একটি Activity এবং এর lifecycle কী?

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

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