# AccessoryFeatureSessionHandler A protocol that receives events from a data provider extension’s feature session. ``` protocol AccessoryFeatureSessionHandler : Sendable ``` ## Overview Conform to this protocol in your data provider extension to respond to the [`AccessoryFeatureSession`](/documentation/AccessoryTransportExtension/AccessoryFeatureSession) life cycle and incoming data. The system creates an instance of your type from the handler factory closure you provide when you declare the feature. Pass the closure to [`init(_:)`](/documentation/AccessoryTransportExtension/AccessoryWorker/init(_:)) once for each session that starts. A handler that serves one feature can’t serve another because each feature pins the `Session` associated type through its own `Handler` type alias (for example, [`AccessoryWorker.Handler`](/documentation/AccessoryTransportExtension/AccessoryWorker/Handler)). Implement [`sessionStarted(_:)`](/documentation/AccessoryTransportExtension/AccessoryFeatureSessionHandler/sessionStarted(_:)) to hold the session weakly, and implement [`dataReceived(_:)`](/documentation/AccessoryTransportExtension/AccessoryFeatureSessionHandler/dataReceived(_:)) to process data that the transport extension sends. ## Topics ### Handling the session life cycle [`func sessionStarted(Self.Session)`](/documentation/AccessoryTransportExtension/AccessoryFeatureSessionHandler/sessionStarted(_:)) Notifies the handler when its feature session starts. [`func sessionInvalidated(error: (any Error)?)`](/documentation/AccessoryTransportExtension/AccessoryFeatureSessionHandler/sessionInvalidated(error:)) Notifies the handler when its feature session ends. ### Receiving data [`func dataReceived(Data) async throws`](/documentation/AccessoryTransportExtension/AccessoryFeatureSessionHandler/dataReceived(_:)) Handles data that the transport extension sends. ### Associated Types [`associatedtype Session : AccessoryFeatureSession`](/documentation/AccessoryTransportExtension/AccessoryFeatureSessionHandler/Session) A session type that the handler serves. ## Relationships ### Inherits From [`Sendable`](/documentation/Swift/Sendable) [`SendableMetatype`](/documentation/Swift/SendableMetatype) --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)