IssueTracker

Please include a sample project using Navigation 2.4.0-rc01 that reproduces your issue.

Thank you but this version can still be reproduced，here is the project that can be reproduced: https://github.com/wanderingGuy/navigation_sample.

1. enable the setting of "don't keep activity" in developer options.
2. start app, click "About" button.
3. press HOME button.
4. click app icon in launcher again.

Thanks, the sample project was really helpful. While the behavior here could be considered 'technically correct', this is unintentional and we'll look to see what we can do to improve this case.

What's actually going on

This looks like a case where there's a marked difference in behavior between fragment lifecycle callbacks and AndroidX Lifecycle.

Specifically, there are three elements in play:

1. The NavHostFragment
2. The NavController and the NavBackStackEntry for each entry in its back stack
3. The child Fragment for each entry in the back stack that corresponds with a fragment destination

When you first load your activity, the NavHostFragment is in its initial state (INITIALIZED in the AndroidX Lifecycle terms). In the NavHostFragment's onCreate(), it sets the graph you've set via the app:navGraph="@navigation/nav_graph" in your activity_main.xml on the NavController it owns. This is what causes Fragments to be added to the NavHostFragment's childFragmentManager.

Because of how Fragments work currently, the fragment lifecycle callbacks for upward transitions (like onCreate()) are called before the AndroidX Lifecycle is moved to that same state:

1. NavHostFragment gets its onCreate() called
2. NavHostFragment inflates its navigation graph and adds the Title fragment to the childFragmentManager
3. Fragments call onCreate() on their child fragments (i.e., Title)
4. NavHostFragment's Lifecycle is moved to CREATED
5. Fragments move their child fragments to CREATED

The same thing happens for onStart()+STARTED and onResume()+RESUMED.

This all works as expected when dealing with only Fragments - the parent fragment gets onCreate() before its child fragments. That nesting is also done on the AndroidX Lifecycle side when it comes to the Fragment's Lifecycle. That idea of nesting Lifecycle changes between parent and child is a key part of the whole infrastructure. The goal is that at no point does a 'child' have a higher Lifecycle state as its parent.

NavController depends solely on AndroidX Lifecycle to drive the Lifecycle of each NavBackStackEntry. And where does that NavController gets its AndroidX Lifecycle callbacks from? From the NavHostFragment that it has been created by. This means that what is actually happening adds an additional line:

1. NavHostFragment gets its onCreate() called
2. NavHostFragment inflates its navigation graph and adds the Title fragment to the childFragmentManager
3. Fragments call onCreate() on their child fragments (i.e., Title)
4. NavHostFragment's Lifecycle is moved to CREATED
5. NavController moves each NavBackStackEntry to CREATED
6. Fragments move their child fragments to CREATED

This is where your error message comes in: you're accessing a NavBackStackEntry's ViewModel in step 3 which is before it actually moves to CREATED (that's step 5).

Caused by: java.lang.IllegalStateException: You cannot access the NavBackStackEntry's ViewModels until it is added to the NavController's back stack (i.e., the Lifecycle of the NavBackStackEntry reaches the CREATED state).

You'd find you'd get the same exact error message if you moved your by navGraphViewModels() code to the Title fragment. You just also see the same issue after turning on 'Don't keep activities' since when recreating the entire activity, all child fragments (including both Title and About) are going through these same steps of having their onCreate() called before the NavHostFragment has had its AndroidX Lifecycle moved to CREATED. This is also explains why it works when you navigate to About manually - by that point, the NavHostFragment is RESUMED and each NavBackStackEntry is able to move beyond the CREATED state immediately as part of that call to navigate().

Workarounds

So if you want to run your code after the NavController moves each NavBackStackEntry to CREATED, you'll want to move your code to step 6 and use an AndroidX Lifecycle driven callback in a LifecycleObserver.

Using the DefaultLifecycleObserver class is by far the easiest way to do this. By upgrading to Lifecycle 2.4.0 (by updating the version in versions.gradle and adding an explicit dependency on implementation deps.lifecycle.runtime), you could rewrite your About fragment as:

class About : Fragment(), DefaultLifecycleObserver {

    private val vm by navGraphViewModels<TestViewModel>(R.id.home)

    // Note how this has a different signature from the Fragment method of the same name
    // this signature is the one from DefaultLifecycleObserver
    override fun onCreate(owner: LifecycleOwner) {
        Log.d("About", "About string ${vm.name}")
    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_about, container, false)
    }
}

By moving to AndroidX Lifecycle callbacks, you ensure that your child fragment calls always happen after the parent NavHostFragment and its NavController have moved through that same Lifecycle state.

Next steps on our side

As per the error message, the intent is that you cannot access the ViewModels until the NavBackStackEntry has been added to the NavController's back stack. However, in this case, the NavBackStackEntry is on the back stack (that's why by navGraphViewModels() was able to find the NavBackStackEntry associated with R.id.home in the first place - you wouldn't have been able to get to this error message if you had chosen an ID that didn't exist on the back stack).

Therefore there might be more clever logic we can do to cover the 'added to back stack but not yet CREATED' case to allow for this type of call from a fragment's onCreate() to succeed. I'll leave this issue open to cover exactly that work.

Future state

There's ongoing work being done in b/127528777 to drive the Fragment lifecycle callbacks via AndroidX Lifecycle transitions, which would automatically have fixed this issue - your fragment lifecycle callback of onCreate() would only occur when your AndroidX Lifecycle has moved to CREATED, effectively removing the difference between the two types of callbacks. I'd strongly suggest staring that issue as well to track when that work will be available.

Thanks, I've learnt a lot and you save my life, I'll keep an eye on this issue until there're any updates.

We've fixed this issue internally and it will be available in the upcoming Navigation 2.5.0-alpha01 and Navigation 2.4.1 releases.