Android JetPack 深入分析ViewModel源码-编程知识

文章目录

- 前言
- 源码分析
- - ViewModel是如何创建的？
  - - ViewModelProvider(this)做了什么？
    - - 小结
    - get(MyViewModel::class.java)做了什么？
    - - 小结
  - ViewModel是如何实现配置更改后数据恢复的？
  - - - 整体时序图
- 结语

前言

本文主要分析ViewModel相关源码，相关使用不再赘述，可参考Android ViewModel使用；

ViewModel 概览

Google官方给的ViewModel定义如下：

ViewModel类旨在`以注重生命周期的方式存储和管理界面相关数据`。ViewModel类`让数据可在发生屏幕旋转等配置更改后继续留存`。

定义主要提到两个关键点：

生命周期

上图是Google官网提供的ViewModel生命周期图示，可以看到ViewModel的生命周期是从onCreate创建到完成并销毁Finished，开发中经常结合LiveData一起使用；
配置更改后数据留存
我们知道当屏幕旋转的时候，Activity会销毁重建,如果我们想恢复之前的数据，之前一般是通过onSaveInstanceState()来实现，内部是通过Bundle来实现，此方法仅适合可以序列化再反序列化的少量数据，而不适合数量可能较大的数据，而ViewModel可以很简单的实现这个功能，并存储较大的数据；

源码分析

ViewModel是如何创建的？

 	val viewModel = ViewModelProvider(this).get(MyViewModel::class.java)

以上是ViewModel创建代码

ViewModelProvider(this)做了什么？

### ViewModelProvider.javapublic ViewModelProvider(@NonNull ViewModelStoreOwner owner) {this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory(): NewInstanceFactory.getInstance());}

ComponentActivity则实现了ViewModelStoreOwner和HasDefaultViewModelProviderFactory接口；
因此我们只需要关注ComponentActivity中对应的方法；

getViewModelStore()；
getDefaultViewModelProviderFactory()；

我们先分析getViewModelStore:

	### ComponentActivity.getViewModelStorepublic ViewModelStore getViewModelStore() {...ensureViewModelStore();return mViewModelStore;}

可以看到getViewModelStore()调用了ensureViewModelStore()，其对应代码如下：

### ComponentActivity.ensureViewModelStorevoid ensureViewModelStore() {if (mViewModelStore == null) {NonConfigurationInstances nc =(NonConfigurationInstances) getLastNonConfigurationInstance();if (nc != null) {// Restore the ViewModelStore from NonConfigurationInstancesmViewModelStore = nc.viewModelStore;}if (mViewModelStore == null) {mViewModelStore = new ViewModelStore();}}}static final class NonConfigurationInstances {Object custom;ViewModelStore viewModelStore;}

在ensureViewModelStore方法中，会判断mViewModelStore是否为null，如果为null的话，会先尝试调用getLastNonConfigurationInstance()方法获取，如果获取不到，则直接new创建一个ViewModelStore；

我们看下ViewModelStore类:

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

其内部创建了一个HashMap，以ViewModel的CanonicalName为key，ViewModel对象为value，存储Activity中创建的ViewModel对象；

我们再看下getDefaultViewModelProviderFactory()方法：

### ComponentActivity.getDefaultViewModelProviderFactory()public ViewModelProvider.Factory getDefaultViewModelProviderFactory() {...if (mDefaultFactory == null) {mDefaultFactory = new SavedStateViewModelFactory(getApplication(),this,getIntent() != null ? getIntent().getExtras() : null);}return mDefaultFactory;}

主要是创建一个SavedStateViewModelFactory对象返回，我们看下SavedStateViewModelFactory的构造方法

    public SavedStateViewModelFactory(@Nullable Application application,@NonNull SavedStateRegistryOwner owner,@Nullable Bundle defaultArgs) {mSavedStateRegistry = owner.getSavedStateRegistry();mLifecycle = owner.getLifecycle();mDefaultArgs = defaultArgs;mApplication = application;mFactory = application != null? ViewModelProvider.AndroidViewModelFactory.getInstance(application): ViewModelProvider.NewInstanceFactory.getInstance();}

我们看下mFactory的创建，application 肯定不为null，因此会调用ViewModelProvider.AndroidViewModelFactory.getInstance(application)返回一个AndroidViewModelFactory对象

    public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {private static AndroidViewModelFactory sInstance;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@Overridepublic <T extends ViewModel> T create(@NonNull Class<T> modelClass) {if (AndroidViewModel.class.isAssignableFrom(modelClass)) {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);}}

可以看到AndroidViewModelFactory主要是用来生产ViewModel的，其create()方法：根据传入的ViewModel类型【继承自AndroidViewModel或ViewModel】，通过反射创建ViewModel对象；

小结

调用ViewModelProvider(this)方法主要做了以下两件事：

获取ViewModelStore对象，如果ViewModelStore为null，会先尝试调用getLastNonConfigurationInstance()方法获取，获取不到则直接通过new创建，内部通过HashMap实现ViewModel对象的存储；
创建AndroidViewModelFactory对象，其内部提供了create()，使用反射创建对应的ViewModel对象；

get(MyViewModel::class.java)做了什么？

	### ViewModelProvider.javapublic <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@MainThreadpublic <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {ViewModel viewModel = mViewModelStore.get(key);if (modelClass.isInstance(viewModel)) {if (mFactory instanceof OnRequeryFactory) {((OnRequeryFactory) mFactory).onRequery(viewModel);}return (T) viewModel;} else {if (viewModel != null) {}}if (mFactory instanceof KeyedFactory) {viewModel = ((KeyedFactory) mFactory).create(key, modelClass);} else {viewModel = mFactory.create(modelClass);}mViewModelStore.put(key, viewModel);return (T) viewModel;}

小结

get(MyViewModel::class.java)方法会根据传入的ViewModel的CanonicalName，先从mViewModelStore缓存中查找，如果找到，则直接返回，如果没找到，则调用前面创建的mFactory.create()方法通过反射创建对应的ViewModel，并添加到 mViewModelStore缓存中；

以上便是ViewModel相关的创建逻辑源码分析！

ViewModel是如何实现配置更改后数据恢复的？

我们知道当屏幕旋转配置发生变化时，会调用onDestroy(),在ComponentActivity构造方法中：

   getLifecycle().addObserver(new LifecycleEventObserver() {@Overridepublic void onStateChanged(@NonNull LifecycleOwner source,@NonNull Lifecycle.Event event) {if (event == Lifecycle.Event.ON_DESTROY) {// Clear out the available contextmContextAwareHelper.clearAvailableContext();// And clear the ViewModelStoreif (!isChangingConfigurations()) {getViewModelStore().clear();}}}});

当监听到Activity调用ON_DESTROY时，会调用getViewModelStore().clear()，清除ViewModelStore内部缓存的ViewModel对象，那为何重新创建后，ViewModel中持有的数据又没有丢失呢？

当Activity销毁时会调用ActivityThread.handleDestroyActivity()方法：

  public void handleDestroyActivity(IBinder token, boolean finishing, int configChanges,boolean getNonConfigInstance, String reason) {ActivityClientRecord r = performDestroyActivity(token, finishing,configChanges, getNonConfigInstance, reason);if (r != null) {...}mSomeActivitiesChanged = true;}

handleDestroyActivity方法中又会调用performDestroyActivity()方法：

  ActivityClientRecord performDestroyActivity(IBinder token, boolean finishing,int configChanges, boolean getNonConfigInstance, String reason) {...ActivityClientRecord r = mActivities.get(token);if (getNonConfigInstance) {try {r.lastNonConfigurationInstances= r.activity.retainNonConfigurationInstances();} catch (Exception e) {if (!mInstrumentation.onException(r.activity, e)) {throw new RuntimeException("Unable to retain activity "+ r.intent.getComponent().toShortString()+ ": " + e.toString(), e);}}}...return r;}

performDestroyActivity方法中会调用activity.retainNonConfigurationInstances()

    NonConfigurationInstances retainNonConfigurationInstances() {Object activity = onRetainNonConfigurationInstance();...return nci;}

retainNonConfigurationInstances()方法中又调用onRetainNonConfigurationInstance(),其在ComponentActivity中具体实现代码如下：

    public final Object onRetainNonConfigurationInstance() {Object custom = onRetainCustomNonConfigurationInstance();ViewModelStore viewModelStore = mViewModelStore;if (viewModelStore == null) {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;}

可以看到会将viewModelStore保存到NonConfigurationInstances对象里返回，结合performDestroyActivity源码，最终会保存到ActivityClientRecord.lastNonConfigurationInstances属性中；

当Activity重新创建的时候，会执行ActivityThread.performLaunchActivity()方法：

    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {...activity.attach(appContext, this, getInstrumentation(), r.token,r.ident, app, r.intent, r.activityInfo, title, r.parent,r.embeddedID, r.lastNonConfigurationInstances, config,r.referrer, r.voiceInteractor, window, r.configCallback,r.assistToken);...return activity;}

会调用 activity.attach()方法将lastNonConfigurationInstances对象传入,并赋值给Activity中的mLastNonConfigurationInstances属性；同时在ComponentActivity的构造方法中找到如下代码：

        getLifecycle().addObserver(new LifecycleEventObserver() {@Overridepublic void onStateChanged(@NonNull LifecycleOwner source,@NonNull Lifecycle.Event event) {ensureViewModelStore();getLifecycle().removeObserver(this);}});

而ensureViewModelStore()方法上面我们已经看到，内部会先调用getLastNonConfigurationInstance()方法获取mViewModelStore对象，getLastNonConfigurationInstance()方法返回的正是mLastNonConfigurationInstances属性；从而实现ViewModel数据的恢复！