27. Exercise: Using Navigation Listeners

You can prevent the drawer from being swiped anywhere other than the startDestination. All we need to do is call addOnDestinationChangedListener with a lambda that sets the DrawerLockMode depending on what destination we’re navigating to. When the id of our NavDestination matches the startDestination of our graph, we’ll unlock the drawerLayout; otherwise, we’ll lock and close the drawerLayout.

// prevent nav gesture if not on start destination
navController.addOnDestinationChangedListener { nc: NavController, nd: NavDestination, args: Bundle? ->
   if (nd.id == nc.graph.startDestination) {
       drawerLayout.setDrawerLockMode(DrawerLayout.LOCK_MODE_UNLOCKED)
   } else {
       drawerLayout.setDrawerLockMode(DrawerLayout.LOCK_MODE_LOCKED_CLOSED)
   }
}

And that’s it! Having listeners like this makes it powerful to do lots of cool stuff when the user navigates, and keeps the code in a single place in the app.