通常我们在Activity/Fragment中创建ViewModel使用如下方式:

NameViewModel nameViewModel=new ViewModelProvider(this).get(NameViewModel.class);

其中ViewModelStore对象由Activity/Fragment提供，并且在Activity/Fragment销毁时调用ViewModel的clear方法

那Activity、Fragment如何创建ViewModelStore的呢？让我我们一起来看看源码，解开疑惑

与Activity关联的ViewModelStore对象

ViewModelStore 用于存储ViewModel实例对象，在androidx.activity.ComponentActivity中实现了ViewModelStoreOwner接口，
实现代码如下：

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;
    }

第13行获取上一次的对象，Android系统提供的一种机制，在系统配置（如屏幕有竖屏变为横屏时）改变时，Activity将会重新创建，通过覆盖onRetainNonConfigurationInstance，将viewModelStore返回，确保ViewModelStore不会从新创建，以此保证与该ViewModelStore关联的ViewModel不因配置改变而丢失,onRetainNonConfigurationInstance代码如下

 @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;
    }

从上面的源码分析可知，只要利用相同的Activity实例调用ViewModelProvider(thisAcitivty).get(viewModelClass) 创建的将会是同一个对象，在该Activity关联的Fragment中也一样。

与androidx.fragment.app.Fragment关联的ViewModelStore对象

同样androidx.fragment.app.Fragment也实现了ViewModelStoreOwner接口
源码如下:

    public ViewModelStore getViewModelStore() {
        if (mFragmentManager == null) {
            throw new IllegalStateException("Can't access ViewModels from detached fragment");
        }
        return mFragmentManager.getViewModelStore(this);
    }

可以看出ViewModelStore的获取委托给了FragmentManager实现

继续看 FragmentManager getViewModelStore 的方法

    
    private FragmentManagerViewModel mNonConfig;
    
    ....省略无关代码

    ViewModelStore getViewModelStore(@NonNull Fragment f) {
        return mNonConfig.getViewModelStore(f);
    }

在FragmentManager中通过mNonConfig获取ViewModelStore对象，看看mNonConfig如果创建的：

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;
        ...省略无关代码

        // 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);
        }
    }

这里在创建mNonConfig时有三种情形：

第一种：当parent!=null说明是在Fragment中又启动的Fragment，调用父Fragment的FragmentManager.getChildNonConfig()创建FragmentManagerViewModel，实际是到查找父Fragment关联的FragmentManagerViewModel 的HashMap中是否有子Fragment的FragmentManagerViewModel，没有就创建，并缓存到mChildNonConfigs中

第二种：当host实现了ViewModelStoreOwner接口，这中情况大多数是在Activity中启动了Fragment，host就是Activity，host.getViewModelStore()获取的即是Activity的ViewModelStore。通过FragmentManagerViewModel.getInstance(viewModelStore)创建FragmentManagerViewModel，实际是利用Activity的ViewModelProvider创建，通过分析我们能构发现，创建的FragmentManagerViewModel也在Activity的ViewModelStore中。这样在Activity/Fragment销毁时FragmentManagerViewModel 会跟着Activity/fragment的销毁

第三中：如果非以上两种就直接创建实例并且不保存FragmentManagerViewModel的状态，通常不会到这个分支

FragmentManagerViewModel 数据结构

final class FragmentManagerViewModel extends ViewModel {
    
    private static final ViewModelProvider.Factory FACTORY = new ViewModelProvider.Factory() {
        @NonNull
        @Override
        @SuppressWarnings("unchecked")
        public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
            FragmentManagerViewModel viewModel = new FragmentManagerViewModel(true);
            return (T) viewModel;
        }
    };

    @NonNull
    static FragmentManagerViewModel getInstance(ViewModelStore viewModelStore) {
        ViewModelProvider viewModelProvider = new ViewModelProvider(viewModelStore,
                FACTORY);
        return viewModelProvider.get(FragmentManagerViewModel.class);
    }

    ...省略无关代码
    private final HashMap<String, Fragment> mRetainedFragments = new HashMap<>();
    private final HashMap<String, FragmentManagerViewModel> mChildNonConfigs = new HashMap<>();
    private final HashMap<String, ViewModelStore> mViewModelStores = new HashMap<>();

    ...省略无关代码

    @NonNull
    FragmentManagerViewModel getChildNonConfig(@NonNull Fragment f) {
        FragmentManagerViewModel childNonConfig = mChildNonConfigs.get(f.mWho);
        if (childNonConfig == null) {
            childNonConfig = new FragmentManagerViewModel(mStateAutomaticallySaved);
            mChildNonConfigs.put(f.mWho, childNonConfig);
        }
        return childNonConfig;
    }

    @NonNull
    ViewModelStore getViewModelStore(@NonNull Fragment f) {
        ViewModelStore viewModelStore = mViewModelStores.get(f.mWho);
        if (viewModelStore == null) {
            viewModelStore = new ViewModelStore();
            mViewModelStores.put(f.mWho, viewModelStore);
        }
        return viewModelStore;
    }
}

总结

• ViewModel通过Activity/Fragment提供的ViewModelStore将ViewModel的生命周期跟Activity/Fragment关联起来（ViewModel 的生命周期非常简单就是onCleared方法)。
• ViewModel具有生命范围，Activity范围和Fragment范围