# BehaviorTreeActionHandler Behavior Tree-specific event handlers that allow an `ActionResult` to be returned from the handler. ``` protocol BehaviorTreeActionHandler : ActionHandlerProtocol ``` ## Overview Behavior trees send Started, Updated and Ended events. Optionally subscribe directly to any of these events in order to override default event handling in cases when conforming a new, custom event handler type. Example: ```swift struct MyAction: BehaviorTreeAction, Codable { // ... } struct MyActionHandler: BehaviorTreeActionHandler { typealias ActionType = MyAction //... add some optional state data here. // Action updated event handler. public mutating func actionUpdatedWithResult(event: EventType) -> ActionResult? { // ... handle the 'updated' event. // Return the action result. return .success } } // The handler must be registered in order to receive events: MyActionHandler.register { event in return MyActionHandler() } ``` ## Topics ### Responding to action progress [`func actionStartedWithResult(event: Self.EventType) -> ActionResult?`](/documentation/RealityKit/BehaviorTreeActionHandler/actionStartedWithResult(event:)) [`func actionUpdatedWithResult(event: Self.EventType) -> ActionResult?`](/documentation/RealityKit/BehaviorTreeActionHandler/actionUpdatedWithResult(event:)) [`func actionPausedWithResult(event: Self.EventType) -> ActionResult?`](/documentation/RealityKit/BehaviorTreeActionHandler/actionPausedWithResult(event:)) [`func actionResumedWithResult(event: Self.EventType) -> ActionResult?`](/documentation/RealityKit/BehaviorTreeActionHandler/actionResumedWithResult(event:)) ### Responding to action completion [`func actionEndedWithResult(event: Self.EventType) -> ActionResult?`](/documentation/RealityKit/BehaviorTreeActionHandler/actionEndedWithResult(event:)) [`func actionTerminatedWithResult(event: Self.EventType) -> ActionResult?`](/documentation/RealityKit/BehaviorTreeActionHandler/actionTerminatedWithResult(event:)) [`func actionSkippedWithResult(event: Self.EventType) -> ActionResult?`](/documentation/RealityKit/BehaviorTreeActionHandler/actionSkippedWithResult(event:)) ## Relationships ### Inherits From [`ActionHandlerProtocol`](/documentation/RealityKit/ActionHandlerProtocol) --- 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)