Navigation graph with multiple top level destinations

You don't have to override AppBarConfiguration. Since version alpha7 AppBarConfiguration has a constructor with a set of ids for all top level destinations.

Set<Integer> topLevelDestinations = new HashSet<>();
topLevelDestinations.add(R.id.fragment1);
topLevelDestinations.add(R.id.fragment2);
appBarConfiguration = new AppBarConfiguration.Builder(topLevelDestinations)
                                             .setDrawerLayout(drawerLayout)
                                             .build();
NavigationUI.setupActionBarWithNavController(this, 
                                             this.navController,
                                             this.appBarConfiguration);

This is not default as the navigation graph has only a single start fragment which should always be the single entry point of the application.

Editing the default behavior with AppBarConfiguration does not make it behave as before, every top level fragment is placed on the back stack so back button will go to all top level fragments. It is unclear how I can make top level fragments as the first element of the back stack.

I made simple example for this issue. https://github.com/isaul32/android-sunflower

Create set of top level destinations at first

val topLevelDestinations = setOf(R.id.garden_fragment,
        R.id.plant_list_fragment)
appBarConfiguration = AppBarConfiguration.Builder(topLevelDestinations)
        .setDrawerLayout(drawerLayout)
        .build()

and then override onSupportNavigateUp function like this

override fun onSupportNavigateUp(): Boolean {
    return NavigationUI.navigateUp(navController, appBarConfiguration)
}