# subscribe(to:_:) Subscribes to a serializable action event and returns a `ActionResult`. ``` @MainActor @preconcurrency static func subscribe(to eventType: ActionEventType, _ handler: @escaping @MainActor (ActionEvent) -> ActionResult) ``` ## Discussion For example, you can call this method to subscribe to the update event, which the system calls each frame it evaluates the action: ```swift struct MyAction: BehaviorTreeAction, Codable { // ... } MyAction.subscribe(to: .updated) { event in // Return value is the action result. return .success } ``` --- 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)