IssueTracker

My solution:

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val appBarConfigurationIds = setOf(R.id.fragmentTab1, R.id.fragmentTab2, R.id.fragmentTab3, R.id.fragmentTab4)
        val appBarConfiguration = AppBarConfiguration(appBarConfigurationIds)

        setupActionBarWithNavController(navController, appBarConfiguration)

        bottomNavigationView.setOnNavigationItemSelectedListener { menuItem ->
            if (menuItem.itemId == navController.currentBackStackEntry?.destination?.id)
                return@setOnNavigationItemSelectedListener false
            appBarConfigurationIds.forEach { fragmentId ->
                if (fragmentId == menuItem.itemId) {
                    navController.navigate(fragmentId, null, NavOptions.Builder() <--- M
                        .setLaunchSingleTop(true)                                 <--- A
                        .setPopUpTo(navController.currentDestination!!.id, true)  <--- G
                        .build())                                                 <--- I
                    return@setOnNavigationItemSelectedListener true               <--- C
                }
            }
            false
        }

        navController.addOnDestinationChangedListener { controller, destination, arguments ->
            if (appBarConfigurationIds.indexOf(destination.id) != -1) {
                controller.graph.startDestination = destination.id// <--- MAGIC
            }
        }
    }

    override fun onSupportNavigateUp() = navController.navigateUp()

    private var lastBackPressed = 0L

    override fun onBackPressed() {
        if (navHostFragment.childFragmentManager.backStackEntryCount == 0) {
            if (lastBackPressed + 1000 > System.currentTimeMillis()) {
                super.onBackPressed()
            } else {
                lastBackPressed = System.currentTimeMillis()
                toast("Double tap for exit")
            }
        } else
            super.onBackPressed()
    }

}

There's a workaround provided the NavigationAdvancedSample, which is basically having multiple NavHostFragment, so as NavController, to work with the BottomNavigationView.

I'd like to know any updates from Google to officially support this kinda very common requirement, thanks

As per comment#77, the 31 changes in FragmentManager to fix the blocking issue turned into literally over 120 changes to make up the rebuilt internals for FragmentManager.

As seen on the Fragment release page that work is nearly complete as we prepare for hopefully one final Fragment 1.3.0-rc02 release before the stable release, thus clearing the way entirely for multiple back stacks (as mentioned in that blog post, in our reddit AMA, and during the MAD Skills Navigation Q&A).

Our own internal API design process has reached the point where we have a firm direction and you should begin to see changes come through attached to this bug as we build up to the final APIs to support multiple back stacks at the Fragment and Navigation level.

This will include:

• The Fragment APIs for supporting multiple back stacks (thus enabling multiple back stacks even if you do not use Navigation)
• The generic Navigation API that will allow any Navigator (be it Fragments, Compose, etc) to support multiple back stacks
• Rework of the existing Navigators to support multiple back stacks out of the box
• The APIs in NavigationUI and other surfaces to allow you to easily opt in to using multiple back stacks

I'd ask that you hold your comments on the intermediate steps along the way as much of it will make more sense when taken as a whole once we release the first alphas (still planned for Fragment 1.4.0-alpha01 and Navigation 2.4.0-alpha01). Definitely expect a more thorough run through of how to use of these APIs via documentation, samples (including updating the NavigationAdvancedSample to use the new APIs), blog posts, videos, and everything else at that point and onward. Besides seeing each commit come through on this issue, we'll also make sure to point out when a first snapshot is available with working APIs for those of you who would like an early preview before the first alpha releases.

Again, please keep in mind the many people who get emailed for each comment on this issue - let's continue to keep this as a good channel for updates and not for general discussion or requests for additional progress updates.

As per comment#94, this issue is covering all of the multiple back stack work across Fragments and Navigation. With this last change in comment#111, the work on the Fragment level has completed and the new saveBackStack(name: String) and restoreBackStack(name: String) APIs have been added to FragmentManager in snapshot build 7248696 of Fragment 1.4. I would still recommend waiting until the actual Fragment 1.4.0-alpha01 release, which will be accompanied with documentation around usage and restrictions (all of which have been thoroughly covered in the detailed descriptions in the changes above).

With the Fragment work complete, the implementation work on the Navigation side of multiple back stacks is next up - you'll continue to see changes added to this CL as that work continues.

As a reminder, that Navigation work includes:

• The generic Navigation API that will allow any Navigator (be it Fragments, Compose, etc) to support multiple back stacks
• Rework of the existing Navigators to support multiple back stacks out of the box
• The APIs in NavigationUI and other surfaces to allow you to easily opt in to using multiple back stacks

Also as mentioned before:

I'd ask that you hold your comments on the intermediate steps along the way as much of it will make more sense when taken as a whole once we release the first alphas (still planned for Fragment 1.4.0-alpha01 and Navigation 2.4.0-alpha01). Definitely expect a more thorough run through of how to use of these APIs via documentation, samples (including updating the NavigationAdvancedSample to use the new APIs), blog posts, videos, and everything else at that point and onward. Besides seeing each commit come through on this issue, we'll also make sure to point out when a first snapshot is available with working APIs for those of you who would like an early preview before the first alpha releases.

Again, please keep in mind the many people who get emailed for each comment on this issue - let's continue to keep this as a good channel for updates and not for general discussion or requests for additional progress updates. If you'd like to receive updates, please star the issue, not comment on it.

A question: with all due respect, are these automatic comments really necessary or useful? I'd like to keep my "star" on this issue, and know when it will be fixed. But the noise doesn't help. Thanks a lot!

Re #131 - yes, they are necessary. As stated multiple times and seen in the hotlists this bug is associated with, this will be fixed with Fragment 1.4.0-alpha01 and Navigation 2.4.0-alpha01, so if you're looking for only an update on when those are released and not for updates on the progress leading up to that point, you can use the AndroidX releases page and the RSS feed it has there.

With these last changes and snapshot build #7351514, we've completed all of the required changes to support multiple back stacks in:

• Fragments (i.e., without using the Navigation Component at all): this is an opt in change by using the new FragmentManager APIs of saveBackStack and restoreBackStack.

• The core Navigation Runtime: adds opt-in new NavOptions methods for restoreState and saveState and a new overload of popBackStack() that also accepts a saveState boolean (defaults to false).

• Navigation with Fragments: the FragmentNavigator now utilizes the new Navigator APIs to properly translate the Navigation Runtime APIs into the Fragment APIs by using the Navigation Runtime APIs.

• NavigationUI: The onNavDestinationSelected(),BottomNavigationView.setupWithNavController() and NavigationView.setupWithNavController() now use the new restoreState and saveState NavOptions by default whenever they would pop the back stack. This means that every app using those NavigationUI APIs will get multiple back stacks without any code changes on their part after upgrading the Navigation 2.4.0-alpha01.

• Navigation Compose: NavHost and the ComposeNavigator have also been updated to support saving and restoring the state by using the Navigation Runtime APIs. An example of using the restoreState+saveState APIs with a bottom nav can be seen in the BottomBarNavDemo.

All of these changes will be available in the upcoming Fragment 1.4.0-alpha01 and Navigation 2.4.0-alpha01 releases. (Note that Navigation Compose versioning has been merged into the rest of Navigation as per b/172823546).

That being said:

• there's a lot more documentation and samples that need to be updated. We'll be tackling as much of this as possible between now and the alpha01 release.
• As you may have seen, there's been a huge number of changes needed to make this happen across all levels. As such, please file separate bugs as you discover any edge cases that we need to handle and we'll make sure those changes are addressed in an alpha02 release.

I'd like to thank you for your patience and respectfulness in not spamming this issue with comments.