ViewModel源码分析 ViewModel源码分析

一、ViewModel的简介
2018年谷歌IO大会上正式发布了JetPack组件，其中包括Databing、Lifecycles、LiveData、Navigation、Paging、Room、ViewModel、WorkManager等一系列框架。并且发布androidx包，这些框架的最新版本组件将引入到androidx包下。
JetPack架构组件的发布，意味着google终于拿出了官方推荐的Android开发架构，一直以来在Android开发上老生常谈三种开发模式，MVC、MVP、MVVM。整体的思想就是做一些代码封装和操作上的分离，具体三种架构的区别不在这详细描述。说一点儿个人对这三中开发模式的理解。
MVC：Activity承担C的任务，强行carry大量业务和数据代码。
MVP：Presenter承担主要代码任务，连接数据层和视图层的交互。
MVVM：Model负责数据、View负责展示、ViewModel负责Model和View的交互，同时，最好的一点是View层自动监听ViewModel的数据变化。
在JetPack组件出现之前，一直以来构建MVVM架构的方式都是代用Databing来充当ViewModel，而Databing是采用标签方式写入布局文件中的，这样一来当出现问题的时候不是太好进行调试。所以ViewMode的出现对于MVVM模式开发具有重大的意义。
二、ViewModel的用法和特点
ViewModel的使用是要结合LiveData框架进行的，LiveData框架也是JetPack组件的一部分，这里先不详细进行介绍。ViewModel有两个特点，一是更加方便的保存数据，第二个特点，也是最重要的特点，就是保证数据不受Activity的销毁重建所影响，当Activity销毁重建后仍然能收到之前的数据。
1）继承ViewModel

class MainViewModel : ViewModel() { private val repertory: MainRepository by lazy { MainRepository() } var data: MutableLiveData = MutableLiveData() fun getDataFromServer(){ repertory.getDataFromServer(data) } }

ViewModel中持有LiveData，LiveData是ViewModel持有数据的载体。
2）Activity通过ViewModelProviders获取ViewModel

class MainActivity : AppCompatActivity() { private lateinit var mModel: MainViewModeloverride fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) initData() }private fun initData() { mModel = ViewModelProviders.of(this)[MainViewModel::class.java] mModel.data?.observe(this, Observer { val mainAdapter = MainAdapter(this, it) val linearLayoutManager = LinearLayoutManager(this) rv.layoutManager = linearLayoutManager rv.adapter = mainAdapter }) mModel.getDataFromServer() } }

3）通过ViewModel中的方法获取数据然后改变LiveData状态，通过响应式的方式通知到Activity进行视图更新。用法上比较简单。
三、源码分析
个人认为对于一个框架的学习仅仅掌握框架的用法是不够的，因为这太简单了，曾经看到过一篇博客，里面有一句话说的很好，”如果你只会用这个框架，而不清楚其中的原理和设计，那么即便这个框架性能再好，再牛逼，那也是写框架的那个人牛逼，而不是你牛逼“。我认为这句话说的很好，所以当我们学习一中新框架的时候，不能单单只会用，一定要清楚其中的原理，尽量去理解作者的设计思想，就像品茶一样，这样才能品出里面的滋味。不扯闲篇了，由于作者能力也有限，所以以上和以下分析，如有不同观点或作者描述有误请给予批评指正，欢迎来喷。
1）先从获取ViewModel的类入手，”ViewModelProviders“从类名中也能分析出这个类是用于提供ViewModel的。源码如下：

public class ViewModelProviders { public ViewModelProviders() { }private static Application checkApplication(Activity activity) { Application application = activity.getApplication(); if (application == null) { throw new IllegalStateException("Your activity/fragment is not yet attached to " + "Application. You can't request ViewModel before onCreate call."); } return application; }private static Activity checkActivity(Fragment fragment) { Activity activity = fragment.getActivity(); if (activity == null) { throw new IllegalStateException("Can't create ViewModelProvider for detached fragment"); } return activity; } @NonNull @MainThread public static ViewModelProvider of(@NonNull Fragment fragment) { ViewModelProvider.AndroidViewModelFactory factory = ViewModelProvider.AndroidViewModelFactory.getInstance( checkApplication(checkActivity(fragment))); return new ViewModelProvider(ViewModelStores.of(fragment), factory); } @NonNull @MainThread public static ViewModelProvider of(@NonNull FragmentActivity activity) { ViewModelProvider.AndroidViewModelFactory factory = ViewModelProvider.AndroidViewModelFactory.getInstance( checkApplication(activity)); return new ViewModelProvider(ViewModelStores.of(activity), factory); } @NonNull @MainThread public static ViewModelProvider of(@NonNull Fragment fragment, @NonNull Factory factory) { checkApplication(checkActivity(fragment)); return new ViewModelProvider(ViewModelStores.of(fragment), factory); } @NonNull @MainThread public static ViewModelProvider of(@NonNull FragmentActivity activity, @NonNull Factory factory) { checkApplication(activity); return new ViewModelProvider(ViewModelStores.of(activity), factory); } @SuppressWarnings("WeakerAccess") @Deprecated public static class DefaultFactory extends ViewModelProvider.AndroidViewModelFactory { /** * Creates a {@code AndroidViewModelFactory} * * @param application an application to pass in {@link AndroidViewModel} * @deprecated Use {@link ViewModelProvider.AndroidViewModelFactory} or * {@link ViewModelProvider.AndroidViewModelFactory#getInstance(Application)}. */ @Deprecated public DefaultFactory(@NonNull Application application) { super(application); } } }

我们从中可以看到，这个类中的方法全是静态的，说明这个类相当于一个工具类，从of方法中我们可以看到真正实现ViewModel创建的类是ViewModelProvider，而这个类的构建又需要通过ViewModelStores的of方法获取，ViewModelStores的代码如下：

public class ViewModelStores { private ViewModelStores() { }@NonNull @MainThread public static ViewModelStore of(@NonNull FragmentActivity activity) { if (activity instanceof ViewModelStoreOwner) { return ((ViewModelStoreOwner) activity).getViewModelStore(); } return holderFragmentFor(activity).getViewModelStore(); }@NonNull @MainThread public static ViewModelStore of(@NonNull Fragment fragment) { if (fragment instanceof ViewModelStoreOwner) { return ((ViewModelStoreOwner) fragment).getViewModelStore(); } return holderFragmentFor(fragment).getViewModelStore(); } }

从这个类中可以发现获取ViewModelStore是通过构建一个HoldFragment，而这个HoldFragment的作用就是持有ViewModelStore的引用，然后将这个Fragment添加到Activity中，而且这个Fragmet有一个重要的特点，在实例化的时候会调用setRetainInstance(true)方法，这个方法的作用是让Fragment不受Activity销毁重建影响，这样一来就能保证ViewModel不会由于Activity的销毁重建导致数据丢失，这是ViewModel的一个重要特性。这种设计确实很巧妙，HolderFragment的代码如下：

@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP) public class HolderFragment extends Fragment implements ViewModelStoreOwner { .......省略部分代码public HolderFragment() { setRetainInstance(true); }@Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); sHolderFragmentManager.holderFragmentCreated(this); } @Override public void onSaveInstanceState(Bundle outState) { super.onSaveInstanceState(outState); } @Override public void onDestroy() { super.onDestroy(); mViewModelStore.clear(); } @NonNull @Override public ViewModelStore getViewModelStore() { return mViewModelStore; } /** * @hide */ @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP) public static HolderFragment holderFragmentFor(FragmentActivity activity) { return sHolderFragmentManager.holderFragmentFor(activity); }/** * @hide */ @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP) public static HolderFragment holderFragmentFor(Fragment fragment) { return sHolderFragmentManager.holderFragmentFor(fragment); } .....省略部分代码 private static HolderFragment createHolderFragment(FragmentManager fragmentManager) { HolderFragment holder = new HolderFragment(); fragmentManager.beginTransaction().add(holder, HOLDER_TAG).commitAllowingStateLoss(); return holder; }HolderFragment holderFragmentFor(FragmentActivity activity) { FragmentManager fm = activity.getSupportFragmentManager(); HolderFragment holder = findHolderFragment(fm); if (holder != null) { return holder; } holder = mNotCommittedActivityHolders.get(activity); if (holder != null) { return holder; }if (!mActivityCallbacksIsAdded) { mActivityCallbacksIsAdded = true; activity.getApplication().registerActivityLifecycleCallbacks(mActivityCallbacks); } holder = createHolderFragment(fm); mNotCommittedActivityHolders.put(activity, holder); return holder; }HolderFragment holderFragmentFor(Fragment parentFragment) { FragmentManager fm = parentFragment.getChildFragmentManager(); HolderFragment holder = findHolderFragment(fm); if (holder != null) { return holder; } holder = mNotCommittedFragmentHolders.get(parentFragment); if (holder != null) { return holder; }parentFragment.getFragmentManager() .registerFragmentLifecycleCallbacks(mParentDestroyedCallback, false); holder = createHolderFragment(fm); mNotCommittedFragmentHolders.put(parentFragment, holder); return holder; } }

之后来到ViewModelProvider，代码量不是很多，但是里面的设计确实很有意思。首先来分析它的成员变量，成员变量只有两个，Factory从字面的意思就能理解，它是生产ViewModel的工厂，所以这里用到了工厂模式。ViewModelStore它用来存储ViewModel，相当于ViewModel的缓存。内部是一个HashMap，key是ViewModel的类名称。ViewModelProvider代码如下：

public class ViewModelProvider { ....省略部分代码 private final Factory mFactory; private final ViewModelStore mViewModelStore; ...... public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) { this(owner.getViewModelStore(), factory); }public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) { mFactory = factory; this.mViewModelStore = store; }public T get(@NonNull Class 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); } public T get(@NonNull String key, @NonNull Class modelClass) { ViewModel viewModel = mViewModelStore.get(key); if (modelClass.isInstance(viewModel)) { //noinspection unchecked return (T) viewModel; } else { //noinspection StatementWithEmptyBody if (viewModel != null) { // TODO: log a warning. } }viewModel = mFactory.create(modelClass); mViewModelStore.put(key, viewModel); //noinspection unchecked return (T) viewModel; .......省略部分代码 }

2）Factory是ViewModelProvider中的一个接口，定义如下：

public interface Factory { @NonNull T create(@NonNull Class modelClass); }

通过create方法创建ViewModel。Factory有两个实现类AndroidViewModelFactory、NewInstanceFactory都是ViewModelProvider中的静态内部类。同时AndroidViewModelFactory又继承于NewInstanceFactory。有一种依赖注入的思想在里面。接下来我们来分析两个Factory的实现类。NewInstanceFactory最终通过newinstance方法创建ViewModel实例。

public static class NewInstanceFactory implements Factory { public T create(@NonNull Class modelClass) { //noinspection TryWithIdenticalCatches try { return modelClass.newInstance(); } catch (InstantiationException e) { throw new RuntimeException("Cannot create an instance of " + modelClass, e); } catch (IllegalAccessException e) { throw new RuntimeException("Cannot create an instance of " + modelClass, e); } } } public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {private static AndroidViewModelFactory sInstance; @NonNull public static AndroidViewModelFactory getInstance(@NonNull Application application) { if (sInstance == null) { sInstance = new AndroidViewModelFactory(application); } return sInstance; } private Application mApplication; public AndroidViewModelFactory(@NonNull Application application) { mApplication = application; }@NonNull @Override public T create(@NonNull Class modelClass) { if (AndroidViewModel.class.isAssignableFrom(modelClass)) { //noinspection TryWithIdenticalCatches try { return modelClass.getConstructor(Application.class).newInstance(mApplication); } catch (NoSuchMethodException e) { throw new RuntimeException("Cannot create an instance of " + modelClass, e); } catch (IllegalAccessException e) { throw new RuntimeException("Cannot create an instance of " + modelClass, e); } catch (InstantiationException e) { throw new RuntimeException("Cannot create an instance of " + modelClass, e); } catch (InvocationTargetException e) { throw new RuntimeException("Cannot create an instance of " + modelClass, e); } } return super.create(modelClass); } }

最后，真正存储ViewModel的容器是ViewModelStore，ViewModelStore中维护了一个HashMap，代码如下：

public class ViewModelStore {private final HashMap mMap = new HashMap<>(); final void put(String key, ViewModel viewModel) { ViewModel oldViewModel = mMap.get(key); if (oldViewModel != null) { oldViewModel.onCleared(); } mMap.put(key, viewModel); }final ViewModel get(String key) { return mMap.get(key); }/** *Clears internal storage and notifies ViewModels that they are no longer used. */ public final void clear() { for (ViewModel vm : mMap.values()) { vm.onCleared(); } mMap.clear(); }

}
【ViewModel源码分析】文章代码库：https://github.com/24KWYL/MVVM.git