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。
