# willChangeValue(forKey:withSetMutation:using:) Informs the observed object that the specified change is about to be made to a specified unordered to-many relationship. ``` func willChangeValue(forKey key: String, withSetMutation mutationKind: NSKeyValueSetMutationKind, using objects: Set) ``` ## Parameters `key` The name of a property that is an unordered to-many relationship `mutationKind` The type of change that will be made. `objects` The objects that are involved in the change (see ). ## Discussion Use this method when implementing key-value observer compliance manually. > Important: > After the values have been changed, a corresponding ``doc://com.apple.objectivec/documentation/ObjectiveC/NSObject-swift.class/didChangeValue(forKey:withSetMutation:using:)`` must be invoked with the same parameters. ### Special Considerations You rarely need to override this method in subclasses, but if you do, be sure to call `super`. --- 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)