ChangeNotifier

ChangeNotifier is a class in the Flutter framework that provides a lightweight mechanism for notifying interested parties about changes. It is part of the foundation library and implements the Listenable interface, allowing one or more listeners to be registered and alerted when a change occurs.

The core API consists of addListener, removeListener, and notifyListeners. Listeners are functions with no arguments that are called synchronously when notifyListeners is invoked. ChangeNotifier manages its internal list of listeners and invokes each registered callback when state changes. A dispose method is also available to release resources and prevent further use of the notifier; it is typically called when the object is no longer needed.

Usage patterns commonly involve extending ChangeNotifier to create a simple state model. When the model’s state changes, the code calls notifyListeners to update all listening UI components or other observers. In practice, ChangeNotifier is frequently used with the Provider package, where ChangeNotifierProvider exposes the notifier to the widget tree and widgets listen via Consumer or similar constructs.

ChangeNotifier offers a straightforward observer pattern suitable for small to medium-scale state management. It is not a full data-binding solution and may not be optimal for highly reactive or large-scale applications without additional patterns or architectures. Developers should consider batching state changes to minimize notifications and call dispose when the notifier is no longer needed to avoid resource leaks.