IssueTracker

I'd like to add that this automatic removal of the NavGraph at the top of the stack is also making it hard for me to implement proper lateral top-level navigation.
A practical case where this kind of navigation happens is with a Material Design bottom navigation bar, where I have a NavHost above the bottom bar.

Given this navigation scheme:
A - B   (top-level)
     B1 (first sub-level)

When starting the Activity, the back stack is
0. A
1. NavGraph

If I wanted to navigate to B laterally, I would do:
navController.popBackStack(R.id.navGraph, false)
navController.navigate(R.id.B)

But the resulting back stack is:
0. B

Now, navigating between top-level destinations actually works (though popBackStack returns false); the problem arises again if I navigate to B1:
0. B1
1. B

When navigating from B1 to a top-level destination like A, the stack becomes:
0. A
1. B1
2. B

That is, A is not a top-level destination in the back stack: if I press back from A, I intend to exit the app, but I'm taken back to B1.

It would be nice to have a fixed reference to the bottom of the stack. Right now I'm working around this with a horrible ` while (navController.popBackStack()) continue`.

Is this going to be fixed?

I have the same issue with a setup as follows:

Navigation graph.

    <navigation xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        app:startDestination="@+id/startupGraph">

        <navigation
            android:id="@+id/startupGraph"
            app:startDestination="@id/launcher_home">
            <fragment
                android:id="@+id/launcher_home"
                android:name="com.example.android.codelabs.navigation.MainFragment"
                android:label="@string/home"
                tools:layout="@layout/main_fragment" />
        </navigation>

        <navigation
            android:id="@+id/homeGraph"
            app:startDestination="@id/flow_step_one">
            <fragment
                android:id="@+id/flow_step_one"
                android:name="com.example.android.codelabs.navigation.FlowStepOneFragment"
                tools:layout="@layout/flow_step_one_fragment" />
            <fragment
                android:id="@+id/flow_step_two"
                android:name="com.example.android.codelabs.navigation.FlowStepTwoFragment"
                tools:layout="@layout/flow_step_two_fragment" />
        </navigation>

    </navigation>
   
MainFragment.

        override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
            super.onViewCreated(view, savedInstanceState)
             view.findViewById<Button>(R.id.navigate_dest_bt).setOnClickListener{
                 Navigation.findNavController(it)
                         .navigate(R.id.homeGraph, null, NavOptions.Builder()
                                 .setPopUpTo(R.id.startupGraph, false)
                                 .build())
             }
        }

FlowStepOneFragment.

        view.findViewById<View>(R.id.next_button).setOnClickListener {
            Navigation.findNavController(it)
                    .navigate(R.id.flow_step_two, null, NavOptions.Builder()
                            .setPopUpTo(R.id.flow_step_one, true)
                            .build())
        }
    }
}

Causes the following error:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.android.codelabs.navigation, PID: 12579
    java.lang.IllegalArgumentException: Navigator androidx.navigation.fragment.FragmentNavigator@d70ec6f reported navigation to unknown destination id com.example.android.codelabs.navigation:id/flow_step_two

The original issue was fixed as a result of https://android-review.googlesource.com/833717 and a number of other internal changes and that will be available in alpha08.

Discovered another issue when doing this specific set of navigation actions, so leaving this open until that part is also fixed.

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

commit d3507cb9dab82264070c72a37dbde1dfb43ef578
Author: Ian Lake <ilake@google.com>
Date:   Tue Dec 04 14:24:32 2018

    Split pop and dispatch logic
   
    When using popUpTo, navigate() would
    internally call popBackStack(), resulting
    in additional dispatch calls to
    OnDestinationChangedListener before the
    navigate() operation itself was done.
   
    By separating the logic, we can ensure that
    we don't prematurely remove NavGraph destinations
    that should still exist after the navigate()
    operation and that developers only see a
    single final OnDestinationChangedListener callback.
   
    Test: new test
    BUG: 113611083
    Change-Id: Ib63e4156521259a945ffecf679ccb404ac192017

M       navigation/runtime/src/androidTest/java/androidx/navigation/NavControllerTest.kt
M       navigation/runtime/src/androidTest/res/navigation/nav_nested_start_destination.xml
M       navigation/runtime/src/main/java/androidx/navigation/NavController.java

https://android-review.googlesource.com/840996
https://goto.google.com/android-sha1/d3507cb9dab82264070c72a37dbde1dfb43ef578