# registerForTraitChanges(_:handler:) Registers a list of traits to observe and a closure to execute when one of the observed traits changes. ``` @discardableResult @MainActor func registerForTraitChanges(_ traits: [UITrait], handler: @escaping Self.TraitChangeHandler) -> any UITraitChangeRegistration where TraitEnvironment : UITraitEnvironment ``` ## Parameters `traits` An array of traits to observe for changes. `handler` A closure that the system executes when one of the registered traits changes. ## Return Value An opaque token you can use to stop observing trait changes by passing to [`unregisterForTraitChanges(_:)`](/documentation/UIKit/UITraitChangeObservable-67e94/unregisterForTraitChanges(_:)). You don’t have to unregister your observations, and you can safely ignore this value. ## Discussion The first parameter of the closure provides access to the observed object whose traits have changed, so you don’t need to create a weak reference. When registering for changes on self, use `self` as the name and `Self` as the type of the first parameter. The following example registers size class traits so that the system executes the closure when a size class trait changes in the view’s trait collection: ```swift let sizeTraits: [UITrait] = [UITraitVerticalSizeClass.self, UITraitHorizontalSizeClass.self] // Register for size class changes on self. Declare the first paramteter as `self: Self`. // Declaring self as the first parameter eliminates the need to capture self from outside the closure, and avoids strong reference cycles. registerForTraitChanges(sizeTraits) { (self: Self, previousTraitCollection: UITraitCollection) in // Handle the trait change. } // Register for size class changes on a different view of class MyView. view.registerForTraitChanges(sizeTraits) { (view: MyView, previousTraitCollection: UITraitCollection) in // Handle the trait change. } ``` --- 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)