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

Can't wait to use this library after the move to AndroidX.
For us is a major blocking feature and we don't want to proceed overriding all the necessary classes.

Thank you Ian and the rest of the navigation team.

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

commit 74c3760e428648d00249dc1d38865b7802f48a9a
Author: Ian Lake <ilake@google.com>
Date:   Wed Mar 13 13:11:57 2019

    Use FragmentFactory in FragmentNavigator
   
    Rely on the FragmentManager's FragmentFactory
    to instantiate Fragments created for
    FragmentNavigator.
   
    Test: new FragmentNavigatorTest
    Fixes: 119054429
    Change-Id: Icbeb2771e29cafdc52e4c4ea770ce89f739921d1

M       navigation/fragment/src/androidTest/java/androidx/navigation/fragment/FragmentNavigatorTest.kt
M       navigation/fragment/src/main/java/androidx/navigation/fragment/FragmentNavigator.java

https://android-review.googlesource.com/928481
https://goto.google.com/android-sha1/74c3760e428648d00249dc1d38865b7802f48a9a

This is fixed internally - FragmentNavigator will use the FragmentFactory to create Fragments in the upcoming 2.1.0-alpha01 release.

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