Android JetPack ViewModel 源码解析

本文主要是介绍Android JetPack ViewModel 源码解析，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

是什么？

ViewModel 用来存储页面相关的数据，当页面销毁的时候，存储数据也会清楚。但是当页面发生旋转的时候，并不会清楚数据。

怎么用？

        UserViewModel userViewModel = new ViewModelProvider(this).get(UserViewModel.class);

UserViewModel 是自己写的ViewModel,就可以拿到页面相关的ViewModel,在这个ViewModel 里我们可以获取LiveData，这样当异步数据加载完成的时候，我们可以更新Ui

public class UserViewModel extends ViewModel {private MutableLiveData<List<User>> users;public LiveData<List<User>> getUsers() {if (users == null) {users = new MutableLiveData<>(new ArrayList<>());}final List<User> value = users.getValue();Thread thread = Executors.defaultThreadFactory().newThread(new Runnable() {@Overridepublic void run() {UseDao useDao = UserDBManager.getInstance().getUseDao();List<User> usersAll = useDao.queryAll();if (value != null) {value.addAll(usersAll);}users.postValue(value);}});thread.start();return users;}}

如何实现的？

1.FragmentActivity 实现了 ViewModelStoreOwner 接口，在ComponentActivity 构造方法里面会监听OnDestory，如果不是页面状态改变，那么就会清楚ViewModelStore 里面的数据

androidx.activity.ComponentActivity#ComponentActivity()

        getLifecycle().addObserver(new LifecycleEventObserver() {@Overridepublic void onStateChanged(@NonNull LifecycleOwner source,@NonNull Lifecycle.Event event) {if (event == Lifecycle.Event.ON_DESTROY) {if (!isChangingConfigurations()) {getViewModelStore().clear();}}}});

2.getViewModelStore 方法

    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) {if (mViewModelStore == null) {mViewModelStore = new ViewModelStore();}}return mViewModelStore;}

判断当前Activity mViewModelStore 是不是空，如果是空就创建一个。

3.androidx.lifecycle.ViewModelProvider#get(java.lang.Class)

    public <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 {//noinspection StatementWithEmptyBodyif (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);return (T) viewModel;}

在这个方法里面会去调用工厂的create 创建ViewModel 对象，默认的实现为NewInstanceFactory：

总结：

ViewModel 最终的数据被存放在ViewModelStore 类 的成员变量Map 里，每一个Activity 都有自己的ViewModelStore. 当Activity 执行OnDestory 的时候，如果不是页面旋转，那么就会清空Map.

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