一、ViewModel原理解析
1.通过ViewModelProviders.of()方法创建ViewModel对象
（1）public static ViewModelProvider of(@NonNull Fragment fragment, @Nullable Factory factory)
（2）public static ViewModelProvider of(@NonNull FragmentActivity activity, @Nullable Factory factory)
（3）public static ViewModelProvider of(@NonNull Fragment fragment)
（4）public static ViewModelProvider of(@NonNull FragmentActivity activity)
在这四个方法中，其实唯一的区别就是要不要传Factory，当没有传自定义的Factory的时候，则会传入默认的Factory，我们看ViewModelProvider构造器的源码和部分of方法的源码：
public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
: NewInstanceFactory.getInstance());
}
在ViewModelProvider中需要传入一个VieModelStore对象，这个对象是由ViewModelStoreOwner来提供的，而在Activity或者Fragment中，是由Activity和Fragment来提供的，因为ViewModelStoreOwner是一个接口，而AppCompatActivity的祖父ComponentActivity和Fragment均实现了ViewModelStoreOwner接口。
但是ViewModelProviders在新的lifecycle-extensions库中，已经是属于被弃用的。新版的API直接使用ViewModelProvider，而不是ViewModelProviders。
比如：
viewModel = ViewModelProvider(this).get(TestViewModel::class.java)
可以如下的方式在baseActivity中添加，由子类Activity调用：
private lateinit var mActivityProvider: ViewModelProvider
protected open fun <T : ViewModel?> getActivityViewModel(modelClass: Class<T>): T {
// 如果使用lateinit，则不能进行判null，kotlin想要判断是否为null，需要使用?
if (mActivityProvider == null) {
mActivityProvider = ViewModelProvider(this)
}
return mActivityProvider.get(modelClass)
}
创建ViewModel对象，首先就需要先初始化一个ViewModelProvider对象
public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
: NewInstanceFactory.getInstance());
}
public ViewModelProvider(@NonNull ViewModelStoreOwner owner, @NonNull Factory factory) {
this(owner.getViewModelStore(), factory);
}
可以看出，ViewModelProvider构造函数其实最终都是需要两个参数，一个是ViewModelStoreOwner对象，一个是Factory。而ViewModelStoreOwner其实就是用来获取一个ViewModelStore对象来保存ViewModel对象的。而Factory就是用来创建ViewModel对象的。
2.ViewModelStoreOwner接口：
public interface ViewModelStoreOwner {
/**
* Returns owned {@link ViewModelStore}
*
* @return a {@code ViewModelStore}
*/
@NonNull
ViewModelStore getViewModelStore();
}
这个接口的主要实现的作用就是返回一个ViewModelStore对象。在Android中，Activity和Fragment都会实现该接口，并且实现getViewModelStore()方法。
比如Activity就是在FragmentActivity的父类ComponentActivity中实现了ViewModelStoreOwner接口
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner,
ViewModelStoreOwner,
HasDefaultViewModelProviderFactory,
SavedStateRegistryOwner,
OnBackPressedDispatcherOwner {
...
@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;
}
...
}
public class Fragment implements ComponentCallbacks, OnCreateContextMenuListener, LifecycleOwner,
ViewModelStoreOwner, HasDefaultViewModelProviderFactory, SavedStateRegistryOwner {
...
@NonNull
@Override
public ViewModelStore getViewModelStore() {
if (mFragmentManager == null) {
throw new IllegalStateException("Can't access ViewModels from detached fragment");
}
return mFragmentManager.getViewModelStore(this);
}
...
}
Fragment的ViewModelStore其实是由FragmentManager进行管理获取
FragmentManager#getViewModelStore
@NonNull
ViewModelStore getViewModelStore(@NonNull Fragment f) {
// 这里的mNonConfig是一个FragmentManagerViewModel对象
return mNonConfig.getViewModelStore(f);
}
每个FragmentActivity都会有一个自己的FragmentManager对象，所以每个FragmentManagerViewModel对象，管理的是一个FragmentActivity中的所有的Fragment对应的ViewModel。具体看FragmentManagerViewModel的getViewModelStore方法
@NonNull
ViewModelStore getViewModelStore(@NonNull Fragment f) {
ViewModelStore viewModelStore = mViewModelStores.get(f.mWho);
if (viewModelStore == null) {
viewModelStore = new ViewModelStore();
mViewModelStores.put(f.mWho, viewModelStore);
}
return viewModelStore;
}
从这里可以看出，每个Fragment都会有自己的ViewModelStore对象，而ViewModelStore对象，是根据每个Fragment的唯一标识进行创建的。
3.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();
}
}
ViewModelStore类对象，是每个Activity或者Fragment都有一个，目的是用于保存该页面的ViewModel对象，方便ViewModel的管理
4.ViewModelProvider的get方法：
@NonNull
@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);
}
@NonNull
@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;
}
从ViewModelProvider的get方法中，可以看出，get方法传入的是一个ViewModel.class的Class类型，然后通过这个类型，得到ViewModel的规范名称。将ViewModel对象缓存在ViewModelStore中的HashMap中。而ViewModel的创建，其实是通过ViewModelProvider.Factory来实现的
5.ViewModelProviders类：
ViewModelProviders的of方法，用于返回一个ViewModelProvider对象
public static ViewModelProvider of(@NonNull FragmentActivity activity,
@Nullable Factory factory) {
if (factory == null) {
factory = activity.getDefaultViewModelProviderFactory();
}
return new ViewModelProvider(activity.getViewModelStore(), factory);
}
从这里我们可以看到，如果传入的Activity或者Fragment有getDefaultViewModelProviderFactory方法实现，而factory为null的时候，则会通过getDefaultViewModelProviderFactory创建对应的Factory，而如果没有getDefaultViewModelProviderFactory的实现，那么就会调用NewInstanceFactory来创建对应的Factory，而NewInstanceFactory其实就是创建AndroidViewModelFactory对象。
最终ViewModel对象，其实就是通过AndroidViewModelFactory的create的方法实现来创建。一般就是通过Class.newInstance或者Class.getConstructor来创建对象。
而ViewModelProvider的第一个参数，其实最终传入的是ViewModelStore对象，这个对象内部是通过一个HashMap来保存ViewModel对象
而新版的源码，ViewModelStore对象是通过Fragment和FragmentActivity对象的getViewModelStore方法来获取，而原先的HolderFragment的功能都移植到了Fragment中
public class HolderFragment extends Fragment implements ViewModelStoreOwner {
private static final String LOG_TAG = "ViewModelStores";
// 这是什么？请看下面注释分析
private static final HolderFragmentManager sHolderFragmentManager = new HolderFragmentManager();
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public static final String HOLDER_TAG = "android.arch.lifecycle.state.StateProviderHolderFragment";
// 这就是我们存放 ViewModel 的 ViewModelStore，就定义在 HolderFragment里
private ViewModelStore mViewModelStore = new ViewModelStore();
public HolderFragment() {
// TODO: 为什么当 activity 由于屏幕旋转等被系统销毁时，
// 这个 fragment 实例也不会被销毁？因为设置了 setRetainInstance(true)
setRetainInstance(true);
}
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// 当 Fragment 的 onCreate 方法执行，说明了 Fragment 已经成功添加到了 Activity，
// sHolderFragmentManager 是 HolderFragmentManager类，它的 holderFragmentCreated()方法
// 是将该 Fragment 从 mNotCommittedActivityHolders 或 mNotCommittedFragmentHolders 中移除
// （HolderFragmentManager 的说明，请看下面的注释）
sHolderFragmentManager.holderFragmentCreated(this);
}
@Override
public void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
}
@Override
public void onDestroy() {
super.onDestroy();
// 当一个设置了 setRetainInstance(true) 的 Fragment 的 onDestroy 方法被调用，
// 证明它依附的 Activity 已经寿终正寝，所以调用 mViewModelStore.clear()，
// 前面我们已经说了，这个 clear 方法会调用所有 ViewModel 对象的 onCleared 方法
// 并且清空它们，我们可以在 ViewModel 的onCleared 方法做一些处理，以免起来不必要的
// 内存泄漏等问题
mViewModelStore.clear();
}
// 该方法用于给外部调用，返回 ViewModelStore
@Override
public ViewModelStore getViewModelStore() {
return mViewModelStore;
}
// 静态方法，没 ViewModelStores.of 方法中被调用
// 作用：在 activity 中添加一个 HolderFragment 用于存储存放了ViewModel对象的ViewModelStore
public static HolderFragment holderFragmentFor(FragmentActivity activity) {
return sHolderFragmentManager.holderFragmentFor(activity);
}
// 静态方法，没 ViewModelStores.of 方法中被调用
// 作用：在 fragment 中添加一个 HolderFragment 用于存储存放了ViewModel对象的ViewModelStore
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public static HolderFragment holderFragmentFor(Fragment fragment) {
return sHolderFragmentManager.holderFragmentFor(fragment);
}
// 上面的大部分操作都是基于HolderFragmentManager，我们来分析下这个类
@SuppressWarnings("WeakerAccess")
static class HolderFragmentManager {
// 存放还没被系统正式添加到 Activity 中的 HolderFragment
private Map<Activity, HolderFragment> mNotCommittedActivityHolders = new HashMap<>();
private Map<Fragment, HolderFragment> mNotCommittedFragmentHolders = new HashMap<>();
// 声明定义了一个能够感知 Activity 生命周期的 ActivityLifecycleCallbacks
private ActivityLifecycleCallbacks mActivityCallbacks =
new EmptyActivityLifecycleCallbacks() {
@Override
public void onActivityDestroyed(Activity activity) {
// 当 Activity destroy 的时候，清除 mNotCommittedActivityHolders 中保存
// 的对应 HolderFragment。前面我们分析了 HolderFragment 的 onCreate 方法中
// 会请一次 mNotCommittedActivityHolders，为什么在这么还要多此一举呢？其实
// 并不是多此一举，因为 Fragment 有可能还没创建完，Activity 就夭折了，那这样子
// HodlerFragment 的 onCreate 就无法调用，所以在加多一层清理机制，确保能够
// 清除掉（不得不感叹，谷歌官方的严谨以及对源码的掌控理解能力）
HolderFragment fragment = mNotCommittedActivityHolders.remove(activity);
if (fragment != null) {
Log.e(LOG_TAG, "Failed to save a ViewModel for " + activity);
}
}
};
private boolean mActivityCallbacksIsAdded = false;
private FragmentLifecycleCallbacks mParentDestroyedCallback =
new FragmentLifecycleCallbacks() {
@Override
public void onFragmentDestroyed(FragmentManager fm, Fragment parentFragment) {
// 与 mActivityCallbacks 的分析同理
super.onFragmentDestroyed(fm, parentFragment);
HolderFragment fragment = mNotCommittedFragmentHolders.remove(
parentFragment);
if (fragment != null) {
Log.e(LOG_TAG, "Failed to save a ViewModel for " + parentFragment);
}
}
};
// HolderFragment 的 onCreate 生命周期被回调，就会调用这个方法，清除
// mNotCommittedActivityHolders 或者 mNotCommittedFragmentHolders 中
// 的引用的 HolderFragment
void holderFragmentCreated(Fragment holderFragment) {
Fragment parentFragment = holderFragment.getParentFragment();
if (parentFragment != null) {
mNotCommittedFragmentHolders.remove(parentFragment);
parentFragment.getFragmentManager().unregisterFragmentLifecycleCallbacks(
mParentDestroyedCallback);
} else {
mNotCommittedActivityHolders.remove(holderFragment.getActivity());
}
}
private static HolderFragment findHolderFragment(FragmentManager manager) {
if (manager.isDestroyed()) {
throw new IllegalStateException("Can't access ViewModels from onDestroy");
}
Fragment fragmentByTag = manager.findFragmentByTag(HOLDER_TAG);
if (fragmentByTag != null && !(fragmentByTag instanceof HolderFragment)) {
throw new IllegalStateException("Unexpected "
+ "fragment instance was returned by HOLDER_TAG");
}
return (HolderFragment) fragmentByTag;
}
private static HolderFragment createHolderFragment(FragmentManager fragmentManager) {
HolderFragment holder = new HolderFragment();
fragmentManager.beginTransaction().add(holder, HOLDER_TAG).commitAllowingStateLoss();
return holder;
}
HolderFragment holderFragmentFor(FragmentActivity activity) {
FragmentManager fm = activity.getSupportFragmentManager();
HolderFragment holder = findHolderFragment(fm);
if (holder != null) {
return holder;
}
holder = mNotCommittedActivityHolders.get(activity);
if (holder != null) {
return holder;
}
if (!mActivityCallbacksIsAdded) {
mActivityCallbacksIsAdded = true;
activity.getApplication().registerActivityLifecycleCallbacks(mActivityCallbacks);
}
holder = createHolderFragment(fm);
// 我们新添加 add 的 Fragment 并不会马上就执行添加完（也就是说，这个方法执行完成后，马上再
// 调用一次，上面的 findHolderFragment 会返回 null。但是这没有关系，因为接下来我们还可
// 从 mNotCommittedActivityHolders 获取到对应的实例），所以我们这里先把他放在
// mNotCommittedActivityHolders 中。Not Committed 表示 fragment 的 commit 还没有完成
mNotCommittedActivityHolders.put(activity, holder);
return holder;
}
HolderFragment holderFragmentFor(Fragment parentFragment) {
FragmentManager fm = parentFragment.getChildFragmentManager();
HolderFragment holder = findHolderFragment(fm);
if (holder != null) {
return holder;
}
holder = mNotCommittedFragmentHolders.get(parentFragment);
if (holder != null) {
return holder;
}
parentFragment.getFragmentManager().registerFragmentLifecycleCallbacks(mParentDestroyedCallback, false);
holder = createHolderFragment(fm);
// 同上
mNotCommittedFragmentHolders.put(parentFragment, holder);
return holder;
}
}
}
HolderFragment通过设置setRetainInstance(true)，使得自身能够不受到屏幕旋转等configuration
changes影响而存活，直到依附的Activity正常结束。
因为HolderFragment的生命周期，ViewModelStore对象保存在HolderFragment中，而ViewModel又存储在ViewModelStore中，这就是为什么我们说ViewModel类能够让数据在屏幕旋转等配置信息改变导致UI重建的情况下不被销毁。
6.根据Factory创建ViewModel
ViewModelProvider的get方法：
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;
}
在ViewModel中，有两种Factory，Factory是的类型是由ViewModelProvider在初始化的时候创建的，所以是由ViewModelProvider决定Factory的类型。在ViewModelProvider中，有两种Factory，一种是默认的Factory，默认的Factory是通过在ComponentActivity或者Fragment中实现HasDefaultViewModelProviderFactory接口，然后在getDefaultViewModelProviderFactory()方法中初始化一个SavedStateViewModelFactory对象；另一种Factory则是NewInstanceFactory，这种是通过NewInstanceFactory.getInstance()的单例方式获取。
（1）SavedStateViewModelFactory创建ViewModel
其实就是通过ViewModel的Class对象，然后通过反射创建ViewModel对象，然后保存到ViewModelStore中的Map集合中
从ViewModelProvider的get方法可以看出，在ViewModelProvider的get方法中会根据Factory的类型，进行不同方法的调用。SavedStateViewModelFactory是实现了ViewModelProvider.KeyedFactory接口的，所以在创建ViewModel的时候，调用的是SavedStateViewModelFactory的create方法。
public <T extends ViewModel> T create(@NonNull String key, @NonNull Class<T> modelClass) {
// 方法参数中的modelClass就是我们要创建的目标ViewModel的Class对象
// 首先判断该ViewModel是否是AndroidViewModel类型
boolean isAndroidViewModel = AndroidViewModel.class.isAssignableFrom(modelClass);
Constructor<T> constructor;
// 如果是AndroidViewModel类型的，则在初始化的时候需要加入Application对象
if (isAndroidViewModel) {
constructor = findMatchingConstructor(modelClass, ANDROID_VIEWMODEL_SIGNATURE);
} else {
constructor = findMatchingConstructor(modelClass, VIEWMODEL_SIGNATURE);
}
// doesn't need SavedStateHandle
if (constructor == null) {
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());
}
}
AndroidViewModel和ViewModel的构造器参数Class
private static final Class<?>[] ANDROID_VIEWMODEL_SIGNATURE = new Class[]{Application.class,
SavedStateHandle.class};
private static final Class<?>[] VIEWMODEL_SIGNATURE = new Class[]{SavedStateHandle.class};
ViewModel保存和恢复数据
ComponentActivity和Fragment都将数据的保存和恢复逻辑转发给了SavedStateRegistryController。在在onCreate方法里通过调用performRestore来恢复数据，在onSaveInstanceState方法里通过调用performSave来保存数据。而SavedStateRegistryController中的SavedStateRegistry对象，就是实际进行数据的保存和恢复的，在SavedStateRegistry通过唯一的key获取到一个SavedStateProvider，而SavedStateProvider其实就是返回需要保存的数据，将对应的需要缓存的数据一一返回，然后保存在系统缓存时的回调到onSaveInstanceState的方法参数Bundle中进行保存。
SavedStateRegistry.performSave()
该方法是由ComponentActivity的onSaveInstanceState方法触发调用SavedStateRegistryController的performSave，进而调用的
@MainThread
void performSave(@NonNull Bundle outBundle) {
// 从SavedStateProvider中取出要保存的数据
Bundle components = new Bundle();
if (mRestoredState != null) {
components.putAll(mRestoredState);
}
for (Iterator<Map.Entry<String, SavedStateProvider>> it =
mComponents.iteratorWithAdditions(); it.hasNext(); ) {
Map.Entry<String, SavedStateProvider> entry1 = it.next();
components.putBundle(entry1.getKey(), entry1.getValue().saveState());
}
// outBundle其实就是Activity要保存的数据
outBundle.putBundle(SAVED_COMPONENTS_KEY, components);
}
在SavedStateRegistry恢复数据的时候，会把恢复后的数据都交给SavedStateHandle。希望保留的数据，可以通过两种方式向mRegular保存数据。
1.通过调用 set(@NonNull String key, @Nullable T value) 方法来实现，该方法就类似于 Map.put(Key,Value) 的方式来存值，但是这种方式并不具备数据变化通知
2.通过 MutableLiveData.setValue的方式来存值。首先通过 getLiveData(@NonNull String key) 方法拿到和特定 Key 绑定的 LiveData 对象，之后向该 LiveData.setValue 时都会同时更新 mRegular
在ComponentActivity恢复数据的时候，会通过SavedStateRegistryController.performSave在Activity的onSaveInstanceState方法中进行数据的保存，然后在ComponentActivity的onCreate方法中，通过调用SavedStateRegistryController.performRestore方法进行数据的恢复，这些恢复的数据都会保存在SavedStateHandleController对象中的SavedStateHandle属性中，然后在Activity重新创建的时候，会通过反射创建对应的ViewModel对象的时候，将SavedStateHandleController中的SavedStateHandle赋值给对应的ViewModel进行数据恢复。
这块的源码分析可以参考：
从源码看 Jetpack（7）-SavedStateHandle源码详解
（2）NewInstanceFactory创建ViewModel
@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);
}
}
这里其实就是直接使用Class的newInstance直接创建对象。Activity和Fragment一般都是使用SavedStateViewModelFactory创建ViewModel对象。
二、ViewModel的销毁
ViewModel的销毁，要分为Activity和Fragment两部分。
首先看下ViewModel在销毁的时候做的事情
protected void onCleared() {
}
@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();
}
而ViewModel的clear()方法的调用，是在ViewModelStore中
public final void clear() {
for (ViewModel vm : mMap.values()) {
vm.clear();
}
mMap.clear();
}
1.Activity中的销毁
Activity的销毁，是通过Lifecycle监听生命周期回调，当生命周期执行到onDestroy的时候，调用ViewModelStore的clear()方法进行ViewModel的销毁。
看ComponentActivity中构造器中的实现：
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_DESTROY) {
// 这里是判断是否是系统配置发生变化，比如手机屏幕旋转
// 这里的配置是在Activity的onStop()方法中进行修改的
if (!isChangingConfigurations()) {
getViewModelStore().clear();
}
}
}
});
protected void onStop() {
if (DEBUG_LIFECYCLE) Slog.v(TAG, "onStop " + this);
if (mActionBar != null) mActionBar.setShowHideAnimationEnabled(false);
mActivityTransitionState.onStop();
dispatchActivityStopped();
mTranslucentCallback = null;
mCalled = true;
if (mAutoFillResetNeeded) {
// If stopped without changing the configurations, the response should expire.
getAutofillManager().onInvisibleForAutofill(!mChangingConfigurations);
} else if (mIntent != null
&& mIntent.hasExtra(AutofillManager.EXTRA_RESTORE_SESSION_TOKEN)
&& mIntent.hasExtra(AutofillManager.EXTRA_RESTORE_CROSS_ACTIVITY)) {
restoreAutofillSaveUi();
}
mEnterAnimationComplete = false;
}
Fragment中的销毁
Fragment的生命周期管理，如下：
static final int INITIALIZING = -1; // Not yet attached.
static final int ATTACHED = 0; // Attached to the host.
static final int CREATED = 1; // Created.
static final int ACTIVITY_CREATED = 2; // Fully created, not started.
static final int STARTED = 3; // Created and started, not resumed.
static final int RESUMED = 4; // Created started and resumed.
Fragment的生命周期，首先会依次增大，然后在从onResume变成onPause的时候，就开始状态码减小。即先升再降的一个状态变化。在当前状态码变成CREATED的时候，就会执行onDestroy。即调用
fragmentStateManager.destroy(mHost, mNonConfig);
FragmentStateManager.destroy
void destroy(@NonNull FragmentHostCallback<?> host,
@NonNull FragmentManagerViewModel nonConfig) {
if (FragmentManager.isLoggingEnabled(Log.DEBUG)) {
Log.d(TAG, "movefrom CREATED: " + mFragment);
}
boolean beingRemoved = mFragment.mRemoving && !mFragment.isInBackStack();
boolean shouldDestroy = beingRemoved || nonConfig.shouldDestroy(mFragment);
if (shouldDestroy) {
boolean shouldClear;
if (host instanceof ViewModelStoreOwner) {
shouldClear = nonConfig.isCleared();
} else if (host.getContext() instanceof Activity) {
Activity activity = (Activity) host.getContext();
shouldClear = !activity.isChangingConfigurations();
} else {
shouldClear = true;
}
if (beingRemoved || shouldClear) {
nonConfig.clearNonConfigState(mFragment);
}
mFragment.performDestroy();
mDispatcher.dispatchOnFragmentDestroyed(mFragment, false);
} else {
mFragment.mState = Fragment.ATTACHED;
}
}
在这里就会调用nonConfig.clearNonConfigState方法，nonConfig其实就是FragmentManagerViewModel对象。
FragmentManagerViewModel.clearNonConfigState
void clearNonConfigState(@NonNull Fragment f) {
if (FragmentManager.isLoggingEnabled(Log.DEBUG)) {
Log.d(TAG, "Clearing non-config state for " + f);
}
// Clear and remove the Fragment's child non config state
FragmentManagerViewModel childNonConfig = mChildNonConfigs.get(f.mWho);
if (childNonConfig != null) {
childNonConfig.onCleared();
mChildNonConfigs.remove(f.mWho);
}
// Clear and remove the Fragment's ViewModelStore
ViewModelStore viewModelStore = mViewModelStores.get(f.mWho);
if (viewModelStore != null) {
viewModelStore.clear();
mViewModelStores.remove(f.mWho);
}
}
三、ViewModel生命周期绑定
按照上面的逻辑，在Activity重建时会执行destory生命周期事件，那么为什么ViewModel没有销毁呢？
其实就是在屏幕旋转的时候，AMS通过Binder回调Activity的retainNonConfigurationInstances()方法，这个时候就会进行数据的保存，保存到一个NonConfigurationInstances对象；而在屏幕翻转结束之后，会再一次调用ViewModelProvider的构造函数，此时就会调用owner.getViewModelStore()，接着就会调用getLastNonConfigurationInstance()，这里就会通过Activity中的NonConfigurationInstances对象取出保存的ViewModelStore对象。
所以数据保存就是通过retainNonConfigurationInstances()方法保存在NonConfigurationInstances对象，而再一次使用取出ViewModel的数据的时候，就是从nc对象中取出ViewModelStore对象，而ViewModelStore对象保存有ViewModel集合
通过对ComponentActivity的getViewModelStore()方法进行分析。可以找到这个问题的答案。
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;
}
当mViewModelStore为null的时候，会从NonConfigurationInstances中获取ViewModelStore对象。
其实在ComponentActivity和Activity中都会有一个NonConfigurationInstances类，而Activity中的NonConfigurationInstances类结构如下：
static final class NonConfigurationInstances {
Object activity;
HashMap<String, Object> children;
FragmentManagerNonConfig fragments;
ArrayMap<String, LoaderManager> loaders;
VoiceInteractor voiceInteractor;
}
这里的Object activity其实就是保存的ComponentActivity中的NonConfigurationInstances类对象，看Activity的下面的方法：
// TODO: retainNonConfigurationInstances()方法，其实是在AMS通过Binder跨进程调用的
// 这是在ActivityThread的performDestroyActivity方法调用
// 而在handleDestroyActivity的时候调用performDestroyActivity
// 而handleRelaunchActivityInner会调用handleDestroyActivity，getNonConfigInstance会传入true
// handleRelaunchActivityInner是在Activity重新启用的时候被调用
// 其实就是旋转的时候，Activity就会被重启，这样就会最终触发retainNonConfigurationInstances()
NonConfigurationInstances retainNonConfigurationInstances() {
Object activity = onRetainNonConfigurationInstance();
HashMap<String, Object> children = onRetainNonConfigurationChildInstances();
FragmentManagerNonConfig fragments = mFragments.retainNestedNonConfig();
// We're already stopped but we've been asked to retain.
// Our fragments are taken care of but we need to mark the loaders for retention.
// In order to do this correctly we need to restart the loaders first before
// handing them off to the next activity.
mFragments.doLoaderStart();
mFragments.doLoaderStop(true);
ArrayMap<String, LoaderManager> loaders = mFragments.retainLoaderNonConfig();
if (activity == null && children == null && fragments == null && loaders == null
&& mVoiceInteractor == null) {
return null;
}
NonConfigurationInstances nci = new NonConfigurationInstances();
nci.activity = activity;
nci.children = children;
nci.fragments = fragments;
nci.loaders = loaders;
if (mVoiceInteractor != null) {
mVoiceInteractor.retainInstance();
nci.voiceInteractor = mVoiceInteractor;
}
return nci;
}
activity这个Object对象，其实是通过onRetainNonConfigurationInstance()方法返回值赋值，而onRetainNonConfigurationInstance()方法的实现是在ComponentActivity中。
看ComponentActivity中的下面方法：
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;
}
因为这里会在ComponentActivity中的NonConfigurationInstances类对象中保存ViewModelStore对象，所以这也是Activity重建时不会销毁ViewModel的原因。
Activity的onRetainNonConfigurationInstance()的调用
onRetainNonConfigurationInstance()方法除了被Activity的retainNonConfigurationInstances()调用以外，还会被LocalActivityManager的dispatchRetainNonConfigurationInstance()方法调用
//在每个子活动上调用onRetainNonConfigurationInstance并将结果按ID存储在HashMap中。
//仅当存在要存储的非null对象时，才构造HashMap。
//请注意，这不支持嵌套的ActivityGroup。
public HashMap<String,Object> dispatchRetainNonConfigurationInstance() {
HashMap<String,Object> instanceMap = null;
final int N = mActivityArray.size();
for (int i=0; i<N; i++) {
LocalActivityRecord r = mActivityArray.get(i);
if ((r != null) && (r.activity != null)) {
Object instance = r.activity.onRetainNonConfigurationInstance();
if (instance != null) {
if (instanceMap == null) {
instanceMap = new HashMap<String,Object>();
}
instanceMap.put(r.id, instance);
}
}
}
return instanceMap;
}
四、Fragment之间共享ViewModel
在分析ViewModel的销毁过程时，我们看到Activity与Fragment存储VieModel是分离的，那么同一个Activity下的Fragment是如何共享ViewModel的呢？
其实共享的是Activity的ViewModel。
而具体的实现逻辑，其实就是在FragmentViewModelLazy.kt中的：
@MainThread
inline fun <reified VM : ViewModel> Fragment.activityViewModels(
noinline factoryProducer: (() -> Factory)? = null
) = createViewModelLazy(VM::class, { requireActivity().viewModelStore },
factoryProducer ?: { requireActivity().defaultViewModelProviderFactory })
在Fragment中可以直接调用，这是一个Fragment的扩展函数，通过实现requireActivity().viewModelStore，获取到了Activity的ViewModelStore对象后，这样就可以实现了Fragment共用Activity的ViewModel，从而实现了Fragment之间共享ViewModel。
Fragment之间共享ViewModel，需要引入
"androidx.fragment:fragment-ktx:1.2.5"
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