在页面（Activity/Fragment）很简单的情况下，通常我们会将UI交互，数据获取与处理等相关业务逻辑，全部写在页面中，但是在页面复杂的情况下，这样做是不合适的，它不符合“单一责任”原则。页面只应该负责接收用户的交互，以及将数据展示到屏幕上，相关数据应该单独存放和处理。
为此，Android为我们提供了ViewModel类，专门用于存放应用程序页面所需的数据。它将页面所需的数据从页面中剥离出来，页面只需要处理用户交互，以及负责展示数据的工作。
另外，如果我们的应用程序支持横竖屏切换，当用户旋转手机屏幕时，我们还需要考虑数据的存储与恢复。如果数据不进行存储，那么通常我们还需要重新去获取一次。
而ViewModel能为我们解决这个问题，它独立于配置变化。也就是说，屏幕旋转导致的Activity重建，并不会影响到ViewModel的生命周期。
目前最新版本为2.3.1
implementation "androidx.lifecycle:lifecycle-extensions:2.3.1"
（1）定义一个User数据类
class User implements Serializable {
public int age;
public String name;
}（2）ViewModel结合LiveData
public class UserModel extends ViewModel {
public final MutableLiveData<User> mUserLiveData = new MutableLiveData<>();
public UserModel() {
//模拟从网络加载用户信息
mUserLiveData.postValue(new User(1, "name1"));
}
//模拟 进行一些数据骚操作
public void doSomething() {
User user = mUserLiveData.getValue();
if (user != null) {
user.age = 15;
user.name = "name15";
mUserLiveData.setValue(user);
}
}
}（3）这时候在Activity中就可以使用ViewModel了. 其实就是一句代码简单实例化,然后就可以使用ViewModel了.
public class MainActivity extends FragmentActivity {
private TextView mContentTv;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mContentTv = findViewById(R.id.tv_content);
//构建ViewModel实例
final UserModel userModel = new ViewModelProvider(this).get(UserModel.class);
//让TextView观察ViewModel中数据的变化,并实时展示
userModel.mUserLiveData.observe(this, new Observer<User>() {
@Override
public void onChanged(User user) {
mContentTv.setText(user.toString());
}
});
findViewById(R.id.btn_test).setOnClickListener(v->{
//点击按钮 更新User数据 观察TextView变化
userModel.doSomething();
});
}
}此时点击按钮后TextView的内容变为15，旋转屏幕后数据依然不变。可见在Activity销毁之后,它才会自动销毁(所以别让ViewModel持有Activity引用,会内存泄露的).
Activity与Fragment可以共享一个ViewModel,因为Fragment是依附在Activity上的,在实例化ViewModel时将Activity传入ViewModelProvider,它会为该Activity已创建好ViewModel,这个Fragment可以方便的访问该ViewModel中的数据.在Activity中修改userModel数据后,该Fragment就能拿到更新后的数据.
public class MyFragment extends Fragment {
public void onStart() {
//这里拿到的ViewModel实例,其实是和Activity中创建的是一个实例
UserModel userModel = new ViewModelProvider(getActivity()).get(UserModel.class);
}
}public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
: NewInstanceFactory.getInstance());
}ViewModelProvider构造方法接受一个ViewModelStoreOwner类型的参数。ViewModelStoreOwner是一个接口，内部只有一个getViewModelStore方法。接下来在构造方法中又调用了重载的构造方法，并且第一个调用了ViewModelStoreOwner的getViewModelStore方法来获取ViewModelStore，第二个参数通过ower判断类型来实例化ViewModelProviderFacory
在中ComponentActivity实现ViewModelStoreOwner接口和HasDefaultViewModelProviderFactory，ComponentActivity重写了getViewModelStore和getDefaultViewModelProviderFactory方法。
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
ViewModelStoreOwner,
HasDefaultViewModelProviderFactory{
@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.");
}
// 确保ViewModelStore不为空
ensureViewModelStore();
return mViewModelStore;
}
@NonNull
@Override
// 创建了一个默认的ViewModelFactory
public ViewModelProvider.Factory getDefaultViewModelProviderFactory() {
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 (mDefaultFactory == null) {
mDefaultFactory = new SavedStateViewModelFactory(
getApplication(),
this,
getIntent() != null ? getIntent().getExtras() : null);
}
return mDefaultFactory;
}
}ensureViewModelStore方法中如果ViewModelStore为null，则会尝试恢复或者实例化ViewModelStore，代码如下：
void ensureViewModelStore() {
if (mViewModelStore == null) {
// 获取上次配置数据实例
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
// 从NonConfigurationInstances中恢复ViewModelStore
mViewModelStore = nc.viewModelStore;
}
// NonConfigurationInstances中没有保存那么就实例化ViewModelStore
if (mViewModelStore == null) {
mViewModelStore = new ViewModelStore();
}
}
}ViewModelProvider的构造方法最终调用了下边的这个重载的构造方法：
public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
mFactory = factory;
mViewModelStore = store;
}从上述代码可以看到，在创建ViewModel的时候，Activity中会首先尝试恢复或者创建一个ViewModelStore。并将其保存到ViewModelProvider中待用。
实例化了ViewModelProvider后会调用ViewModelProvider的get方法。代码如下：
public <T extends ViewModel> T get(@NonNull Class<T> modelClass) {
String canonicalName = modelClass.getCanonicalName();
// ... 省略校验
return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
}
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 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);
return (T) viewModel;
}get方法中首先调用了mViewModelStore的get方法，并通过key来获取ViewModel。可见ViewModel本身应该是一个K-V的集合类。暂且不管ViewModelStore。接下来，如果modelClass已经实例化了，就直接返回ViewModel，否则就调用Factory来创建ViewModel，并将ViewModel存入ViewModelStore中。很显然，ViewModel的实例化时在Factory中进行的。ViewModelProvider的构造方法是允许我们自己传入一个工厂的，如果不传也会创建默认的工厂。本章的第一小节已经看到会默认实例化一个SavedStateViewModelFactory，这个工厂继承了KeyedFactory，我们来看下它的create是如何实现的：
public <T extends ViewModel> T create(@NonNull String key, @NonNull Class<T> modelClass) {
// 判断是不是AndroidViewModel类型
boolean isAndroidViewModel = AndroidViewModel.class.isAssignableFrom(modelClass);
Constructor<T> constructor;
// 获取ViewModel的构造方法
if (isAndroidViewModel && mApplication != null) {
// 查找ANDROID_VIEWMODEL_SIGNATURE对应的构造方法
constructor = findMatchingConstructor(modelClass, ANDROID_VIEWMODEL_SIGNATURE);
} else {
// 查找VIEWMODEL_SIGNATURE的构造方法
constructor = findMatchingConstructor(modelClass, VIEWMODEL_SIGNATURE);
}
// doesn't need SavedStateHandle
if (constructor == null) {
// 通过Factory去创建ViewModel
return mFactory.create(modelClass);
}
SavedStateHandleController controller = SavedStateHandleController.create(
mSavedStateRegistry, mLifecycle, key, mDefaultArgs);
try {
T viewmodel;
// 通过构造方法反射实例化ViewModel
if (isAndroidViewModel && mApplication != null) {
viewmodel = constructor.newInstance(mApplication, controller.getHandle());
} else {
viewmodel = constructor.newInstance(controller.getHandle());
}
viewmodel.setTagIfAbsent(TAG_SAVED_STATE_HANDLE_CONTROLLER, controller);
return viewmodel;
} catch (IllegalAccessException e) {
throw new RuntimeException("Failed to access " + modelClass, e);
} catch (InstantiationException e) {
throw new RuntimeException("A " + modelClass + " cannot be instantiated.", e);
} catch (InvocationTargetException e) {
throw new RuntimeException("An exception happened in constructor of "
+ modelClass, e.getCause());
}
}这里的mFactory是在SavedStateViewModelFactory实例化时创建的，可能是AndroidViewModelFactory类型或者NewInstanceFactory类型。
public SavedStateViewModelFactory(@Nullable Application application,
@NonNull SavedStateRegistryOwner owner,
@Nullable Bundle defaultArgs) {
// ...
mFactory = application != null
? ViewModelProvider.AndroidViewModelFactory.getInstance(application)
: ViewModelProvider.NewInstanceFactory.getInstance();
}以AndroidViewModelFactory为例来看create
public <T extends ViewModel> T create(@NonNull Class<T> 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);
}其实就是通过构造方法反射创建了ViewModel而已。
总体来看ViewModelProvider的任务就是创建ViewModel并将ViewModel存储到ViewModelStore中。而ViewModel的创建都是通过Model.class的构造方法反射创建的。
上一节猜测ViewModelStore是一个存储K-V结构的数据类型，那么来看下它的源码。
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();
}
}原来ViewModelStore内部就是封装了一下HashMap，实例化的ViewModel都被存储到了HashMap中。另外注意到ViewModelStore中还有一个clear方法，用来清理ViewMode和Map集合。在哪里对ViewModelStore进行清楚呢？这个还需要到Activity中找答案了，注意到在ComponentActivity的构造方法中有如下代码：
public ComponentActivity() {
Lifecycle lifecycle = getLifecycle();
if (Build.VERSION.SDK_INT >= 19) {
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_STOP) {
Window window = getWindow();
final View decor = window != null ? window.peekDecorView() : null;
if (decor != null) {
decor.cancelPendingInputEvents();
}
}
}
});
}
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
// Activity的销毁事件
if (event == Lifecycle.Event.ON_DESTROY) {
// Clear out the available context
mContextAwareHelper.clearAvailableContext();
// And clear the ViewModelStore
if (!isChangingConfigurations()) {
getViewModelStore().clear();
}
}
}
});
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
ensureViewModelStore();
getLifecycle().removeObserver(this);
}
});
}全局搜素发现只有在ComponentActivity构造方法中有一处调用了ViewModelStore的clear方法，很显然，这里是通过LifeCycle监听Activity的销毁事件，然后才对ViewModel进行了清理。
我们知道，屏幕旋转时候Activity会经历销毁和重建的过程，那么ViewModelStore是什么时候被保存的呢？其实是在ComponentActivity的onRetainNonConfigurationInstance方法中，代码如下：
// ComponentActivity
public final Object onRetainNonConfigurationInstance() {
// Maintain backward compatibility.
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;
}ViewModelStore的恢复则是在getLastNonConfigurationInstance中，代码如下：
// Activity
public Object getLastNonConfigurationInstance() {
return mLastNonConfigurationInstances != null
? mLastNonConfigurationInstances.activity : null;
}getLastNonConfigurationInstance的调用其实前边代码已经有提到，就是在ensureViewModelStore中。ensureViewModelStore除了在getViewModelProvider中调用外，还会在生命周期的回调中被调用，代码在上边也有贴出。
最后，还有一点比较重要，就是onRetainNonConfigurationInstance的调用时机，这点涉及到的东西也比较多，就不再这里赘述了，可以参考ViewModel源码研究之聊聊onSaveInstanceState和onRetainNonConfigurationInstance的区别这篇文章
参考：
https://juejin.cn/post/6987566061499449357