Android 9 Development Cookbook - Third Edition

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With the basics covered for handling the back stack, it's time to discuss the other callback: onBackStackChanged(). This is where you can implement custom behavior when the stack changes. One common example is changing the Home icon to a back arrow. We get this behavior automatically with an Activity when we set the parent property (in AndroidManifest), but Android doesn't do this for fragments. What if we wanted to have a back arrow on FragmentTwo? Add this line of code to the NextButton onClick():

getSupportActionBar().setDisplayHomeAsUpEnabled(true);

If you run the app now, you'll see the back arrow when you go to FragmentTwo. The problem is, the back arrow doesn't actually do anything. The next problem you may notice is ...