title: Get Backstack status using android navigation component
body: <p>I want to implement backpress behaviour such that it prompts a confirmation popup when you press back with the backstack empty, otherwise it pops the next fragment in the stack.</p>
  
  <p>I'm trying to get the backstack count, but i always get 0 from both fragment managers</p>
  
  <pre><code> getSupportFragmentManager().getBackStackEntryCount();
   getFragmentManager().getBackStackEntryCount();
  </code></pre>
  
  <p>I guess it should work since i checked the google code of fragment navigator and it adds to backstack through the canonical fragment transaction:</p>
  
  <p>FragmentNavigator.java:</p>
  
  <pre><code> if (initialNavigation || isClearTask) {
          backStackEffect = BACK_STACK_DESTINATION_ADDED;
      } else if (isSingleTopReplacement) {
          // Single Top means we only want one instance on the back stack
          if (mBackStack.size() &gt; 1) {
              // If the Fragment to be replaced is on the FragmentManager's
              // back stack, a simple replace() isn't enough so we
              // remove it from the back stack and put our replacement
              // on the back stack in its place
              mFragmentManager.popBackStack();
              ft.addToBackStack(Integer.toString(destId));
              mIsPendingBackStackOperation = true;
          }
          backStackEffect = BACK_STACK_UNCHANGED;
      } else {
          ft.addToBackStack(Integer.toString(destId));
          mIsPendingBackStackOperation = true;
          backStackEffect = BACK_STACK_DESTINATION_ADDED;
      }
      ft.setReorderingAllowed(true);
      ft.commit();
  </code></pre>
  
  <p>I didn't find any API to retrieve this information through NavController or Navigator neither.</p>
  
  <p>Thanks</p>