为什么要使用ViewModel？
在日常开发中，横竖屏切换是非常常见的功能，由于横竖屏切换会使得
Activity重建，导致界面相关数据都会丢失。为了避免这种情况，一般会有两种做法：第一种是在AndroidManifest.xml文件中，将Activity的一个属性设置为android:configChanges="orientation|keyboardHidden|screenSize"；第二种方案是使用onSaveInstanceState方法保存数据，但此方法仅适合可以序列化和反序列化的少量数据，而不适合数量可能较大的数据，如用户列表或位图。在使用MVP架构时，当View层使用Presenter层去异步加载一个耗时任务，若在任务结果返回回来之前，界面已经被销毁，我们都需要手动判断View层的状态；若Presenter层还有持有了View层的
Context，若不清理掉的话可能会造成内存泄露。
ViewModel的出现能够很优雅地解决上述两个问题。
简单用法
// 1. 创建一个继承ViewModel的类
public class MyVideoModel extends ViewModel {}
// 2. 创建一个ViewModelProvider实例
ViewModelProvider provider = new ViewModelProvider(this);
// 3. 通过provider获取MyVideoModel实例
MyVideoModel videoModel = provider.get(MyVideoModel.class);
源码分析
先来看看第2步创建ViewModelProvider时做了些什么事
public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
: NewInstanceFactory.getInstance());
}
第一个参数调用了owner.getViewModelStore()，这个ViewModelStoreOwner是一个接口，我们继承的父类Activity--ComponentActivity实现了这个接口。
@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;
}
当我们在自己的Activity#onCreate方法内new ViewModelProvider(this);时，这个
mViewModelStore就已经不为空了，所以直接返回。ViewModelStore就是用来保存我们自己创建的
ViewModel对象。那么它是什么时候被创建赋值的呢？在FragmentActivity#onCreate方法内调用了
mFragments.attachHost(null /*parent*/);
// FragmentActivity.java
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
mFragments.attachHost(null /*parent*/);
...
super.onCreate(savedInstanceState);
...
}
// FragmentController.java
public void attachHost(@Nullable Fragment parent) {
mHost.mFragmentManager.attachController(
mHost, mHost /*container*/, parent);
}
// FragmentManagerImpl.java
public void attachController(@NonNull FragmentHostCallback host,
@NonNull FragmentContainer container, @Nullable final Fragment parent) {
...
// Get the FragmentManagerViewModel
if (parent != null) {
mNonConfig = parent.mFragmentManager.getChildNonConfig(parent);
} else if (host instanceof ViewModelStoreOwner) {
ViewModelStore viewModelStore = ((ViewModelStoreOwner) host).getViewModelStore();
mNonConfig = FragmentManagerViewModel.getInstance(viewModelStore);
} else {
mNonConfig = new FragmentManagerViewModel(false);
}
}
attachController方法中的parent为空，所以不会走第一个if条件；第一个参数host，它的实现类是
HostCallbacks。
class HostCallbacks extends FragmentHostCallback implements
ViewModelStoreOwner,
OnBackPressedDispatcherOwner {
...
@NonNull
@Override
public ViewModelStore getViewModelStore() {
return FragmentActivity.this.getViewModelStore();
}
...
}
再回到ComponentActivity#getViewModelStore方法
@NonNull
@Override
public ViewModelStore getViewModelStore() {
...
if (mViewModelStore == null) {
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
// Activity第一次启动时，得到nc为null。但是在横竖屏切换时，这个nc不为空了
if (nc != null) {
// Restore the ViewModelStore from NonConfigurationInstances
// 所以这里就拿到了之前保存的ViewModelStore，也就意味着拿到之前我们的ViewModel实例。
// 后面会继续分析到
mViewModelStore = nc.viewModelStore;
}
// 执行这里
if (mViewModelStore == null) {
mViewModelStore = new ViewModelStore();
}
}
return mViewModelStore;
}
此时再回到上面创建ViewModelProvider时的第二个参数，因为ComponentActivity并没有实现
HasDefaultViewModelProviderFactory，所以第二个参数的类型是NewInstanceFactory。第2步源码分析完成。
接下来看第3步源码实现。
@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);
}
继续追踪get的重载函数
@NonNull
@MainThread
public T get(@NonNull String key, @NonNull Class modelClass) {
// 注释1
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.
}
}
// 注释2
if (mFactory instanceof KeyedFactory) {
viewModel = ((KeyedFactory) mFactory).create(key, modelClass);
} else {
// 注释3
viewModel = mFactory.create(modelClass);
}
mViewModelStore.put(key, viewModel);
return (T) viewModel;
}
方法中的key为androidx.lifecycle.ViewModelProvider.DefaultKey:xxx.xxx.MyViewModel，
modelClass就是我们自己传的MyViewModel.class。注释1处代码第一次执行时肯定为空，从第2步源码分析可知mFactory的类型为NewInstanceFactory，所以会执行到第注释3。通过反射获取到我们自己的
ViewModel对象，然后将其保存到ViewModelStore中。
当因配置更新导致界面重建时如何做到保存数据的？
当界面发生横竖屏切换时，Activity会被销毁再重建，在调用onDestroy方法之前系统会调用一个方法
// Activity.java
/**
* Called by the system, as part of destroying an
* activity due to a configuration change, when it is known that a new
* instance will immediately be created for the new configuration. You
* can return any object you like here, including the activity instance
* itself, which can later be retrieved by calling
* {@link #getLastNonConfigurationInstance()} in the new activity
* instance.
*/
public Object onRetainNonConfigurationInstance() {
return null;
}
来看看ComponentActivity中是如何将其重写的
@Override
@Nullable
public final Object onRetainNonConfigurationInstance() {
// 这里直接返回的是null
Object custom = onRetainCustomNonConfigurationInstance();
ViewModelStore viewModelStore = mViewModelStore;
// 正常情况下，此时的mViewModelStore不可能为空
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;
}
根据代码中的注释，最终ViewModelStore对象被保存在ComponentActivity的内部类
NonConfigurationInstances对象中。这个方法是在Activity中被调用的。
// Activity.java
NonConfigurationInstances retainNonConfigurationInstances() {
Object activity = onRetainNonConfigurationInstance();
...
NonConfigurationInstances nci = new NonConfigurationInstances();
nci.activity = activity;
...
return nci;
}
将ComponentActivity#NonConfigurationInstances对象赋值给了
Activity#NonConfigurationInstances对象的activity属性。而retainNonConfigurationInstances方法是在Activity执行onDestroy生命周期方法之前执行的。
// ActivityThread.java
/** Core implementation of activity destroy call. */
ActivityClientRecord performDestroyActivity(IBinder token, boolean finishing,
int configChanges, boolean getNonConfigInstance, String reason) {
ActivityClientRecord r = mActivities.get(token);
...
if (getNonConfigInstance) {
try {
// 将其返回给了ActivityClientRecord属性
r.lastNonConfigurationInstances
= r.activity.retainNonConfigurationInstances();
} catch (Exception e) {
...
}
}
try {
r.activity.mCalled = false;
// 执行onDestroy方法
mInstrumentation.callActivityOnDestroy(r.activity);
...
} catch (Exception e) {
...
}
return r;
}
最终mViewModelStore对象被保存在ActivityClientRecord对象的
lastNonConfigurationInstances属性上。很显然，mActivities属性的生命周期与ActivityThread保持一致，那么lastNonConfigurationInstances也一样，因此不会受Activity的销毁再重建的影响。
当Activity重建时，又会执行onCreate方法，此时，我们再回到getViewModelStore方法
@NonNull
@Override
public ViewModelStore getViewModelStore() {
...
if (mViewModelStore == null) {
// 会执行到这里
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
// Restore the ViewModelStore from NonConfigurationInstances
mViewModelStore = nc.viewModelStore;
}
if (mViewModelStore == null) {
mViewModelStore = new ViewModelStore();
}
}
return mViewModelStore;
}
@Nullable
public Object getLastNonConfigurationInstance() {
return mLastNonConfigurationInstances != null
? mLastNonConfigurationInstances.activity : null;
}
mLastNonConfigurationInstances不为空，所以直接返回它的属性activity的值。还记得这个属性保存的是什么值吗？上文已经提到过了，它就是ComponentActivity的NonConfigurationInstances对象，而它的viewModelStore就保存着横竖屏销毁前的那个ViewModelStore对象，那也就意味着拿到了之前的ViewModel对象。
那mLastNonConfigurationInstances这个属性是在哪里被赋值的呢？它是Activity对象在ActivityThread中被创建后，调用其attach方法时进行赋值的。上文也提到过，我们的ViewModel被保存在ViewModelStore，而ViewModelStore又被保存在CompponentActivity#NonConfigurationInstances对象的viewModelStore属性上，而这个对象又被保存到了Activity#NonConfigurationInstances对象的activity属性上，它又被保存到了ActivityThread#ActivityClientRecord对象的lastNonConfigurationInstances属性上。