Navigation graph with multiple top level destinations

问题

I am implementing an android app (in Kotlin, but that is not relevant to the Problem) in my free time and I try to use android jetpack and new libraries. I have a single Activity with a navigation drawer. I try to follow the sample sunflower app. It uses the following combination in the main activity to enable the logic behind the navigation drawer:

appBarConfiguration = AppBarConfiguration(navController.graph, drawerLayout)
setSupportActionBar(findViewById(R.id.toolbar))
setupActionBarWithNavController(navController, appBarConfiguration)

Note on this code: This automatically will navigate to the correct fragments when clicked in the navigation drawer and close the drawer and keep them selected etc. All that boilerplate code. That is pretty neat and also works. As far as I understand this, the IDs of the navigation drawer menu items have to match the ids of the fragments in the navigation graph and this way they are connected.

The problem I have: When I use the navigation drawer to go to any fragment other than the starting fragment of the navigation graph, it will display a back button instead of the hamburger item. That is not what I expect, I would expect it still to be the hamburger item since the navigation drawer is for navigating between views on an equal level and not nested in each other, right? I expect a back button if I navigate to a subfragment of any fragment by clicking on elements in that fragment (for example list -> detail) but not if I navigate using the navigation drawer.

Now I traced that problem back to the AppBarConfiguration builder which reads on the constructor taking a navgraph The NavGraph whose start destination should be considered the only top level destination. I can fairly easily fix that by overriding AppBarConfiguration to return different top level destinations than just the starting destination of the navigation graph.

However my question is, why is there this behaviour default? Is it a bug? If I override this will I violate some design guidelines by Google? Should not every element in the navigation drawer be on the same level how I expect it? Is there a different solution inteded for I want to do?

回答1:

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.

回答2:

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)
}

回答3:

To get a correct behavior of toolbar and drawer with multiple top level destinations, you can use following code:

val navController = Navigation.findNavController(this, R.id.nav_host_fragment)
val toolbar = findViewById<Toolbar>(R.id.toolbar)
val drawerLayout = findViewById<DrawerLayout>(R.id.drawer_layout)

/*
Create AppBarConfiguration with set of top level destinations and drawerLayout
Set contains ids of your navigation graph screens
*/
val appBarConfiguration = AppBarConfiguration(
    setOf(R.id.defaultFragment, R.id.firstFragment, R.id.secondFragment), 
    drawer_layout
)

//finally configure toolbar
toolbar.setupWithNavController(navController, appBarConfiguration)

This code will ensure that hamburger icon is displayed on all of your top level destinations, and back button will appear when you navigate deeper.