Android Activity & Fragment Lifecycle

Sources: Activity Lifecycle · Fragment Lifecycle

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🔵 Activity Lifecycle

An Activity has 6 core lifecycle callbacks invoked by the system as it transitions between states.

Lifecycle Flow

onCreate() → onStart() → onResume()
                             ↓
                         [App Running]
                             ↓
onPause() → onStop() → onDestroy()
     ↑            ↑
     └── onResume()  └── onRestart() → onStart()

Callback Reference

Callback	State	Description
onCreate()	Created	First callback. Perform one-time setup: inflate layout (setContentView), bind data, init ViewModels. Receives savedInstanceState (null on first creation).
onStart()	Started	Activity becomes visible to the user. Initialize UI maintenance code. Completes quickly.
onResume()	Resumed	Activity is in the foreground and interactive. Start camera, animations, or other resources needed while visible.
onPause()	Paused	User is leaving the activity (not always destroyed). Activity may still be visible (e.g., multi-window). Release sensors (GPS, camera), pause animations. Do NOT do heavy work here — it’s brief.
onStop()	Stopped	Activity is no longer visible. Perform heavy-load shutdowns (e.g., save to DB). Use this instead of onPause() for UI-related releasing in multi-window mode.
onDestroy()	Destroyed	Final callback. Called when: (1) user dismisses the activity / finish() is called, or (2) system destroys due to a config change (rotation, multi-window). Use ViewModel to survive config changes.

Key Tips

• onRestart() is called when the activity returns from the Stopped state (not Destroyed) back to visible.
• Never save user data or make network calls in onPause() — use onStop() instead.
• Configuration changes (rotation) destroy and recreate the Activity; use ViewModel to preserve UI data across this.
• Memory management: The system kills processes (not activities directly) to free RAM. A Stopped activity is more likely to be killed than a Paused one.

State & Memory Kill Likelihood

Likelihood of being killed	Process state	Activity state
Least	Foreground	Resumed
More	Background (invisible)	Stopped
Most	—	Created / Started (before save)

Saving & Restoring State

Use a combination of strategies:

• ViewModel — survives config changes; preferred for complex UI state.
• onSaveInstanceState(outState: Bundle) — for lightweight transient state across process death.
• onRestoreInstanceState(savedInstanceState: Bundle) — called after onStart() when state is available.

override fun onSaveInstanceState(outState: Bundle) {
    outState.putString("GAME_STATE_KEY", gameState)
    super.onSaveInstanceState(outState)
}

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    gameState = savedInstanceState?.getString("GAME_STATE_KEY")
}

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🟢 Fragment Lifecycle

A Fragment lifecycle is closely tied to its host Activity but has its own additional states and callbacks, especially around its View.

Lifecycle States

Fragments move through these 5 states (managed by FragmentManager):

INITIALIZED → CREATED → STARTED → RESUMED
                ↓
            DESTROYED

A fragment progresses upward (e.g., added to back stack) and downward (e.g., popped from back stack).

Rule: A child fragment’s state can never exceed its parent fragment’s or host activity’s state.

Callback Reference

⬆️ Upward Transitions (Creating)

Callback	When called	Use for
onAttach(context)	Fragment attached to host Activity via FragmentManager	Access host context; always called before any lifecycle state change
onCreate(savedInstanceState)	Fragment reaches CREATED state	Restore non-view state; fragment’s view is NOT yet created here
onCreateView(inflater, container, savedInstanceState)	View is being created	Inflate or programmatically create the fragment’s View
onViewCreated(view, savedInstanceState)	View fully created (View INITIALIZED)	Set up initial view state, observe LiveData, set adapters (RecyclerView, ViewPager2)
onViewStateRestored(savedInstanceState)	View state restored (View CREATED)	Restore additional view-related state
onStart()	Fragment & View reach STARTED	Safe to perform FragmentTransaction on child FragmentManager
onResume()	Fragment & View reach RESUMED	Fragment is fully visible, all animations done — user can interact

⬇️ Downward Transitions (Destroying)

Callback	When called	Use for
onPause()	Fragment & View move back to STARTED	User is leaving fragment but it may still be visible
onStop()	Fragment & View move to CREATED	Fragment is no longer visible; last safe point for FragmentTransaction
onDestroyView()	View reaches DESTROYED	Remove all references to the fragment’s View (allow GC)
onDestroy()	Fragment reaches DESTROYED	Clean up fragment-level resources
onDetach()	Fragment removed from FragmentManager	Always called after lifecycle state changes

Fragment vs View Lifecycle

A Fragment has two Lifecycle objects:

• Fragment’s own Lifecycle — tied to the fragment instance.
• Fragment’s View Lifecycle (viewLifecycleOwner) — tied to the view, only exists when a view is present.

✅ Best practice: Observe LiveData using viewLifecycleOwner (not this) to avoid memory leaks when the view is destroyed but the fragment instance remains.

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    // Use viewLifecycleOwner, NOT 'this'
    viewModel.data.observe(viewLifecycleOwner) { data ->
        // update UI
    }
}

onSaveInstanceState() vs onStop() ordering

API Level	Order
< API 28	onSaveInstanceState() → onStop()
≥ API 28	onStop() → onSaveInstanceState()

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📊 Activity vs Fragment Lifecycle Comparison

Activity	Fragment
onCreate()	onCreate()
—	onCreateView()
—	onViewCreated()
onStart()	onStart()
onResume()	onResume()
onPause()	onPause()
onStop()	onStop()
—	onDestroyView()
—	onDestroy()
onDestroy()	onDetach()

Fragment has more granular callbacks, especially around its View lifecycle (onCreateView, onViewCreated, onDestroyView), because the View can be destroyed and recreated independently of the Fragment instance (e.g., when the fragment is on the back stack).