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Android Lifecycle

Understanding lifecycles is crucial for avoiding memory leaks, crashes, and unpredictable behavior. Both Activity and Fragment have structured lifecycles.

Activity Lifecycle​

  1. onCreate(): Called when the activity is first created. Setup UI, initialize ViewModels, bind data.
  2. onStart(): Activity becomes visible to the user.
  3. onResume(): Activity comes to the foreground and becomes interactive.
  4. onPause(): Activity loses focus but might still be partially visible (e.g., dialer popup). Pause animations, camera previews.
  5. onStop(): Activity is no longer visible. Save state, release heavy resources.
  6. onDestroy(): Activity is destroyed. Release all references.

Fragment Lifecycle​

Fragments have two distinct lifecycles: the Fragment instance lifecycle and the View lifecycle.

  1. onAttach(): Attached to the host Activity.
  2. onCreate(): Fragment instance is created.
  3. onCreateView(): Inflate the UI view.
  4. onViewCreated(): Safe to start manipulating views (e.g., findViewById, ViewBinding).
  5. onStart() / onResume(): Mirrored from Activity.
  6. onPause() / onStop(): Mirrored from Activity.
  7. onDestroyView(): The view is destroyed. Null out ViewBindings here to prevent memory leaks!
  8. onDestroy() / onDetach(): The instance is destroyed and detached.

ViewModel Lifecycle​

ViewModel survives configuration changes (like screen rotations).

  • Created when the Activity/Fragment is first created.
  • Lives throughout rotations.
  • onCleared() is called when the Activity is genuinely finished or the Fragment is permanently removed. This is where you should cancel Coroutines/RxJava streams.

Saving UI State​

Apps might be killed by the system to free up resources.

  • ViewModel: Handles rotation but not system process death.
  • SavedStateHandle: Integrated inside ViewModel to save and restore data across system process death.
  • onSaveInstanceState(): Legacy way in Activities/Fragments to store lightweight UI data (e.g., scroll position, entered text) in a specific Bundle.

Interview Questions​

Q: Why do we null out ViewBinding in Fragment's onDestroyView? Answer: A Fragment's instance might outlive its view (e.g., when it is in the back stack but not visible). If you keep a reference to the ViewBinding variable, the Views cannot be garbage collected, leading to a memory leak.

Q: What is the lifecycle order when navigating from Activity A to Activity B? Answer: A.onPause() -> B.onCreate() -> B.onStart() -> B.onResume() -> A.onStop(). Activity A only drops to onStop() after B has completed its layout and rendered its first frame.