title: Dynamic ActionBar title from a Fragment using AndroidX Navigation
body: <p>I am using the new <a href="https://developer.android.com/topic/libraries/architecture/navigation/" rel="noreferrer">Navigation</a> component from Android Jetpack.</p>
  
  <p>The root Activity setup is quite simple:</p>
  
  <pre><code>override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.activity_main)
      setSupportActionBar(toolbar)
  
      val navController = findNavController(R.id.navigationFragment)
      setupActionBarWithNavController(navController)
  
      bottomNavigationView.setupWithNavController(navController)
  }
  </code></pre>
  
  <p>It works well when the Fragment's title is defined in the navigation graph. But for one Fragment, I want to set the title dynamically.</p>
  
  <p>I tried with <code>findNavController().currentDestination.label = "Hello world"</code> but it does nothing.</p>
  
  <p>I could of course use a trick like <code>(activity as? AppCompatActivity)?.supportActionBar?.title = "Hello world"</code>, but I feel that it will break the magic that <code>setupActionBarWithNavController()</code> does for me. It there any way to update the Action Bar title dynamically? </p>