1. 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。
//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？
为了搞清楚这些问题，首先我在 "小站交流群" 提出了这些问题，幸运得是得到了一些积极的反馈。得到了一些结论和线索之后，便开始从源码中寻找答案，期间也遇到了一些问题，比如：ActivityManagerService的activityStopped方法的远程代理调用找不到，在群友们的帮助下，最终顺利找到，交流的过程中还是有不少收获。
2. onSaveInstanceState(Bundle outState)方法详解
首先在ComponentActivity的onSaveInstanceState(Bundle outState) 方法中加个断点。调用栈如下：
重点关注ActivityThread.callActivityOnSaveInstanceState(Bundle outState)
2.1 ActivityThread.callActivityOnSaveInstanceState(Bundle outState)
//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。
重点关注ActivityThread.handleStopActivity()
2.2 ActivityThread.handleStopActivity()
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);
if (localLOGV) Slog.v(
TAG, "Finishing stop of " + r + ": show=" + show
+ " win=" + r.window);
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;
}
注意到pendingActions.setStopInfo(stopInfo)
2.3 PendingTransactionActions$StopInfo.run()
@Override
public void run() {
// Tell activity manager we have been stopped.
try {
if (DEBUG_MEMORY_TRIM) Slog.v(TAG, "Reporting activity stopped: " + mActivity);
// TODO(lifecycler): Use interface callback instead of AMS.
ActivityManager.getService().activityStopped(
mActivity.token, mState, mPersistentState, mDescription);
} catch (RemoteException ex) {
// Dump statistics about bundle to help developers debug
final LogWriter writer = new LogWriter(Log.WARN, TAG);
final IndentingPrintWriter pw = new IndentingPrintWriter(writer, " ");
pw.println("Bundle stats:");
Bundle.dumpStats(pw, mState);
pw.println("PersistableBundle stats:");
Bundle.dumpStats(pw, mPersistentState);
if (ex instanceof TransactionTooLargeException
&& mActivity.packageInfo.getTargetSdkVersion() < Build.VERSION_CODES.N) {
Log.e(TAG, "App sent too much data in instance state, so it was ignored", ex);
return;
}
throw ex.rethrowFromSystemServer();
}
}
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()
//ActivityManagerService.java
@Override
public final void activityStopped(IBinder token, Bundle icicle,
PersistableBundle persistentState, CharSequence description) {
if (DEBUG_ALL) Slog.v(TAG, "Activity stopped: token=" + token);
// 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);
}
重点关注r.activityStoppedLocked(icicle, persistentState, description)
2.5 ActivityRecord.activityStoppedLocked
final void activityStoppedLocked(Bundle newIcicle, PersistableBundle newPersistentState,
CharSequence description) {
final ActivityStack stack = getStack();
if (mState != STOPPING) {
Slog.i(TAG, "Activity reported stop, but no longer stopping: " + this);
stack.mHandler.removeMessages(STOP_TIMEOUT_MSG, this);
return;
}
if (newPersistentState != null) {
persistentState = newPersistentState;
service.notifyTaskPersisterLocked(task, false);
}
if (DEBUG_SAVED_STATE) Slog.i(TAG_SAVED_STATE, "Saving icicle of " + this + ": " + icicle);
if (newIcicle != null) {
icicle = newIcicle;
haveState = true;
launchCount = 0;
updateTaskDescription(description);
}
if (!stopped) {
if (DEBUG_STATES) Slog.v(TAG_STATES, "Moving to STOPPED: " + this + " (stop complete)");
stack.mHandler.removeMessages(STOP_TIMEOUT_MSG, this);
stopped = true;
setState(STOPPED, "activityStoppedLocked");
mWindowContainerController.notifyAppStopped();
if (finishing) {
clearOptionsLocked();
} else {
if (deferRelaunchUntilPaused) {
stack.destroyActivityLocked(this, true /* removeFromApp */, "stop-config");
mStackSupervisor.resumeFocusedStackTopActivityLocked();
} else {
mStackSupervisor.updatePreviousProcessLocked(this);
}
}
}
}
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
2.7 ActivityStackSupervisor.realStartActivityLocked
//ActivityStackSupervisor.java
final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
boolean andResume, boolean checkConfig) throws RemoteException {
// 忽略其它代码
// Create activity launch transaction.
final ClientTransaction clientTransaction = ClientTransaction.obtain(app.thread,
r.appToken);
clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
System.identityHashCode(r), r.info,
// TODO: Have this take the merged configuration instead of separate global
// and override configs.
mergedConfiguration.getGlobalConfiguration(),
mergedConfiguration.getOverrideConfiguration(), r.compat,
r.launchedFromPackage, task.voiceInteractor, app.repProcState, r.icicle,
r.persistentState, results, newIntents, mService.isNextTransitionForward(),
profilerInfo));
// 忽略其它代码
}
我们看到最终是通过ActivityRecord.icicle恢复数据。
2.8 LaunchActivityItem.execute()
//LaunchActivityItem.java
@Override
public void execute(ClientTransactionHandler client, IBinder token,
PendingTransactionActions pendingActions) {
Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
ActivityClientRecord r = new ActivityClientRecord(token, mIntent, mIdent, mInfo,
mOverrideConfig, mCompatInfo, mReferrer, mVoiceInteractor, mState, mPersistentState,
mPendingResults, mPendingNewIntents, mIsForward,
mProfilerInfo, client);
client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
}
2.9 ActivityThread.performLaunchActivity(ActivityClientRecord r, Intent customIntent)
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);
}
}
3. onRetainNonConfigurationInstance()
该方法是在重建Activity时调用performDestoryActivity时会保存数据。
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) {
if (!mInstrumentation.onException(r.activity, e)) {
throw new RuntimeException(
"Unable to retain activity "
+ r.intent.getComponent().toShortString()
+ ": " + e.toString(), e);
}
}
}
我们可以看到onRetainNonConfigurationInstance方法返回的Object会赋值给ActivityClientRecord的lastNonConfigurationInstances。
4. 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。
欢迎关注“字节小站”同名公众号
