Activity and Fragment Lifecycle

The Activity Lifecycle

An Activity goes through states as the user navigates the app. Each state has a callback method you can override.

Diagram: The Activity lifecycle states and their transitions.

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Log Each Callback

import android.os.Bundle
import android.util.Log
import androidx.activity.ComponentActivity

class MainActivity : ComponentActivity() {
    private val tag = "Lifecycle"

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        Log.d(tag, "onCreate")
    }

    override fun onStart() {
        super.onStart()
        Log.d(tag, "onStart")
    }

    override fun onResume() {
        super.onResume()
        Log.d(tag, "onResume")
    }

    override fun onPause() {
        super.onPause()
        Log.d(tag, "onPause")
    }

    override fun onStop() {
        super.onStop()
        Log.d(tag, "onStop")
    }

    override fun onDestroy() {
        super.onDestroy()
        Log.d(tag, "onDestroy")
    }
}

Rotate the device and watch Logcat (filter by Lifecycle):

// Launch app:
// onCreate → onStart → onResume
// Rotate device:
// onPause → onStop → onDestroy → onCreate → onStart → onResume

Rotation destroys and recreates the Activity. This is why you must not keep state in the Activity itself — use a ViewModel instead.

Lifecycle-Aware Components

Instead of overriding every callback, observe the lifecycle with DefaultLifecycleObserver. Your code stays clean and reusable.

import androidx.lifecycle.DefaultLifecycleObserver
import androidx.lifecycle.LifecycleOwner
import android.util.Log

class MyObserver : DefaultLifecycleObserver {
    override fun onStart(owner: LifecycleOwner) {
        Log.d("Observer", "Started — connect to server")
    }

    override fun onStop(owner: LifecycleOwner) {
        Log.d("Observer", "Stopped — disconnect")
    }
}

Attach it to an Activity:

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        lifecycle.addObserver(MyObserver())
    }
}

The observer now runs its onStart/onStop automatically when the Activity changes state.

Fragment Lifecycle

A Fragment lives inside an Activity. It has extra callbacks because it handles its own view creation:

In a Compose-only app, you rarely use Fragments directly. Compose handles UI state for you. But you will meet Fragments in older code or third-party libraries.

Practice Exercise

Add a DefaultLifecycleObserver that logs "Foreground" in onResume and "Background" in onPause. Press the Home button and return to the app. Confirm the log order is onPause → Background, then onResume → Foreground.