异常在这里抛出(在FragmentActivity中):

@Override
public void onBackPressed() {
    if (!mFragments.getSupportFragmentManager().popBackStackImmediate()) {
        super.onBackPressed();
    }
}

在FragmentManager.popBackStatckImmediate()中，FragmentManager.checkStateLoss()首先被调用。这就是IllegalStateException的原因。参见下面的实现:

private void checkStateLoss() {
    if (mStateSaved) { // Boom!
        throw new IllegalStateException(
                "Can not perform this action after onSaveInstanceState");
    }
    if (mNoTransactionsBecause != null) {
        throw new IllegalStateException(
                "Can not perform this action inside of " + mNoTransactionsBecause);
    }
}

我通过使用一个标志来标记Activity的当前状态来解决这个问题。以下是我的解决方案:

public class MainActivity extends AppCompatActivity {
    /**
     * A flag that marks whether current Activity has saved its instance state
     */
    private boolean mHasSaveInstanceState;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        mHasSaveInstanceState = true;
        super.onSaveInstanceState(outState);
    }

    @Override
    protected void onResume() {
        super.onResume();
        mHasSaveInstanceState = false;
    }

    @Override
    public void onBackPressed() {
        if (!mHasSaveInstanceState) {
            // avoid FragmentManager.checkStateLoss()'s throwing IllegalStateException
            super.onBackPressed();
        }
    }
}

这是2017年10月，谷歌制作了Android支持库与新事物称为生命周期组件。它为“onSaveInstanceState后不能执行此操作”的问题提供了一些新的思路。

简而言之:

使用生命周期组件来确定弹出片段的时间是否正确。

带解释的较长版本:

why this problem come out? It's because you are trying to use FragmentManager from your activity(which is going to hold your fragment I suppose?) to commit a transaction for you fragment. Usually this would look like you are trying to do some transaction for an up coming fragment, meanwhile the host activity already call savedInstanceState method(user may happen to touch the home button so the activity calls onStop(), in my case it's the reason) Usually this problem shouldn't happen -- we always try to load fragment into activity at the very beginning, like the onCreate() method is a perfect place for this. But sometimes this do happen, especially when you can't decide what fragment you will load to that activity, or you are trying to load fragment from an AsyncTask block(or anything will take a little time). The time, before the fragment transaction really happens, but after the activity's onCreate() method, user can do anything. If user press the home button, which triggers the activity's onSavedInstanceState() method, there would be a can not perform this action crash. If anyone want to see deeper in this issue, I suggest them to take a look at this blog post. It looks deep inside the source code layer and explain a lot about it. Also, it gives the reason that you shouldn't use the commitAllowingStateLoss() method to workaround this crash(trust me it offers nothing good for your code) How to fix this? Should I use commitAllowingStateLoss() method to load fragment? Nope you shouldn't; Should I override onSaveInstanceState method, ignore super method inside it? Nope you shouldn't; Should I use the magical isFinishing inside activity, to check if the host activity is at the right moment for fragment transaction? Yeah this looks like the right way to do. Take a look at what Lifecycle component can do. Basically, Google makes some implementation inside the AppCompatActivity class(and several other base class you should use in your project), which makes it a easier to determine current lifecycle state. Take a look back to our problem: why would this problem happen? It's because we do something at the wrong timing. So we try not to do it, and this problem will be gone. I code a little for my own project, here is what I do using LifeCycle. I code in Kotlin.

val hostActivity: AppCompatActivity? = null // the activity to host fragments. It's value should be properly initialized.

fun dispatchFragment(frag: Fragment) {
    hostActivity?.let {
       if(it.lifecyclecurrentState.isAtLeast(Lifecycle.State.RESUMED)){
           showFragment(frag)
       }
    }
}

private fun showFragment(frag: Fragment) {
    hostActivity?.let {
        Transaction.begin(it, R.id.frag_container)
                .show(frag)
                .commit()
    }

如上所示。我将检查主机活动的生命周期状态。对于支持库中的生命周期组件，这可能更具体。代码lifecyclecurrentState.isAtLeast(lifeccycle . state . resumed)的意思是，如果当前状态至少是onResume，不晚于它?这确保我的方法不会在其他生命状态(如onStop)期间执行。

Is it all done? Of course not. The code I have shown tells some new way to prevent application from crashing. But if it do go to the state of onStop, that line of code wont do things and thus show nothing on your screen. When users come back to the application, they will see an empty screen, that's the empty host activity showing no fragments at all. It's bad experience(yeah a little bit better than a crash). So here I wish there could be something nicer: app won't crash if it comes to life state later than onResume, the transaction method is life state aware; besides, the activity will try continue to finished that fragment transaction action, after the user come back to our app. I add something more to this method:

class FragmentDispatcher(_host: FragmentActivity) : LifecycleObserver {
    private val hostActivity: FragmentActivity? = _host
    private val lifeCycle: Lifecycle? = _host.lifecycle
    private val profilePendingList = mutableListOf<BaseFragment>()

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun resume() {
        if (profilePendingList.isNotEmpty()) {
            showFragment(profilePendingList.last())
        }
    }

    fun dispatcherFragment(frag: BaseFragment) {
        if (lifeCycle?.currentState?.isAtLeast(Lifecycle.State.RESUMED) == true) {
            showFragment(frag)
        } else {
            profilePendingList.clear()
            profilePendingList.add(frag)
        }
    }

    private fun showFragment(frag: BaseFragment) {
        hostActivity?.let {
            Transaction.begin(it, R.id.frag_container)
                    .show(frag)
                    .commit()
        }
    }
}

我在这个dispatcher类中维护了一个列表，来存储那些没有机会完成事务操作的片段。当用户从主屏幕返回并发现仍然有片段等待启动时，它将转到@OnLifecycleEvent(lifeccycle . event . on_resume)注释下的resume()方法。现在我认为它应该像我预期的那样工作。

如果你在onActivityResult中做一些FragmentTransaction，你可以做什么，你可以在onActivityResult中设置一些布尔值，然后在onResume中，你可以根据布尔值做你的FragmentTransaction。请参考下面的代码。

@Override
protected void onResume() {
    super.onResume;
    if(isSwitchFragment){
        isSwitchFragment=false;
        bottomNavigationView.getTabAt(POS_FEED).select();
    }
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    if (requestCode == FilterActivity.FILTER_REQUEST_EVENT && data != null) {
        isSwitchFragment=true;
    }
}

把这个添加到你的活动中

@Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    if (outState.isEmpty()) {
        // Work-around for a pre-Android 4.2 bug
        outState.putBoolean("bug:fix", true);
    }
}

注意，使用transaction.commitAllowingStateLoss()可能会给用户带来糟糕的体验。有关为什么会抛出这个异常的更多信息，请参阅这篇文章。

提供:IllegalStateException的解决方案

这个问题困扰了我很久，但幸运的是，我想出了一个具体的解决方案。这里有详细的解释。

使用commitAllowStateloss()可能会防止这种异常，但会导致UI不规范。到目前为止，我们已经了解到，当我们在Activity状态丢失后试图提交一个片段时，会遇到IllegalStateException—所以我们应该延迟事务，直到状态恢复。可以像这样简单地做

声明两个私有布尔变量

 public class MainActivity extends AppCompatActivity {

    //Boolean variable to mark if the transaction is safe
    private boolean isTransactionSafe;

    //Boolean variable to mark if there is any transaction pending
    private boolean isTransactionPending;

现在在onPostResume()和onPause中，我们设置和取消设置布尔变量isTransactionSafe。想法是只有当活动在前台时才标记事务安全，这样就不会有无状态的机会。

/*
onPostResume is called only when the activity's state is completely restored. In this we will
set our boolean variable to true. Indicating that transaction is safe now
 */
public void onPostResume(){
    super.onPostResume();
    isTransactionSafe=true;
}
/*
onPause is called just before the activity moves to background and also before onSaveInstanceState. In this
we will mark the transaction as unsafe
 */

public void onPause(){
    super.onPause();
    isTransactionSafe=false;

}

private void commitFragment(){
    if(isTransactionSafe) {
        MyFragment myFragment = new MyFragment();
        FragmentManager fragmentManager = getFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.add(R.id.frame, myFragment);
        fragmentTransaction.commit();
    }
}

-我们到目前为止所做的将从IllegalStateException保存，但我们的事务将丢失，如果它们在活动移动到后台后完成，有点像commitAllowStateloss()。为了解决这个问题，我们有一个isTransactionPending布尔变量

public void onPostResume(){
   super.onPostResume();
   isTransactionSafe=true;
/* Here after the activity is restored we check if there is any transaction pending from
the last restoration
*/
   if (isTransactionPending) {
      commitFragment();
   }
}


private void commitFragment(){

 if(isTransactionSafe) {
     MyFragment myFragment = new MyFragment();
     FragmentManager fragmentManager = getFragmentManager();
     FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
     fragmentTransaction.add(R.id.frame, myFragment);
     fragmentTransaction.commit();
     isTransactionPending=false;
 }else {
     /*
     If any transaction is not done because the activity is in background. We set the
     isTransactionPending variable to true so that we can pick this up when we come back to
foreground
     */
     isTransactionPending=true;
 }
}