# Android Jetpack系列–3.ViewModel使用及源码解析

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原文：https://www.dandroid.cn/archives/19311
Canonical：https://www.dandroid.cn/archives/19311
作者：admin
发布日期：2023-09-10
最后更新：2023-09-10 03:32
分类：Jetpack组件 / ViewModel
内容类型：post
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## 正文

ViewModel

 LiveData和ViewModel是一对好搭档

定义

 视图模型，以感知生命周期的形式来存储和管理视图相关的数据，让数据可在发生屏幕旋转等配置更改后继续留存；

特点

 页面数据丢失（转屏、闪退等生命周期重建现象），可以使用onSaveInstanceState()保存数据，单仅适用于数据量少(IPC对Bundle有1M的限制)，且需要支持序列化，而ViewModel对数据量和序列化均没有要求（ViewModel生命周期长于Activity）；

 有效的将逻辑代码和视图控制器分开，防止视图控制器臃肿；

 逻辑层往往持有UI层引用并进行异步调用，而UI需要管理这些请求，确保界面销毁后不会存在内存泄露，而ViewModel可以避免（ViewModel不持有UI层引用）；

生命周期

 ViewModelScope

 Activity created

 onCreated()

 onStart()

 onResume()

 Activity rotated

 onPause()

 onStop()

 onDestroy()

 onCreated()

 onStart()

 onResume()

 Activity finish()

 onPause()

 onStop()

 onDestroy()

 onCleared

 Activity finished

简单使用

//1. 自定义ViewModel
//class MyViewModel(application: Application) : AndroidViewModel(application) {
class UserViewModel : ViewModel() {
 var userName: MutableLiveData = MutableLiveData()
 var isLoading: MutableLiveData = MutableLiveData()
 //2. 获取数据的异步请求
 fun getUserName() {
 isLoading.value = true
 GlobalScope.launch {
 delay(1000)
 //3.使用LiveData将数据抛出
 isLoading.postValue(false)
 userName.postValue("欢迎关注公众号：今阳说")
 }
 }
}

class ViewModelActivity : AppCompatActivity() {
 override fun onCreate(savedInstanceState: Bundle?) {
 super.onCreate(savedInstanceState)
 setContentView(R.layout.activity_view_model)
 //4. 获取ViewModel实例
 val userViewModel: UserViewModel = ViewModelProvider(
 this, ViewModelProvider.NewInstanceFactory()
// this, ViewModelProvider.AndroidViewModelFactory(application)
 ).get(UserViewModel::class.java)
 //5. 观察ViewModel中的LiveData数据，更新UI
 userViewModel.userName.observe(this, Observer {
 tv_view_model.text=it
 })
 userViewModel.isLoading.observe(this, Observer {
 progressBar.visibility=if (it) View.VISIBLE else View.GONE
 })
 userViewModel.getUserName()
 }
}

 还可以用来在不同Fragment/Activity之间共享数据，类似于RxBus等事件总线，都是基于观察者模式

class SharedViewModel :ViewModel(){
 var isReadFinish: MutableLiveData = MutableLiveData()
}
class DetailFragment : Fragment(){
 fun onReadFinish(){
 val viewModel=ViewModelProvider(
 this, ViewModelProvider.NewInstanceFactory()
 ).get(SharedViewModel::class.java)
 viewModel.isReadFinish.value=true
 }
}
class ListFragment : Fragment(){
 override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
 super.onViewCreated(view, savedInstanceState)
 val viewModel=ViewModelProvider(
 this, ViewModelProvider.NewInstanceFactory()
 ).get(SharedViewModel::class.java)
 viewModel.isReadFinish.observe(this, Observer {
 //更新列表UI
 })
 }
}

ViewModel原理解析

ViewModel类

 先看一下ViewModel类的代码

public abstract class ViewModel {
 // Can't use ConcurrentHashMap, because it can lose values on old apis (see b/37042460)
 @Nullable
 private final Map mBagOfTags = new HashMap<>();
 private volatile boolean mCleared = false;

 /**
 * This method will be called when this ViewModel is no longer used and will be destroyed.
 *

* It is useful when ViewModel observes some data and you need to clear this subscription to * prevent a leak of this ViewModel. */ @SuppressWarnings("WeakerAccess") protected void onCleared() { //在ViewModel将被清除时调用 //当ViewModel观察了一些数据，可以在这里做解注册 防止内存泄漏 } //在ViewModel将被清除时调用 @MainThread final void clear() { mCleared = true; // Since clear() is final, this method is still called on mock objects // and in those cases, mBagOfTags is null. It'll always be empty though // because setTagIfAbsent and getTag are not final so we can skip // clearing it if (mBagOfTags != null) { synchronized (mBagOfTags) { for (Object value : mBagOfTags.values()) { // see comment for the similar call in setTagIfAbsent closeWithRuntimeException(value); } } } onCleared(); } ... }

ViewModelProvider类

 使用ViewModel时我们是通过ViewModelProvider获取其实例对象的，那么我们来看一下ViewModelProvider的构造方法;

 由下面代码可以看出就是通过反射生成ViewModel的实现类；

public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) {
 this(owner.getViewModelStore(), factory);
}

public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
 mFactory = factory;
 mViewModelStore = store;
}

ViewModelProvider如何通过ViewModelStore实现配置更改重建后ViewModel依然存在的

 上面ViewModelProvider构造方法的第一个参数通过owner.getViewModelStore()获取ViewModelStore对象，那么我们来看一下ComponentActivity中对ViewModelStoreOwner的实现:

@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.");
 }
 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;
}

 上面代码先从NonConfigurationInstance从获取ViewModelStore实例，如果不存在，就new ViewModelStore();

 而在onRetainNonConfigurationInstance()方法中会把mViewModelStore赋值给NonConfigurationInstances;

 在Activity因配置改变 而正要销毁时，且新Activity会立即创建，那么系统就会调用此方法。

 也就说配置改变时系统把viewModelStore存在了NonConfigurationInstances中。

@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;
}

 可以看到getViewModelStore和onRetainNonConfigurationInstance中都是通过getLastNonConfigurationInstance获取NonConfigurationInstances对象的

@Nullable
public Object getLastNonConfigurationInstance() {
 return mLastNonConfigurationInstances != null
 ? mLastNonConfigurationInstances.activity : null;
}

 那么mLastNonConfigurationInstances是哪来的呢？是Activity的attach方法

 attach方法是为Activity关联上下文环境，是在Activity 启动的核心流程——ActivityThread的performLaunchActivity方法中调用，这里的lastNonConfigurationInstances是存在 ActivityClientRecord中的一个组件信息;

 那么ActivityThread 中的 ActivityClientRecord 是不受 activity 重建的影响，那么ActivityClientRecord中lastNonConfigurationInstances也不受影响，那么其中的Object activity也不受影响，那么ComponentActivity中的NonConfigurationInstances的viewModelStore不受影响，那么viewModel也就不受影响了。

final void attach(Context context, ActivityThread aThread, ...
 NonConfigurationInstances lastNonConfigurationInstances,... ) {
 ...
 mLastNonConfigurationInstances = lastNonConfigurationInstances;
 ...
}

ViewModelProvider的Factory如何使用

 ViewModelProvider构造方法的第二个入参Factory，在ViewModelProvider中已经实现了两个供我们使用：NewInstanceFactory和AndroidViewModelFactory

/**
 * Simple factory, which calls empty constructor on the give class.
 */
public static class NewInstanceFactory implements Factory {

 @SuppressWarnings("ClassNewInstance")
 @NonNull
 @Override
 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);
 }
 }
}

/**
 * {@link Factory} which may create {@link AndroidViewModel} and
 * {@link ViewModel}, which have an empty constructor.
 */
public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {

 private static AndroidViewModelFactory sInstance;

 /**
 * Retrieve a singleton instance of AndroidViewModelFactory.
 *
 * @param application an application to pass in {@link AndroidViewModel}
 * @return A valid {@link AndroidViewModelFactory}
 */
 @NonNull
 public static AndroidViewModelFactory getInstance(@NonNull Application application) {
 if (sInstance == null) {
 sInstance = new AndroidViewModelFactory(application);
 }
 return sInstance;
 }

 private Application mApplication;

 /**
 * Creates a {@code AndroidViewModelFactory}
 *
 * @param application an application to pass in {@link AndroidViewModel}
 */
 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);
 }
}

 如果需要参数，也可以自己实现Factory,例如：

public class MainViewModelFactory implements ViewModelProvider.Factory {
 private String mValue;

 public MainViewModelFactory(String value){
 mValue = value;
 }

 @NonNull
 @Override
 public T create(@NonNull Class modelClass) {
 try {
 Class[] parameterTypeArray = new Class[]{String.class};
 return modelClass.getConstructor(parameterTypeArray).newInstance(mValue);
 } catch (IllegalAccessException e) {
 e.printStackTrace();
 } catch (InstantiationException e) {
 e.printStackTrace();
 } catch (NoSuchMethodException e) {
 e.printStackTrace();
 } catch (InvocationTargetException e) {
 e.printStackTrace();
 }
 return null;

 }
}
//或者多个参数的
class ParametrizedFactory implements ViewModelProvider.Factory {
 private final Object[] mConstructorParams;

 ParametrizedFactory(Object... constructorParams) {
 mConstructorParams = constructorParams;
 }

 @Override
 public T create(Class modelClass) {
 if (modelClass == null) {
 throw new IllegalArgumentException("Target ViewModel class can not be null")
 }
 Log.w("ParametrizedFactory", "Don't use callbacks or Context parameters in order to avoid leaks!!")
 try {
 if (mConstructorParams == null || mConstructorParams.length == 0) {
 return modelClass.newInstance();
 } else {
 Class>[] classes = new Class>[mConstructorParams.length];
 for (int i = 0; i < mConstructorParams.length; i++) {
 classes[i] = mConstructorParams[i].getClass();
 }
 return modelClass.getConstructor(classes).newInstance(mConstructorParams);
 }
 } catch (InstantiationException e) {
 e.printStackTrace();
 } catch (IllegalAccessException e) {
 e.printStackTrace();
 } catch (NoSuchMethodException e) {
 e.printStackTrace();
 } catch (InvocationTargetException e) {
 e.printStackTrace();
 }
 return null;
 }
}

ViewModelProvider的get()是如何获取ViewModel的

 下面让我们再看一下ViewModelProvider的get()是如何实现的

@NonNull
@MainThread
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);
}

//带key创建ViewModel，会让使用相同key的Activity或者Fragment，创建的ViewModel数据独立
@NonNull
@MainThread
public T get(@NonNull String key, @NonNull Class modelClass) {
 //1.通过key在ViewModelStore中取得ViewModel;
 //2.如果这个ViewModel能转换为modelClass类的对象，直接返回该ViewModel;
 //3.否则会通过Factory创建一个ViewModel，并将其存储到ViewModelStore中;
 ViewModel viewModel = mViewModelStore.get(key);
 if (modelClass.isInstance(viewModel)) {
 //noinspection unchecked
 return (T) viewModel;
 } else {
 //noinspection StatementWithEmptyBody
 if (viewModel != null) {
 // TODO: log a warning.
 }
 }
 if (mFactory instanceof KeyedFactory) {
 viewModel = ((KeyedFactory) (mFactory)).create(key, modelClass);
 } else {
 viewModel = (mFactory).create(modelClass);
 }
 mViewModelStore.put(key, viewModel);
 //noinspection unchecked
 return (T) viewModel;
}

ViewModelStore类

 上面提到将ViewModel存储到ViewModelStore中，那么我们来看一下是如何存储的

public class ViewModelStore {

 private final HashMap 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 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();
 }
}

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由 答源 DAN 底座 1.4.1 生成 · 2026-11-10T17:04:17+00:00