Jetpack源码解析之ViewModel
ViewModel 最早是在 2017 年的 Google I/O 大会上被提出，同时间问世的还有 Lifecycle 和 LiveData。
Google开发团队希望我们用ViewModel来管理页面状态相关信息，并建议我们搭配LiveData一起食用。
基本使用
依赖（可选），activity-ktx artifact。
implementation "androidx.fragment:fragment-ktx:1.2.5"//by viewModels()
ViewModel和Activity/Fragment相绑定，被视图持有。ViewModel存在的时间范围是从首次请求ViewModel直到 Activity 完成并销毁,所以ViewModel类让数据可在发生屏幕旋转等配置更改后继续留存。
class MyViewModel : ViewModel() {
private val users: MutableLiveData<List<User>> by lazy {
MutableLiveData().also {
loadUsers()
}
}
fun getUsers(): LiveData<List<User>> {
return users
}
private fun loadUsers() {
// Do an asynchronous operation to fetch users.
}
}
然后，您可以从 Activity 访问该列表，如下所示：
class MyActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
// Create a ViewModel the first time the system calls an activity's onCreate() method.
// Re-created activities receive the same MyViewModel instance created by the first activity.
// Use the 'by viewModels()' Kotlin property delegate
// from the activity-ktx artifact
val model: MyViewModel by viewModels()
//等同于 val model = ViewModelProvider(this).get(MyViewModel::class.java)
model.getUsers().observe(this, Observer<List<User>>{ users ->
// update UI
})
}
}
by viewModels()为Kotlin语法糖，Fragment中用by viewModels获取到的是属于该Fragment的model,如果在Fragment中想要获取activity的ViewModel,则要使用by activityViewModels()
原理解析
ViewModel创建
by viewModels()是ktx包的便便用法，实际内部调用到的还是ViewModelProvider(store, factory).get(viewModelClass.java)。所以我们直接看ViewModelProvider。
public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
: NewInstanceFactory.getInstance());
}
public ViewModelProvider(@NonNull ViewModelStore store, @NonNull Factory factory) {
mFactory = factory;
mViewModelStore = store;
}
传入了store和factory。
store
getViewModelStore()获取的。ComponentActivity实现了ViewModelStoreOwner接口，所以getViewModelStore的实现在ComponentActivity中,每个Activity中会持有一个viewModel实例。
compomentActivity.getViewModelStore()
@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;
}
factory
public interface Factory {
/**
* Creates a new instance of the given {@code Class}.
* <p>
*
* @param modelClass a {@code Class} whose instance is requested
* @param <T> The type parameter for the ViewModel.
* @return a newly created ViewModel
*/
@NonNull
<T extends ViewModel> T create(@NonNull Class<T> modelClass);
}
可以看到Factory就一个create方法，注释有说用来创建ViewModel实例的。
HasDefaultViewModelProviderFactory
如果activity实现了HasDefaultViewModelProviderFactory接口，则factory在getDefaultViewModelProviderFactory中：
@Override
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;
}
public final class SavedStateViewModelFactory extends ViewModelProvider.KeyedFactory { ... }
abstract static class KeyedFactory extends OnRequeryFactory implements Factory { ... }
返回的是个SavedStateViewModelFactory，其create方法：
@Override
public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
// ViewModelProvider calls correct create that support same modelClass with different keys
// If a developer manually calls this method, there is no "key" in picture, so factory
// simply uses classname internally as as key.
String canonicalName = modelClass.getCanonicalName();
if (canonicalName == null) {
throw new IllegalArgumentException("Local and anonymous classes can not be ViewModels");
}
return create(canonicalName, modelClass);
}
private static final Class<?>[] ANDROID_VIEWMODEL_SIGNATURE = new Class[]{Application.class,SavedStateHandle.class};
private static final Class<?>[] VIEWMODEL_SIGNATURE = new Class[]{SavedStateHandle.class};
@Override
public <T extends ViewModel> T create(@NonNull String key, @NonNull Class<T> modelClass) {
//AndroidViewModel 继承ViewModel,内部持有一个application
boolean isAndroidViewModel = AndroidViewModel.class.isAssignableFrom(modelClass);
//判断是不是AndroidViewModel的子类，用不同的构造器
Constructor<T> constructor;
if (isAndroidViewModel) {
constructor = findMatchingConstructor(modelClass, ANDROID_VIEWMODEL_SIGNATURE);
} else {
constructor = findMatchingConstructor(modelClass, VIEWMODEL_SIGNATURE);
}
// doesn't need SavedStateHandle
if (constructor == null) {
//到这里表示是无参数的构造函数，会调到NewInstanceFactory中的create，和第二种情况一样
return mFactory.create(modelClass);
}
SavedStateHandleController controller = SavedStateHandleController.create(
mSavedStateRegistry, mLifecycle, key, mDefaultArgs);
try {
T viewmodel;
if (isAndroidViewModel) {
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());
}
}
先判断传入的类是不是AndroidViewModel的子类，然后调用findMatchingConstructor,传入的signature不同。
private static final Class<?>[] ANDROID_VIEWMODEL_SIGNATURE = new Class[]{Application.class,SavedStateHandle.class};
private static final Class<?>[] VIEWMODEL_SIGNATURE = new Class[]{SavedStateHandle.class};
private static <T> Constructor<T> findMatchingConstructor(Class<T> modelClass,
Class<?>[] signature) {
for (Constructor<?> constructor : modelClass.getConstructors()) {
Class<?>[] parameterTypes = constructor.getParameterTypes();
if (Arrays.equals(signature, parameterTypes)) {
return (Constructor<T>) constructor;
}
}
return null;
}
这堆代码说白了就是：要么返回一个形参为SavedStateHandle.class的构造函数，要么返回一个形参为（Application.class,SavedStateHandle.class）的构造函数。没有这两样构造函数的中的任何一种的话，返回null。如果为null，则处理方式和第二种情况一样了。
如果不为null，则证明构造函数中传入了SavedStateHandle，即需要保存状态。后续代码暂且不表。
!HasDefaultViewModelProviderFactory
如果activity没有实现HasDefaultViewModelProviderFactory接口，返回的直接是个NewInstanceFactory。且NewInstanceFactory中,其create方法如下：
@NonNull
@Override
public <T extends ViewModel> T create(@NonNull Class<T> 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);
}
}
直接拿到类后通过反射就new出了Instance。
get(VM.class)
@MainThread
public <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);
}
@MainThread
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;
}
如果能在mViewModelStore中通过“包名”为Key获取到viewModel则将其返回；否则用上一步创造出来的Factory来创建viewModel，然后存入mViewModelStore中。
ViewModelStore
通过以上的描述，猜想ViewModelStore应该是个类似map的东西。所以看下源码：
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();
}
}
果然，内部维护了一个map，Key就是我们class的完整包名,在同一个activity下重复获取获取到的都是是同一个viewModel。
ViewModel销毁
ViewModel销毁的相关代码在ComponentActivity的初始化方法中：
public ComponentActivity() {
...
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,@NonNull Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_DESTROY) {
if (!isChangingConfigurations()) {
getViewModelStore().clear();
}
}
}
});
}
利用LifecycleObserver,当执行ON_DESTROY方法时，调用到viewModelStore的clear()。
保存状态
ViewModel 对象存在的时间范围是获取 ViewModel 时传递给 ViewModelProvider 的 Lifecycle。
ViewModel 将一直留在内存中，直到限定其存在时间范围的 Lifecycle 永久消失：对于 Activity，是在 Activity Finish时；而对于 Fragment，是在 Fragment 分离时。
有一种情况比较特殊，屏幕旋转（不配置configChanges）的时候，activity生命周期是会经过销毁重建的，即：
onPause--onSaveInstanceState--onStop--onRetainCustomNonConfigurationInstance--onDestroy--onCreate--onStart--onRestoreInstanceState--onResume
而ViewModel不会再onDestroy过程中销毁，原因有二：
- 在onDestroy判断中，有一个判断：
if (!isChangingConfigurations()) {
getViewModelStore().clear();
}
isChangingConfigurations表示此次销毁是否是为了改变配置而重建的一次操作，如果是的，就先不销毁了。
2.NonConfigurationInstances
我们在onStop和onDestroy中会调用一个onRetainCustomNonConfigurationInstance【为了便于区分简写onRCustomNC】方法，这个方法原是为了保留自定义非配置实例，它的前身是onRetainNonConfigurationInstance【简写为onRNC】，但是现在onRNC不能被重写【加了final】，onRCustomNC即将被弃用，官方推荐我们用ViewModel来代替。
因为在onRetainNonConfigurationInstance【onRNC】中，源码中将mViewModelStore保存了起来。
public final Object onRetainNonConfigurationInstance() {
Object custom = onRetainCustomNonConfigurationInstance();//空方法，你可以写，不过即将被弃用。
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;
}
这里面用来保存viewModelStore的载体是NonConfigurationInstances。NonConfigurationInstances是ComponentActivity中的一个静态内部类，里面存了viewModelStore;
static final class NonConfigurationInstances {
Object custom;
ViewModelStore viewModelStore;
}
也就是在destory之前，viewmodelStory就已被保存了。
再回顾getViewModelStore方法，在获取viewModelStore时，新建之前，从NonConfigurationInstances的中试图获取viewmodelStory；
if (mViewModelStore == null) {
NonConfigurationInstances nc = (NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
// Restore the ViewModelStore from NonConfigurationInstances
mViewModelStore = nc.viewModelStore;
}
if (mViewModelStore == null) {
mViewModelStore = new ViewModelStore();
}
}
这里的nc便是上面保存的nci。
面试问题
viewModelStore何时初始化
调试过程中无意发现，就算你新建一个Activity什么都不做,也会调用到getViewModelStore方法，其原因在于viewModelStory在onCreate中就被初始化了。在FragmentActivity中：
final FragmentController mFragments = FragmentController.createController(new HostCallbacks());
protected void onCreate(@Nullable Bundle savedInstanceState) {
mFragments.attachHost(null /*parent*/);
...
}
FragmentControl:
final FragmentManager mFragmentManager = new FragmentManagerImpl();
public void attachHost(@Nullable Fragment parent) {
mHost.mFragmentManager.attachController(
mHost, mHost /*container*/, parent);
}
FragmentManager:
void attachController(@NonNull FragmentHostCallback<?> host,
@NonNull FragmentContainer container, @Nullable final Fragment parent) {
if (mHost != null) throw new IllegalStateException("Already attached");
mHost = host;
mContainer = container;
mParent = parent;
if (mParent != null) {
// Since the callback depends on us being the primary navigation fragment,
// update our callback now that we have a parent so that we have the correct
// state by default
updateOnBackPressedCallbackEnabled();
}
// Set up the OnBackPressedCallback
if (host instanceof OnBackPressedDispatcherOwner) {
OnBackPressedDispatcherOwner dispatcherOwner = ((OnBackPressedDispatcherOwner) host);
mOnBackPressedDispatcher = dispatcherOwner.getOnBackPressedDispatcher();
LifecycleOwner owner = parent != null ? parent : dispatcherOwner;
mOnBackPressedDispatcher.addCallback(owner, mOnBackPressedCallback);
}
// Get the FragmentManagerViewModel
if (parent != null) {
mNonConfig = parent.mFragmentManager.getChildNonConfig(parent);
} else if (host instanceof ViewModelStoreOwner) {
//这里，已经被初始化了
ViewModelStore viewModelStore = ((ViewModelStoreOwner) host).getViewModelStore();
mNonConfig = FragmentManagerViewModel.getInstance(viewModelStore);
} else {
mNonConfig = new FragmentManagerViewModel(false);
}
}
