# UITraitChangeObservable.TraitChangeHandler A closure the system executes when observed traits change. ``` typealias TraitChangeHandler = (TraitEnvironment, UITraitCollection) -> Void where TraitEnvironment : UITraitEnvironment ``` ## Parameters `traitEnvironment` The observed object containing the updated trait collection. `previousTraitCollection` The trait collection prior to the changes that triggered the execution of the handler. ## Discussion Use [`registerForTraitChanges(_:handler:)`](/documentation/UIKit/UITraitChangeObservable-67e94/registerForTraitChanges(_:handler:)) to register a list of traits to observe and a handler to execute. If the closure captures a strong reference to the object receiving the registration, it creates a strong reference cycle. Use the `traitEnvironment` parameter to refer to the observed object inside the closure. --- 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)