Android ViewModel如何经受配置更改

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经受住配置更改。我读了很多博客帖子，上面说“

它将创建一个HolderFragment，添加到您的活动或活动中 片段，它是不可见的，当配置更改时，活动 摧毁，但持有者碎片仍然活着

这是有道理的。但是我试图对此进行更多的探索，发现在支持库27.1.0+中，他们删除了HolderFragment，并使用了描述

弃用ViewModelStores.of（）及其所依赖的HolderFragment 因为不再需要它们

---

现在的问题是他们现在是如何做同样的事情的？

使用

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，此逻辑非常简单。

ViewModelStore

使用

onretainonconfigurationinstance

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;
    }

    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);
        }
    }

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；

它存储片段的UUID的hashmap和

ViewModelStore

此

mNonConfig

字段在

FragmentManagerImpl#attachController

方法中初始化：

@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;
    }

    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

活动中检索
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，重要的部分是这个方法-
activity.getViewModelStore（）
，因为它为所有
ViewModel
对象返回一个包装器对象（
HashMap
holder），如果它能够在配置更改后存活，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
还原或从头创建。基本上，
NonConfigurationInstances
是在配置更改后仍然存在的对象，因此用于存储
ViewModelStore
。这就是返回相同
ViewModelStore
对象的原因ter rotation-它存储在独立于配置更改的
非配置实例中
：

如果查看
onretainonconfigurationinstance（）
，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，仅当调用非配置更改原因的
onDestroy
时，才会清除：

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

类似的技巧用于存储
片段的
视图模型
，
很好的答案，但是
OnRetainonConfiguration实例
在API 15中不是被去除了吗？