Event Handling with NotificationListener in Flutter

NotificationListener<T> is a widget in Flutter that listens for notifications of type T and allows you to respond to events (e.g., scroll events, layout changes) within a widget subtree.

Example: Listening to Scroll Events

In this example, we listen to ScrollNotification to detect scroll changes in a ListView:

NotificationListener<ScrollNotification>(
  onNotification: (notification) {
    if (notification is ScrollUpdateNotification) {
      final scrollPosition = notification.metrics.pixels;
      print("Scroll position: $scrollPosition");
    }
    return false; // Allow the notification to propagate
  },
  child: ListView.builder(
    itemCount: 100,
    itemBuilder: (context, index) => ListTile(title: Text('Item $index')),
  ),
);        

How It Works:

• ScrollNotification: The NotificationListener listens for scroll-related notifications like ScrollUpdateNotification.
• Callback: When a scroll event occurs, the onNotification callback is triggered, where you can capture the scroll position.
• Returning false: This allows the notification to continue propagating.

Example: Custom Notification

You can also create custom notifications to handle specific events.

class MyCustomNotification extends Notification {
  final String message;
  MyCustomNotification(this.message);
}

NotificationListener<MyCustomNotification>(
  onNotification: (notification) {
    print('Custom notification: ${notification.message}');
    return true; // Stop propagation
  },
  child: ElevatedButton(
    onPressed: () {
      MyCustomNotification('Hello!').dispatch(context);
    },
    child: Text('Send Notification'),
  ),
);        

How It Works:

• Custom Notification: MyCustomNotification is created to carry a message.
• Dispatch: The notification is dispatched when the button is pressed, and the listener reacts to it.

Key Benefits:

• Event Propagation: Capture events like scrolling or custom actions across widget hierarchies.
• Decoupling Widgets: Parent widgets don’t need to be aware of child widget events.

In short, NotificationListener allows you to easily handle various events and notifications in Flutter apps.