# priority The priority of the constraint. ``` var priority: NSLayoutConstraint.Priority { get set } ``` ## Discussion By default, all constraints are required; this property is set to [`required`](/documentation/AppKit/NSLayoutConstraint/Priority-swift.struct/required) in macOS or `UILayoutPriorityRequired` in iOS. If a constraint’s priority level is less than [`required`](/documentation/AppKit/NSLayoutConstraint/Priority-swift.struct/required) in macOS or `UILayoutPriorityRequired` in iOS, then it is optional. Higher priority constraints are satisfied before lower priority constraints; however, optional constraint satisfaction is not all or nothing. If a constraint `a == b` is optional, the constraint-based layout system will attempt to minimize `abs(a-b)`. Priorities may not change from nonrequired to required, or from required to nonrequired. An exception will be thrown if a priority of [`required`](/documentation/AppKit/NSLayoutConstraint/Priority-swift.struct/required) in macOS or `UILayoutPriorityRequired` in iOS is changed to a lower priority, or if a lower priority is changed to a required priority after the constraints is added to a view. Changing from one optional priority to another optional priority is allowed even after the constraint is installed on a view. Priorities must be greater than 0 and less than or equal to [`required`](/documentation/AppKit/NSLayoutConstraint/Priority-swift.struct/required) in macOS or `UILayoutPriorityRequired` in iOS. --- 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)