Activity Lifecycle

An Activity moves through a well-defined set of states as the user navigates, rotates the device, or switches apps. Understanding these callbacks is key to saving state and releasing resources correctly.

The lifecycle

flowchart TB
    CREATE["onCreate()"] --> START["onStart()"]
    START --> RESUME["onResume()"]
    RESUME --> RUNNING["Activity running (visible, focused)"]
    RUNNING --> PAUSE["onPause()"]
    PAUSE --> STOP["onStop()"]
    STOP --> DESTROY["onDestroy()"]
    PAUSE --> RESUME
    STOP --> START

Callback	Called when	Typical use
`onCreate`	Activity is created	Set content, initialize
`onStart`	Becoming visible	Start UI updates
`onResume`	Gains focus	Begin animations, acquire camera
`onPause`	Losing focus	Pause, save light state
`onStop`	No longer visible	Release resources
`onDestroy`	Being destroyed	Final cleanup

Configuration changes

By default, a configuration change (like rotation) destroys and recreates the Activity. Anything held only in the Activity is lost. There are two robust ways to survive this:

Save UI state with rememberSaveable in Compose (next chapters).
Hold state in a ViewModel, which survives configuration changes (see Coroutines on Android).

@Composable
fun NameField() {
    // survives rotation
    var text by rememberSaveable { mutableStateOf("") }
    TextField(value = text, onValueChange = { text = it })
}

Saving instance state (classic)

In the View system you override onSaveInstanceState:

override fun onSaveInstanceState(outState: Bundle) {
    super.onSaveInstanceState(outState)
    outState.putString("draft", currentDraft)
}

In Compose, prefer rememberSaveable and ViewModel instead.

Rule of thumb: transient UI state → rememberSaveable; screen/business state that should survive rotation → ViewModel.

Exercises

Add Log statements to each lifecycle callback and observe the order when you rotate the device.
Use rememberSaveable for a text field and confirm its value survives a rotation.

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