Activities and the lifecycle

What is an Activity?

An Activity is one of Android's app components: an entry point that the system can start, which shows a user interface. Older apps had one activity per screen. Modern Compose apps usually have a single activity (MainActivity) and switch screens inside it with navigation (module 7). Your MainActivity extends ComponentActivity.

Why a lifecycle?

Phones are busy. A call comes in, the user switches to WhatsApp, the screen turns off, memory runs low. Android must tell your activity what is happening so it can pause a video, save a draft or stop using the camera. These notifications are the lifecycle callbacks: functions Android calls at each change.

        onCreate()      created: set up the UI (setContent)
            |
        onStart()       visible
            |
        onResume()      in front, user can interact       <--+
            |                                                  |
        onPause()       partly hidden or losing focus  -------+ (back to onResume)
            |
        onStop()        no longer visible          --> onRestart() --> onStart()
            |
        onDestroy()     finishing, or recreated after a configuration change

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent { Text("Hello") }
        Log.d("Life", "onCreate")
    }
    override fun onStart() { super.onStart(); Log.d("Life", "onStart") }
    override fun onStop() { super.onStop(); Log.d("Life", "onStop") }
}

Configuration changes

A configuration change is a change to the device setup: rotating the screen, switching to dark mode, changing the language, resizing on a foldable. By default Android destroys and recreates the activity, calling onDestroy then onCreate again, so the UI can load resources for the new configuration. Anything stored only in the activity's variables is lost.

Process death

When memory is low, Android may kill your app's process while it is in the background. When the user returns, Android recreates the activity and passes a savedInstanceState Bundle. Small UI state saved with rememberSaveable or SavedStateHandle is restored; everything else must be reloaded, for example from Room.

Common mistakes

• Keeping important state in plain activity variables: lost on rotation.
• Doing heavy work in onCreate on the main thread: slow app start. Load data with coroutines.
• Forgetting to stop location or sensor updates in onStop, draining the battery.
• Relying on onDestroy to save data: it may never run if the process is killed. Save earlier.