“不保留活动”打开，导致app返回前台崩溃问题解决

问题描述

不保留活动开关打开，把app切入后台，会导致当前展示的Activity被回收，切到前台后重建。
我们有个业务场景是，Activity里面有个ViewPager2，VP里面放Fragment，Fragment的展示需要在Activity中做一些逻辑判断，然后才把ViewPager2的adapter set给ViewPager2，进而Fragment才展示出来（在showContent函数中做的）。在正常情况下，是没有问题的，看log1，A表示Activity，F表示Fragment。

2023-04-24 11:48:14.880 27631-27631/com.sohu.sohuvideo E/lzy: A onCreate
2023-04-24 11:48:14.929 27631-27631/com.sohu.sohuvideo E/lzy: A onStart
2023-04-24 11:48:14.934 27631-27631/com.sohu.sohuvideo E/lzy: A onResume
2023-04-24 11:48:14.970 27631-27631/com.sohu.sohuvideo E/lzy: A showContent
2023-04-24 11:48:14.981 27631-27631/com.sohu.sohuvideo E/lzy: A showContent
2023-04-24 11:48:14.996 27631-27631/com.sohu.sohuvideo E/lzy: F onCreate
2023-04-24 11:48:14.996 27631-27631/com.sohu.sohuvideo E/lzy: F onCreateView
2023-04-24 11:48:14.999 27631-27631/com.sohu.sohuvideo E/lzy: F onViewCreated
2023-04-24 11:48:15.003 27631-27631/com.sohu.sohuvideo E/lzy: F onStart
2023-04-24 11:48:15.004 27631-27631/com.sohu.sohuvideo E/lzy: F onResume()

我们把“不保留活动”开关打开，点击home键切入后台，执行流程如下。
可以看到Activity和Fragment都被回收了

2023-04-24 11:49:37.176 27631-27631/com.sohu.sohuvideo E/lzy: F onStop
2023-04-24 11:49:37.176 27631-27631/com.sohu.sohuvideo E/lzy: A onSaveInstanceState
2023-04-24 11:49:37.181 27631-27631/com.sohu.sohuvideo E/lzy: F onSaveInstanceState
2023-04-24 11:49:37.203 27631-27631/com.sohu.sohuvideo E/lzy: A onDestroy
2023-04-24 11:49:37.210 27631-27631/com.sohu.sohuvideo E/lzy: F onDestroy()

我们再把app切出到前台，Activity和Fragment都会重建，结果app崩溃了

2023-04-23 14:11:35.851 11874-11874/? E/AndroidRuntime: FATAL EXCEPTION: mainProcess: com.sohu.sohuvideo, PID: 11874kotlin.UninitializedPropertyAccessException: lateinit property firstPageSessionListForeverLiveData has not been initializedat com.sohu.sohuvideo.chat.fragment.SessionListFragment.subscribeToModel(SessionListFragment.kt:3)at com.sohu.sohuvideo.chat.fragment.SessionListFragment.onViewCreated(SessionListFragment.kt:8)at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:21)at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:25)at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:69)at androidx.fragment.app.FragmentManager.moveFragmentToExpectedState(FragmentManager.java:4)at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:75)at androidx.fragment.app.FragmentManager.dispatchStateChange(FragmentManager.java:3)at androidx.fragment.app.FragmentManager.dispatchActivityCreated(FragmentManager.java:3)at androidx.fragment.app.FragmentController.dispatchActivityCreated(FragmentController.java:1)at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:5)at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1433)at android.app.Activity.performStart(Activity.java:7976)at android.app.ActivityThread.handleStartActivity(ActivityThread.java:3544)at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:226)at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:206)at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:178)at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:102)at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2245)at android.os.Handler.dispatchMessage(Handler.java:107)at android.os.Looper.loop(Looper.java:237)at android.app.ActivityThread.main(ActivityThread.java:7840)at java.lang.reflect.Method.invoke(Native Method)at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:985)

崩溃的原因是，Fragment里有个lateinit属性没有被赋值，看代码逻辑是这个属性是在Activity中创建Fragment的时候传进来的，Fragment是在ViewPager2的Adapter中创建的。说明切到前台，执行流程是正常打开是不一样的，所以才导致这个问题。
切到前台执行流程如下

2023-04-24 11:50:28.542 27631-27631/com.sohu.sohuvideo E/lzy: A onCreate
2023-04-24 11:50:28.547 27631-27631/com.sohu.sohuvideo E/lzy: F onCreate
2023-04-24 11:50:28.576 27631-27631/com.sohu.sohuvideo E/lzy: A onStart
2023-04-24 11:50:28.609 27631-27631/com.sohu.sohuvideo E/lzy: F onCreateView
2023-04-24 11:50:28.610 27631-27631/com.sohu.sohuvideo E/lzy: F onViewCreated
2023-04-24 11:50:28.612 27631-27631/com.sohu.sohuvideo E/lzy: F onStart
2023-04-24 11:50:28.614 27631-27631/com.sohu.sohuvideo E/lzy: A onRestoreInstanceState
2023-04-24 11:50:28.616 27631-27631/com.sohu.sohuvideo E/lzy: A onResume
2023-04-24 11:50:28.621 27631-27631/com.sohu.sohuvideo E/lzy: F onResume()
2023-04-24 11:50:28.656 27631-27631/com.sohu.sohuvideo E/lzy: A showContent
2023-04-24 11:50:30.143 27631-27631/com.sohu.sohuvideo E/lzy: A showContent

我们发现切到前台，Fragment和Activity会重建，生命周期的执行流程如上，这个是因为系统知道这些被回收了，自动重建的。而Fragment中使用firstPageSessionListForeverLiveData是在onViewCreated，所以就报错了。

问题的解决

其实这个问题拔高一点，是activity和Fragment、Fragment和Fragment之前共享数据的问题，我们可以把需要共享的数据，放到依附Activity的ViewModel中，这样只要能获取到这个ViewModel实例就能拿到共享数据，而不是放在Activity中让Activity传给Fragment。

以上面的问题为例，firstPageSessionListForeverLiveData放在ViewModel里面，Fragment被重建的时候，可以直接从Activity的ViewModel中直接拿到这firstPageSessionListForeverLiveData。就可以解决这个问题。

这里需要说明的是，Activity重建后，按照执行流程还是会执行到给ViewPager2设置Adapter的流程中，但是，实际上已经不会再去场景Fragment了，也就是不会再执行Adapter的createFragment函数了。这个不知道为什么，应该是这种销毁重建有特殊处理吧，能把重建的Fragment和ViewPager2能自动关联起来，这块可以以后研究。其实也就是应该这个原因才有的崩溃。
locateTab()是去定位展示哪个Fragment的，是可以正常执行的

    private void showContent(boolean isDataNotEmpty) {Log.e("lzy", "A showContent");PullListMaskController.ListViewState newState = isDataNotEmpty ? PullListMaskController.ListViewState.LIST_REFRESH_COMPLETE : PullListMaskController.ListViewState.EMPTY_BLANK;LogUtils.d(TAG, "fyf--------showContent(): mCurrentState = " + mCurrentState + ", newState = " + newState);if (mCurrentState != newState) {mCurrentState = newState;if (mCurrentState == PullListMaskController.ListViewState.LIST_REFRESH_COMPLETE) {viewBinding.fragmentViewPager.setAdapter(new SessionPagerAdapter(this));viewBinding.slidingTabLayout2.setViewPager(viewBinding.fragmentViewPager, new String[]{"我关注的", "粉丝来信"});viewPagerReady = true;locateTab();showViewStatusWhenResponse(PullListMaskController.ListViewState.LIST_REFRESH_COMPLETE);onFollowUnReadChange();onOtherUnReadChange();} else {showViewStatusWhenResponse(PullListMaskController.ListViewState.EMPTY_BLANK);}}}private class SessionPagerAdapter extends FragmentStateAdapter {public SessionPagerAdapter(@NonNull FragmentActivity fragmentActivity) {super(fragmentActivity);}@NonNull@Overridepublic Fragment createFragment(int position) {SessionListFragment mFragment = new SessionListFragment();Bundle bundle = new Bundle();LogUtils.d(TAG, "fyf--------createFragment(): ");if (position == 0) {bundle.putString(SessionListFragment.TAB_TYPE, SessionListFragment.MY_ATTENTION);followFragment = mFragment;
//                followFragment.setSessionLiveData(followTabLiveData);
//                followFragment.setFirstPageSessionLiveData(followTabFirstPageLiveData);} else {bundle.putString(SessionListFragment.TAB_TYPE, SessionListFragment.OTHER);otherFragment = mFragment;
//                otherFragment.setSessionLiveData(otherTabLiveData);
//                otherFragment.setFirstPageSessionLiveData(otherTabFirstPageLiveData);}mFragment.setArguments(bundle);mFragment.setCallBack(new SessionListFragment.ICallback() {@Overridepublic int getTotalItemCount() {int totalCount = 0;if (followFragment != null) {totalCount += followFragment.getItemCount();}if (otherFragment != null) {totalCount += otherFragment.getItemCount();}return totalCount;}});return mFragment;}

还有一点需要说明的是，ViewPager2是有两个tab，也就是有两个Fragment，如果两个Fragment都创建过，销毁重建的时候，两个Fragment都会重建出来，确定了展示那个Fragment了之后，另一个Fragment又被销毁了，不知道为什么。

2023-04-24 14:54:39.100 7400-7400/? E/lzy: A onCreate
2023-04-24 14:54:39.106 7400-7400/? E/lzy: F onCreate
2023-04-24 14:54:39.106 7400-7400/? E/lzy: F onCreate
2023-04-24 14:54:39.138 7400-7400/? E/lzy: A onStart
2023-04-24 14:54:39.186 7400-7400/? E/lzy: F onCreateView
2023-04-24 14:54:39.187 7400-7400/? E/lzy: F onViewCreated
2023-04-24 14:54:39.190 7400-7400/? E/lzy: F onCreateView
2023-04-24 14:54:39.191 7400-7400/? E/lzy: F onViewCreated
2023-04-24 14:54:39.193 7400-7400/? E/lzy: F onStart
2023-04-24 14:54:39.193 7400-7400/? E/lzy: F onStart
2023-04-24 14:54:39.198 7400-7400/? E/lzy: A onRestoreInstanceState
2023-04-24 14:54:39.199 7400-7400/? E/lzy: A onResume
2023-04-24 14:54:39.203 7400-7400/? E/lzy: F onResume()
2023-04-24 14:54:39.232 7400-7400/? E/lzy: A showContent
2023-04-24 14:54:39.236 7400-7400/? E/lzy: F onPause
2023-04-24 14:54:39.236 7400-7400/? E/lzy: F onResume()
2023-04-24 14:54:39.330 7400-7400/? E/lzy: A showContent
2023-04-24 14:54:49.237 7400-7400/? E/lzy: F onSaveInstanceState
2023-04-24 14:54:49.243 7400-7400/? E/lzy: F onStop
2023-04-24 14:54:49.247 7400-7400/? E/lzy: F onDestroy()

看上面的log，最后两行log，切出前台后，两个Fragment都被重建了，最后有一个被销毁了

参考

Android开发之InstanceState详解
Fragment 的过去、现在和将来