ViewModel 对象的包装器对象（HashMap 持有者），如果它可以在配置更改后继续存在，所以您的所有 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 是在配置更改后仍然存在的对象，因此用于存储 ViewModelStore。这就是旋转后返回相同 ViewModelStore 对象的原因 - 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;
}

另外，只有当 onDestroy 由于非配置更改原因被调用时才会被清除：

...
    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();
                }
            }
        }
    });
...

类似的技巧用于为 Fragment 存储 ViewModel。

ViewModel 如何在配置更改后存活下来

How ViewModel survives configuration change

android

mvvm

android-viewmodel

android-architecture-components