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);
    }
}
复制代码

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。

ViewModel源码研究之聊聊onSaveInstanceState和onRetainNonConfigurationInstance的区别

1. 前言

2. onSaveInstanceState(Bundle outState)方法详解

2.1 ActivityThread.callActivityOnSaveInstanceState(Bundle outState)

2.2 ActivityThread.handleStopActivity()

2.3 PendingTransactionActions$StopInfo.run()

2.4 ActivityManagerService.activityStopped()

2.5 ActivityRecord.activityStoppedLocked

2.6 被杀端后恢复数据过程

2.7 ActivityStackSupervisor.realStartActivityLocked

2.8 LaunchActivityItem.execute()

2.9 ActivityThread.performLaunchActivity(ActivityClientRecord r, Intent customIntent)

3. onRetainNonConfigurationInstance()

4. 答案

热门文章

最新文章

相关电子书

热门

活动广场

任务中心

开发者评测

高校计划

乘风者计划

训练营

阿里云MVP

话题

直播

下载

镜像站

技术资料

插件

ViewModel源码研究之聊聊onSaveInstanceState和onRetainNonConfigurationInstance的区别

1. 前言

2. onSaveInstanceState(Bundle outState)方法详解

2.1 ActivityThread.callActivityOnSaveInstanceState(Bundle outState)

2.2 ActivityThread.handleStopActivity()

2.3 PendingTransactionActions$StopInfo.run()

2.4 ActivityManagerService.activityStopped()

2.5 ActivityRecord.activityStoppedLocked

2.6 被杀端后恢复数据过程

2.7 ActivityStackSupervisor.realStartActivityLocked

2.8 LaunchActivityItem.execute()

2.9 ActivityThread.performLaunchActivity(ActivityClientRecord r, Intent customIntent)

3. onRetainNonConfigurationInstance()

4. 答案

热门文章

最新文章

相关电子书