IssueTracker

Welcome to 2020, multiple back stacks issue. Throughout this update I'll be using Markdown syntax, so make sure to hit the 'Use the new UI' button on the top of the page to switch over to the new UI and the markdown goodness.

As mentioned in comment#65, our goal was to ship multiple back stacks in Navigation 2.3.0-alpha01 and the required support in Fragment 1.3.0-alpha01 once the blocking issue in Fragments was fixed. While we've made great progress on it (31 CLs touching some gnarly internals of FragmentManager), fixing that has exposed a number of other issues so far (many of which were released in Fragment 1.2.0). Unfortunately, one of those found issues does block our original restructuring work needed to get multiple back stacks working at the Fragment level. We have a plan to tackle that work and the initial work (another 5 CLs) is proceeding as expected.

However, this additional layer of work was not anticipated in the above comment#65. With some substantial Navigation features planned for Navigation 2.3.0-alpha01 and more to come, I do not feel comfortable holding up that release for an indeterminate amount of time and delaying those other features.

Instead, we'll be releasing Navigation 2.3.0-alpha01 without multiple back stack support. Our plan is to finish up the Fragment rework in the upcoming Fragment 1.3.0-alpha01 release and ensure that everything works better than ever in the single back stack world that is FragmentManager today. Oh, and get Navigation 2.3.0 to stable as soon as possible.

I've updated the public hotlists to denote that multiple back stack support is now tentatively scheduled for Navigation 2.4.0-alpha01 and Fragment 1.4.0-alpha01, respectively. If everything goes better than expected, it may be earlier than that.

Please be considerate when replying to this issue. It does notify everyone who has starred this issue.

Please be considerate when replying to this issue, because hundreds of people will get emails for unhelpful contributions.

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.