ViewModel源码研究之聊聊onSaveInstanceState和onRetainNonConfigurationInstance的区别
本文转载自 ViewModel源码研究之聊聊onSaveInstanceState和onRetainNonConfigurationInstance的区别（作者：字节小站）。版权归原作者所有，此处仅作个人学习备份。
前言
最近在研究ViewModel实现原理。ViewModel有两个特性。
- 当配置发生改变时（例如：旋转屏幕），重新创建的Activity能够通过ViewModel将数据还原回来，
- 当按返回键或者调用finish方法时，ViewModel能够感知到onDestroy事件，同时将ViewModel保存的Closeable对象关闭掉（例如：主动关闭协程）
当屏幕旋转时，会调用Activity的onRetainNonConfigurationInstance方法。ViewModel组件正是通过该方法将ViewModel保存起来，给重建的Activity使用。
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//androidx.activity:activity:1.2.2@aar
//ComponentActivity.java
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;
}
Activity还有个类似的方法onSaveInstanceState() ，onSaveInstanceState() 和onRetainNonConfigurationInstance() 的区别是：
- onSaveInstanceState() 调用的场景是：activity1启动activity2。生命周期调用顺序如下：
activity1.onPause()->activity2.onCreate()->activity2.onStart()->activity2.onResume()->activity1.onStop()->activity1.onSaveInstanceState()，
- onRetainNonConfigurationInstance() 调用场景是当configuration发生改变时，例如：旋转屏幕。
那么问题来了，一共有三个
- 它们存储的状态数据颗粒度一样吗？
- 它们把状态数据存储到哪里去了？
- 如果系统后台将Activity杀掉后，它们都能把状态恢复回来吗？
onSaveInstanceState(Bundle outState)方法详解
首先在ComponentActivity的onSaveInstanceState(Bundle outState) 方法中加个断点，重点关注 ActivityThread.callActivityOnSaveInstanceState(Bundle outState)。
2.1 ActivityThread.callActivityOnSaveInstanceState(Bundle outState)
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//ActivityThread.java
private void callActivityOnSaveInstanceState(ActivityClientRecord r) {
r.state = new Bundle();
r.state.setAllowFds(false);
if (r.isPersistable()) {
r.persistentState = new PersistableBundle();
mInstrumentation.callActivityOnSaveInstanceState(r.activity, r.state,
r.persistentState);
} else {
mInstrumentation.callActivityOnSaveInstanceState(r.activity, r.state);
}
}
我们注意到r.state = new Bundle()， 原来outState参数是在这里创建的。Bundle可以用来组件间传递数据，也可以用来进程间传递数据。
2.2 ActivityThread.handleStopActivity()
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public void handleStopActivity(IBinder token, boolean show, int configChanges,
PendingTransactionActions pendingActions, boolean finalStateRequest, String reason) {
final ActivityClientRecord r = mActivities.get(token);
r.activity.mConfigChangeFlags |= configChanges;
final StopInfo stopInfo = new StopInfo();
performStopActivityInner(r, stopInfo, show, true /* saveState */, finalStateRequest,
reason);
updateVisibility(r, show);
// Make sure any pending writes are now committed.
if (!r.isPreHoneycomb()) {
QueuedWork.waitToFinish();
}
stopInfo.setActivity(r);
stopInfo.setState(r.state);
stopInfo.setPersistentState(r.persistentState);
pendingActions.setStopInfo(stopInfo);
mSomeActivitiesChanged = true;
}
2.3 PendingTransactionActions$StopInfo.run()
该方法调用了 ActivityManager.getService().activityStopped(mActivity.token, mState, mPersistentState, mDescription) 方法，还将 2.1 中创建的 mState 当参数传进来了。
2.4 ActivityManagerService.activityStopped()
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//ActivityManagerService.java
@Override
public final void activityStopped(IBinder token, Bundle icicle,
PersistableBundle persistentState, CharSequence description) {
// Refuse possible leaked file descriptors
if (icicle != null && icicle.hasFileDescriptors()) {
throw new IllegalArgumentException("File descriptors passed in Bundle");
}
final long origId = Binder.clearCallingIdentity();
synchronized (this) {
final ActivityRecord r = ActivityRecord.isInStackLocked(token);
if (r != null) {
r.activityStoppedLocked(icicle, persistentState, description);
}
}
trimApplications();
Binder.restoreCallingIdentity(origId);
}
2.5 ActivityRecord.activityStoppedLocked
我们注意到最终bundle数据会保存在ActivityRecord的icicle对象中。
总结：onSaveInstanceState方法是当Activity调用了onStop后，会调用到ActivityThread的callActivityOnSaveInstanceState()方法，把Activity需要保存的数据放入Bundle对象中，并且随后通过IPC进程间通信机制，调用ActivityManagerService的activityStopped方法，将Bundle对象保存到AMS端的ActivityRecord中。
2.6 被杀端后恢复数据过程
最终是通过 ActivityStackSupervisor.realStartActivityLocked → LaunchActivityItem.execute() → ActivityThread.performLaunchActivity 用 r.state 恢复数据：
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private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent){
activity.mCalled = false;
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
}
onRetainNonConfigurationInstance()
该方法是在重建Activity时调用performDestoryActivity时会保存数据。
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ActivityClientRecord performDestroyActivity(IBinder token, boolean finishing,
int configChanges, boolean getNonConfigInstance, String reason) {
ActivityClientRecord r = mActivities.get(token);
//省略一些代码
if (getNonConfigInstance) {
try {
r.lastNonConfigurationInstances
= r.activity.retainNonConfigurationInstances();
} catch (Exception e) {
// ...
}
}
}
我们可以看到onRetainNonConfigurationInstance方法返回的Object会赋值给ActivityClientRecord的lastNonConfigurationInstances。
答案
- 颗粒度不一样。onSaveInstanceState()是保存到Bundle中，只能保存Bundle能接受的数据类型，比如一些基本类型的数据。而onRetainNonConfigurationInstance() 可以保存任何类型的数据，数据类型是Object
- onSaveInstanceState()数据最终存储到ActivityManagerService的ActivityRecord中了，也就是存到系统进程中去了。而onRetainNonConfigurationInstance() 数据是存储到ActivityClientRecord中，也就是存到应用本身的进程中了
- onSaveInstanceState存到系统进程中，所以App被杀之后还是能恢复的。而onRetainNonConfigurationInstance存到本身进程中，App被杀是没法恢复的。