Activity lifecycle

Activity Lifecycle Deep Dive¶

Overview¶

The Activity lifecycle is the set of states an Activity moves through, managed by the Android OS to optimize: - Memory Management: Reclaiming RAM from background apps. - Battery Usage: Stopping sensors/animations when not visible. - UI Restoration: Preserving user input during interruptions. - Multitasking: Handling seamless transitions between apps. - Process Recovery: Recreating the UI after the system kills a task.

Core Lifecycle States¶

1. Created (`onCreate`)¶

The entry point. Perform one-time setup here: inflate UI, initialize ViewModels, and bind data. - State: The activity exists but is not visible.

2. Started (`onStart`)¶

The activity becomes visible to the user. The app prepares to enter the foreground. - State: Visible, but not yet interactive.

3. Resumed (`onResume`)¶

The activity is in the foreground and has user focus. - Action: Start animations, camera previews, or high-frequency updates. - State: Interactive.

4. Paused (`onPause`)¶

The activity is partially obscured (e.g., by a dialog or multi-window mode). - Critical: Keep this lightweight. The next activity cannot start until this finishes. - State: Partially visible, losing focus.

5. Stopped (`onStop`)¶

The activity is no longer visible. - Action: Save persistent data (database/network), stop heavy CPU tasks. - State: Hidden, resides in the backstack.

6. Destroyed (`onDestroy`)¶

The final cleanup. Occurs when the user finishes the activity or the system needs space. - State: Removed from memory.

Common Lifecycle Triggers¶

Screen Rotation: Destroys and recreates the activity (Configuration Change).
Incoming Calls: Moves activity to onPause or onStop.
Split-Screen Mode: Triggers lifecycle changes as focus shifts between windows.
App Switching: Moves the current app to onStop.
Process Death: The OS kills a Stopped activity to free RAM.

Typical Lifecycle Implementation¶

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        Log.d("Lifecycle", "onCreate: Activity created")
    }

    override fun onStart() {
        super.onStart()
        Log.d("Lifecycle", "onStart: Activity visible")
    }

    override fun onResume() {
        super.onResume()
        Log.d("Lifecycle", "onResume: Activity interactive")
    }

    override fun onPause() {
        super.onPause()
        Log.d("Lifecycle", "onPause: Focus lost")
    }

    override fun onStop() {
        super.onStop()
        Log.d("Lifecycle", "onStop: No longer visible")
    }

    override fun onDestroy() {
        super.onDestroy()
        Log.d("Lifecycle", "onDestroy: Activity destroyed")
    }
}

Activity lifecycle