文章目录
- 前言
- 源码分析
- ViewModel是如何创建的？
- ViewModelProvider(this)做了什么？
- 小结
- get(MyViewModel::class.java)做了什么？
- 小结
- ViewModel是如何实现配置更改后数据恢复的？
- 整体时序图
- 结语
前言
本文主要分析ViewModel相关源码，相关使用不再赘述，可参考Android ViewModel使用；
ViewModel 概览
Google官方给的ViewModel定义如下：
ViewModel类旨在`以注重生命周期的方式存储和管理界面相关数据`。
ViewModel类`让数据可在发生屏幕旋转等配置更改后继续留存`。
定义主要提到两个关键点：
-
生命周期
上图是Google官网提供的ViewModel生命周期图示，可以看到ViewModel的生命周期是从onCreate创建到完成并销毁Finished，开发中经常结合LiveData一起使用； -
配置更改后数据留存
我们知道当屏幕旋转的时候，Activity会销毁重建,如果我们想恢复之前的数据，之前一般是通过onSaveInstanceState()来实现，内部是通过Bundle来实现，此方法仅适合可以序列化再反序列化的少量数据，而不适合数量可能较大的数据，而ViewModel可以很简单的实现这个功能，并存储较大的数据；
源码分析
ViewModel是如何创建的？
val viewModel = ViewModelProvider(this).get(MyViewModel::class.java)
以上是ViewModel创建代码
ViewModelProvider(this)做了什么？
### ViewModelProvider.java
public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
: NewInstanceFactory.getInstance());
}
ComponentActivity则实现了ViewModelStoreOwner和HasDefaultViewModelProviderFactory接口；
因此我们只需要关注ComponentActivity中对应的方法；
getViewModelStore()；getDefaultViewModelProviderFactory()；
我们先分析getViewModelStore:
### ComponentActivity.getViewModelStore
public ViewModelStore getViewModelStore() {
...
ensureViewModelStore();
return mViewModelStore;
}
可以看到getViewModelStore()调用了ensureViewModelStore()，其对应代码如下：
### ComponentActivity.ensureViewModelStore
void ensureViewModelStore() {
if (mViewModelStore == null) {
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
// Restore the ViewModelStore from NonConfigurationInstances
mViewModelStore = nc.viewModelStore;
}
if (mViewModelStore == null) {
mViewModelStore = new ViewModelStore();
}
}
}
static final class NonConfigurationInstances {
Object custom;
ViewModelStore viewModelStore;
}
在ensureViewModelStore方法中，会判断mViewModelStore是否为null，如果为null的话，会先尝试调用getLastNonConfigurationInstance()方法获取，如果获取不到，则直接new创建一个ViewModelStore；
我们看下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();
}
}
其内部创建了一个HashMap，以ViewModel的CanonicalName为key，ViewModel对象为value，存储Activity中创建的ViewModel对象；
我们再看下getDefaultViewModelProviderFactory()方法：
### ComponentActivity.getDefaultViewModelProviderFactory()
public ViewModelProvider.Factory getDefaultViewModelProviderFactory() {
...
if (mDefaultFactory == null) {
mDefaultFactory = new SavedStateViewModelFactory(
getApplication(),
this,
getIntent() != null ? getIntent().getExtras() : null);
}
return mDefaultFactory;
}
主要是创建一个SavedStateViewModelFactory对象返回，我们看下SavedStateViewModelFactory的构造方法
public SavedStateViewModelFactory(@Nullable Application application,
@NonNull SavedStateRegistryOwner owner,
@Nullable Bundle defaultArgs) {
mSavedStateRegistry = owner.getSavedStateRegistry();
mLifecycle = owner.getLifecycle();
mDefaultArgs = defaultArgs;
mApplication = application;
mFactory = application != null
? ViewModelProvider.AndroidViewModelFactory.getInstance(application)
: ViewModelProvider.NewInstanceFactory.getInstance();
}
我们看下mFactory的创建，application 肯定不为null，因此会调用ViewModelProvider.AndroidViewModelFactory.getInstance(application)返回一个AndroidViewModelFactory对象
public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {
private static AndroidViewModelFactory sInstance;
public static AndroidViewModelFactory getInstance(@NonNull Application application) {
if (sInstance == null) {
sInstance = new AndroidViewModelFactory(application);
}
return sInstance;
}
private Application mApplication;
public AndroidViewModelFactory(@NonNull Application application) {
mApplication = application;
}
@NonNull
@Override
public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
if (AndroidViewModel.class.isAssignableFrom(modelClass)) {
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);
}
}
可以看到AndroidViewModelFactory主要是用来生产ViewModel的，其create()方法：根据传入的ViewModel类型【继承自AndroidViewModel或ViewModel】，通过反射创建ViewModel对象；
小结
调用ViewModelProvider(this)方法主要做了以下两件事：
-
获取
ViewModelStore对象，如果ViewModelStore为null，会先尝试调用getLastNonConfigurationInstance()方法获取，获取不到则直接通过new创建，内部通过HashMap实现ViewModel对象的存储； -
创建
AndroidViewModelFactory对象，其内部提供了create()，使用反射创建对应的ViewModel对象；
get(MyViewModel::class.java)做了什么？
### ViewModelProvider.java
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 {
if (viewModel != null) {
}
}
if (mFactory instanceof KeyedFactory) {
viewModel = ((KeyedFactory) mFactory).create(key, modelClass);
} else {
viewModel = mFactory.create(modelClass);
}
mViewModelStore.put(key, viewModel);
return (T) viewModel;
}
小结
get(MyViewModel::class.java)方法会根据传入的ViewModel的CanonicalName，先从mViewModelStore缓存中查找，如果找到，则直接返回，如果没找到，则调用前面创建的mFactory.create()方法通过反射创建对应的ViewModel，并添加到 mViewModelStore缓存中；
以上便是ViewModel相关的创建逻辑源码分析！
ViewModel是如何实现配置更改后数据恢复的？
我们知道当屏幕旋转配置发生变化时，会调用onDestroy(),在ComponentActivity构造方法中：
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (event == Lifecycle.Event.ON_DESTROY) {
// Clear out the available context
mContextAwareHelper.clearAvailableContext();
// And clear the ViewModelStore
if (!isChangingConfigurations()) {
getViewModelStore().clear();
}
}
}
});
当监听到Activity调用ON_DESTROY时，会调用getViewModelStore().clear()，清除ViewModelStore内部缓存的ViewModel对象，那为何重新创建后，ViewModel中持有的数据又没有丢失呢？
当Activity销毁时会调用ActivityThread.handleDestroyActivity()方法：
public void handleDestroyActivity(IBinder token, boolean finishing, int configChanges,
boolean getNonConfigInstance, String reason) {
ActivityClientRecord r = performDestroyActivity(token, finishing,
configChanges, getNonConfigInstance, reason);
if (r != null) {
...
}
mSomeActivitiesChanged = true;
}
handleDestroyActivity方法中又会调用performDestroyActivity()方法：
ActivityClientRecord performDestroyActivity(IBinder token, boolean finishing,
int configChanges, boolean getNonConfigInstance, String reason) {
...
ActivityClientRecord r = mActivities.get(token);
if (getNonConfigInstance) {
try {
r.lastNonConfigurationInstances
= r.activity.retainNonConfigurationInstances();
} catch (Exception e) {
if (!mInstrumentation.onException(r.activity, e)) {
throw new RuntimeException(
"Unable to retain activity "
+ r.intent.getComponent().toShortString()
+ ": " + e.toString(), e);
}
}
}
...
return r;
}
performDestroyActivity方法中会调用activity.retainNonConfigurationInstances()
NonConfigurationInstances retainNonConfigurationInstances() {
Object activity = onRetainNonConfigurationInstance();
...
return nci;
}
retainNonConfigurationInstances()方法中又调用onRetainNonConfigurationInstance(),其在ComponentActivity中具体实现代码如下：
public final Object onRetainNonConfigurationInstance() {
Object custom = onRetainCustomNonConfigurationInstance();
ViewModelStore viewModelStore = mViewModelStore;
if (viewModelStore == null) {
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;
}
可以看到会将viewModelStore保存到NonConfigurationInstances对象里返回，结合performDestroyActivity源码，最终会保存到ActivityClientRecord.lastNonConfigurationInstances属性中；
当Activity重新创建的时候，会执行ActivityThread.performLaunchActivity()方法：
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
...
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.referrer, r.voiceInteractor, window, r.configCallback,
r.assistToken);
...
return activity;
}
会调用 activity.attach()方法将lastNonConfigurationInstances对象传入,并赋值给Activity中的mLastNonConfigurationInstances属性；同时在ComponentActivity的构造方法中找到如下代码：
getLifecycle().addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
ensureViewModelStore();
getLifecycle().removeObserver(this);
}
});
而ensureViewModelStore()方法上面我们已经看到，内部会先调用getLastNonConfigurationInstance()方法获取mViewModelStore对象，getLastNonConfigurationInstance()方法返回的正是mLastNonConfigurationInstances属性；从而实现ViewModel数据的恢复！
整体时序图
结语
如果以上文章对您有一点点帮助，希望您不要吝啬的点个赞加个关注，您每一次小小的举动都是我坚持写作的不懈动力！ღ( ´･ᴗ･` )
