Which method must be overridden in the host activity for fragment-based navigation?

In fragment-based navigation within an Android application, overriding the onSupportNavigateUp() method in the host activity is essential for correctly handling navigation when the user interacts with the Up button in the action bar. This method is responsible for defining the behavior that occurs when the user attempts to navigate "up" in the app's hierarchy.

When you implement a fragment-based architecture, the Up button should take the user back to the previous fragment or activity, maintaining a proper navigation flow that aligns with the navigation stack. By overriding onSupportNavigateUp(), you can specify how the activity should respond, typically involving fragment transactions that restore the previous fragment on the stack.

In contrast, the other methods mentioned are vital to the activity lifecycle but do not specifically address the navigation behavior through fragments. onStart() and onResume() are lifecycle methods that manage the activity's visibility and state but do not handle up navigation. onCreate() initializes the activity but does not handle navigation logic directly. Thus, overriding onSupportNavigateUp() is crucial for enabling and customizing fragment navigation in an Android app.