android - ViewModel 如何在配置更改中幸存

Question

我正在尝试在我的应用程序中使用 ViewModel。我想到的问题是 View 模型如何在配置更改中幸存下来。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

这是有道理的。但我尝试对此进行更多探索，发现在支持库 27.1.0+ 中，他们已经删除了 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，因此真正的问题是ViewModelStore如何存储。

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

私有(private) FragmentManagerViewModel mNonConfig;

它存储 Fragment 的 UUID 的 HashMap 和 ViewModelStore。

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);
        }
    }