En Activity repræsenterer en enkelt skærm med en brugerinterface — en fundamental Android-komponent. Dens livscyklus (sekvensen af tilstande og callbacks, når den oprettes, vises, skjules og ødelægges) er afgørende at forstå for korrekt at håndtere ressourcer og tilstand.
Hvad en Activity er
An ACTIVITY = one screen of the app (a UI the user interacts with):
→ an app is typically several activities (a login screen, a home screen, etc.)
→ the OS manages activities and calls LIFECYCLE methods as their state changes
Activity-livscyklussen
Key lifecycle callbacks (the OS calls these as the activity's state changes):
onCreate() → activity is being CREATED → initialize UI, set up (called once)
onStart() → becoming VISIBLE to the user
onResume() → now in the FOREGROUND, interactive (user can interact)
onPause() → losing focus (another activity coming in front) → pause/save lightly
onStop() → no longer VISIBLE → release resources, stop heavy work
onDestroy() → being DESTROYED → final cleanup
→ Flow: onCreate → onStart → onResume → [running] → onPause → onStop → onDestroy
