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。

聊聊onSaveInstanceState和onRetainNonConfigurationInstance的区别