LifecycleObserver

LifecycleObserver is an interface from the Android Jetpack Lifecycle library that marks a class as capable of observing the lifecycle events of a LifecycleOwner, such as an Activity or Fragment. Observers are registered with a Lifecycle by calling addObserver on the owner’s Lifecycle, enabling decoupled components to respond to lifecycle transitions without tightly coupling to the owner.

Historically, observers implemented LifecycleObserver and used methods annotated with @OnLifecycleEvent(Event) to receive callbacks for specific lifecycle events. The common events include ON_CREATE, ON_START, ON_RESUME, ON_PAUSE, ON_STOP, ON_DESTROY, and a catch-all ON_ANY. The methods annotated with these events are invoked as the owner transitions through its lifecycle, typically on the main thread.

In modern usage, the LifecycleObserver interface and the OnLifecycleEvent annotation have been deprecated in favor of newer APIs. The recommended approaches are DefaultLifecycleObserver, which provides per-event callback methods with default implementations (for example onCreate, onStart, onResume, onPause, onStop, onDestroy), and LifecycleEventObserver, which implements onStateChanged(LifecycleOwner, Event) to handle events more flexibly. These alternatives reduce the need for annotation-based wiring and improve type safety and readability.

Usage basics include:

- Implement a observer (older) or one of the recommended interfaces.

- Register with a Lifecycle.Owner’s lifecycle via lifecycle.addObserver(observer).

- Optionally remove the observer with lifecycle.removeObserver(observer).

LifecycleObserver serves to decouple business logic from lifecycle-aware components, facilitating safer resource management, such as pausing or releasing resources when the owner stops or destroys.