IssueTracker

Dialogs are separate windows that always sit above your activity's window. This means that the dialog will continue to intercept the system back button no matter what state the underlying FragmentManager is in, or what code you run in your Activity's onBackPressed() - which is where the OnBackPressedDispatcher plugs into.

Re comment#4 - as part of b/205992875 and Activity 1.5.0-alpha01, a new subclass of Dialog, ComponentDialog was added that has its own OnBackPressedDispatcher that is tied into the Dialog's onBackPressed().

As part of b/217620781 and AppCompat 1.5.0-alpha01, AppCompatDialog now extends ComponentDialog, thus allowing any subclass of AppCompatDialog to also have access to an appropriate OnBackPressedDispatcher.

As BottomSheetDialogFragment uses BottomSheetDialog internally and BottomSheetDialog extends AppCompatDialog, upgrading to AppCompat 1.5.0-alpha01 or higher, then your BottomSheetDialogFragment can indeed access a valid OnBackPressedDispatcher by getting it from the dialog:

This allows you to cast the result of super.onCreateDialog() to a BottomSheetDialog as part of your overload to onCreateDialog:

override fun onCreateDialog(savedInstanceState: Bundle?) = super.onCreateDialog(savedInstanceState).also { dialog ->
  (dialog as BottomSheetDialog).onBackPressedDispatcher.addCallback(this) {
    // Handle back
  }
}

Or access the dialog via requireDialog() from your overload of onViewCreated():

override fun onViewCreated(view: View, savedInstanceState: Bundle?) : Dialog {
  (requireDialog() as BottomSheetDialog).onBackPressedDispatcher.addCallback(this) {
    // Handle back
  }
}

Here is an extension to resolve the correct onBackPressedDispatcher for any fragment:

  fun Fragment.resolveOnBackPressedDispatcher(): OnBackPressedDispatcher {
    val dialogFragment = findDialogFragment(this)
    return if (dialogFragment != null) {
      // DialogFragments are running in a separate window
      // so they have their own back pressed dispatcher
      // https://issuetracker.google.com/issues/149173280
      (dialogFragment.requireDialog() as ComponentDialog)
          .onBackPressedDispatcher
    } else {
      requireActivity()
          .onBackPressedDispatcher
    }
  }

  private tailrec fun findDialogFragment(fragment: Fragment): DialogFragment? {
    return if (fragment is DialogFragment) {
      fragment
    } else {
      val parentFragment = fragment.parentFragment
      if (parentFragment != null) {
        findDialogFragment(parentFragment)
      } else {
        null
      }
    }
  }