IssueTracker

Yes, this is correct. Having individual navigation graphs on each activities overrides or disallow up navigation to the main activity.

I believe I have found the potential problem that is causing this:
// @ androidx.navigation.NavController
// line 124

    // Now record the pop operation that we were sent
    if (!mBackStack.isEmpty()) {
        mBackStack.removeLast();
    }
    // We never want to leave NavGraphs on the top of the stack
    while (!mBackStack.isEmpty()
            && mBackStack.peekLast() instanceof NavGraph) {
        popBackStack();
    }

the condition in while loop is never satisfied because of the above `if` statement above it has already removed the last item of the mBackStack (which was the NavGraph) and thus popBackStack() is never called again.

According to Ian Lake, NavigationUI does not support up navigation yet, that is why this is a feature request.

+Pedro Varela - we don't support navigating up through activities as part of NavigationUI yet, please star the feature request: https://issuetracker.google.com/issues/79993862 (this issue)

Ian Lake said Up navigation is not supported through activities, only through fragments.

With the introduction of AppBarConfiguration, this is now possible with the setupActionBarWithNavController method by using an empty set of top level destinations (to always show the up button) and then overriding onSupportNavigateUp() as per the documentation: https://developer.android.com/topic/libraries/architecture/navigation/navigation-ui#action_bar

I'll leave this bug open as we still need an equivalent API for the Toolbar versions.

https://android-review.googlesource.com/846338 adds a setFallbackOnNavigateUpListener method on AppBarConfiguration that allows you to register an OnNavigateUpListener to handle cases where navController.navigateUp() return false.

This allows you to call onSupportNavigateUp() (or super.onSupportNavigateUp() if you're using an ActionBar) from that method to have your Up button transition between activities.

This will be available in 1.0.0-alpha09.

There is Jarks when implementing this way to navigate up between activities.

Thank you Ian :)

Project: platform/frameworks/support
Branch: androidx-master-dev

commit 3dd6d1fcdb3ffa7d4bd265a023f0ac7c94e14d19
Author: Ian Lake <ilake@google.com>
Date:   Mon Mar 18 09:36:37 2019

    Deprecate FragmentNavigator.instantiateFragment
   
    Instead of providing a custom subclass of
    FragmentNavigator, instantiation of Fragments
    should be controlled by setting a custom
    FragmentFactory, which is the default behavior
    of instantiateFragment() now.
   
    By deprecating the class, we ensure that
    developers who were overriding this method
    are informed of the changes and can change
    their code to use FragmentFactory.
   
    Test: ./gradlew checkApi
    BUG: 119054429
    Change-Id: Iff46919face98baa6dc62835ca3fc105d987d141

M       navigation/fragment/api/2.1.0-alpha01.txt
M       navigation/fragment/api/current.txt
M       navigation/fragment/src/main/java/androidx/navigation/fragment/FragmentNavigator.java

https://android-review.googlesource.com/930120
https://goto.google.com/android-sha1/3dd6d1fcdb3ffa7d4bd265a023f0ac7c94e14d19