提纲

1. ViewModel是什么
2. ViewModel的优势
3. ViewModel基本用法
4. ViewModel相关方法说明及实现原理

一、ViewModel是什么

ViewModel 具备宿主生命周期感知能力的数据存储组件，使用ViewModel保存的数据，在页面因配置变更导致页面销毁重建之后依然也是存在的。

配置变更：横竖屏切换、分辨率调整、权限变更、系统字体样式变更等

二、ViewModel的优势

2.1 页面配置更改数据不丢失

当设备因配置更改导致 Activity/Fragment 重建，ViewModel 中的数据并不会因此而丢失；
配合 LiveData 可以在页面重建后立马能收到最新保存的数据用以重新渲染页面。

2.2 生命周期感应

当在 ViewModel 中做一些网络请求或数据的处理时，可以复写 onCleared() 方法，终止清理一些操作，释放内存。该方法在宿主 onDestroy 时被调用。

2.2 数据共享

• Activity + Fragment 的页面，可以使用 ViewModel 实现页面之间的数据共享；
• 不同的 Activity也可以实现数据共享。

三、ViewModel基本用法

data class MyData(var name:String,var age:Int,var sex:Int)

class MyModel : ViewModel(){
    private val liveData = MutableLiveData<List<MyData>>()

    fun loadData(): LiveData<List<MyData>> {
        if (liveData.value == null){
            val list = fromRemote()
            liveData.postValue(list)
        }
        return liveData
    }

    fun fromRemote() = listOf(MyData("1",1,1),MyData("2",2,2))
}

class MainActivity2 : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main2)

        val viewModel = ViewModelProvider(this).get(MyModel::class.java)
        viewModel.loadData().observe(this, Observer {
            // TODO: do something
        })
    }
}

四、ViewModel相关方法说明及实现原理

4.1 相关方法说明

4.1.1 ViewModelProvider(@NonNull ViewModelStoreOwner owner) 构造方法

   public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
        this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
                ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
                : NewInstanceFactory.getInstance());
    } 
    
    // 赋值mFactory和mViewModelStore
    public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
        mFactory = factory;
        mViewModelStore = store;
    }

4.1.2 T get(@NonNull String key, @NonNull Class<T> modelClass) 获取ViewModel实例

    @NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull Class<T> modelClass) {
        String canonicalName = modelClass.getCanonicalName();
        if (canonicalName == null) {
            throw new IllegalArgumentException("Local and anonymous classes can not be ViewModels");
        }
        return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
    } 

    @NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
        // 通过key在ViewModelStore中取Value(ViewModel)
        // ViewModelStore是真正存储ViewModel的地方，其内部维护了一个HashMap<String,ViewModel>
        ViewModel viewModel = mViewModelStore.get(key);
        // viewModel是否存在
        if (modelClass.isInstance(viewModel)) {
            if (mFactory instanceof OnRequeryFactory) {
                ((OnRequeryFactory) mFactory).onRequery(viewModel);
            }
            // 存在 返回
            return (T) viewModel;
        } else {
            //noinspection StatementWithEmptyBody
            if (viewModel != null) {
                // TODO: log a warning.
            }
        }
        // 不存在 创建viewModel实例
        if (mFactory instanceof KeyedFactory) {
            viewModel = ((KeyedFactory) (mFactory)).create(key, modelClass);
        } else {
            viewModel = (mFactory).create(modelClass);
        }
        // 添加到Map中
        mViewModelStore.put(key, viewModel);
        // 返回
        return (T) viewModel;
    }

4.2 实现原理

ViewModel 可以实现因配置变更导致页面销毁重建之后依然可以复用。准确点来说，应该是页面恢复重建前后获取到的是同一个 ViewModel 实例对象。
ViewModelProvider本质是从传递进去的 ViewModelStore来获取实例。如果没有传递，则利用 factory 去创建一个新的，并存储到 ViewModelStore。

4.2.1 ViewModelStoreOwner在Activity中的实现

   public class ComponentActivity extends androidx.core.app.ComponentActivity implements ViewModelStoreOwner,... {

    static final class NonConfigurationInstances {
        Object custom;
        ViewModelStore viewModelStore;
    }

    // Lazily recreated from NonConfigurationInstances by getViewModelStore()
    private ViewModelStore mViewModelStore;

    /**
     * 应为配置变更导致的act重建时回调的方法
     * getLastNonConfigurationInstance() 方法，是在窗口重建后获取重建前保存的数据
     */
    @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;
        }
        // 将viewModelStore包装成NonConfigurationInstances对象中保存
        NonConfigurationInstances nci = new NonConfigurationInstances();
        nci.custom = custom;
        nci.viewModelStore = viewModelStore;
        return nci;
    }

    @NonNull
    @Override
    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.");
        }
        // mViewModelStore为null，可能窗口已经被重建
        if (mViewModelStore == null) {
            // 获取重建前保存的NonConfigurationInstances对象，从而获取mViewModelStore，并赋值当前mViewModelStore
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            // 如果还为null，说明窗口是第一次被创建
            if (mViewModelStore == null) {
                // 直接new一个对象
                mViewModelStore = new ViewModelStore();
            }
        }
        // 返回ViewModelStore
        return mViewModelStore;
    }
} 

4.2.2 ViewModelStoreOwner在Fragment中的实现

# Fragment
public class Fragment implements ViewModelStoreOwner {
    // Internal unique name for this fragment;
    @NonNull
    String mWho = UUID.randomUUID().toString();

    // The fragment manager we are associated with.  Set as soon as the
    // fragment is used in a transaction; cleared after it has been removed
    // from all transactions.
    FragmentManagerImpl mFragmentManager;

    // Host this fragment is attached to.
    FragmentHostCallback mHost;

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

# FragmentManagerImpl
/**
 * Container for fragments associated with an activity.
 */
final class FragmentManagerImpl extends FragmentManager implements LayoutInflater.Factory2 {
    FragmentHostCallback mHost;
    private FragmentManagerViewModel mNonConfig;

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

    @NonNull
    FragmentManagerViewModel getChildNonConfig(@NonNull Fragment f) {
        return mNonConfig.getChildNonConfig(f);
    }
}

# FragmentManagerViewModel
/**
 * FragmentManagerViewModel is the always up to date view of the Fragment's
 * non configuration state
 */
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);
    }
    
    @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;
    }
}