Dynamic ActionBar title from a Fragment using AndroidX Navigation

As of 1.0.0-alpha08, you can have the NavigationUI bits dynamically set the title... if the dynamic bits are arguments on the navigation action.

So, for example, in your navigation graph, you could have something like this:

  <fragment    android:id="@+id/displayFragment"    android:name="com.commonsware.jetpack.sampler.nav.DisplayFragment"    android:label="Title: {title}" >    <argument      android:name="modelId"      app:argType="string" />    <argument      android:name="title"      app:argType="string" />  </fragment>

Here, the android:label attribute for our <fragment> has an argument name wrapped in braces ({title} in "Title: {title}". The app bar's title will then be set to the value of the label, with {title} replaced by the value of the title argument.

If you need something more elaborate than that — for example, you want to look up the model by ID and read a property from it — you will need to use more manual approaches, such as those outlined in other answers to this question.

The title can be changed in the fragment by casting the activity as AppCompatActivity.

Kotlin

(requireActivity() as AppCompatActivity).supportActionBar?.title = "Hello"

Java

((AppCompatActivity) requireActivity()).getSupportActionBar().setTitle("Hello");

Taking consideration that your host activity is MainActivity, just add the following code to your MainActivity's onCreate fun

val navController = Navigation.findNavController(this, R.id.nav_host_fragment)// setting title according to fragmentnavController.addOnDestinationChangedListener {     controller, destination, arguments ->        toolbar.title = navController.currentDestination?.label}