---
title: priority
framework: appkit
role: symbol
role_heading: Instance Property
path: appkit/nslayoutconstraint/priority-swift.property
---

# priority

The priority of the constraint.

## Declaration

```swift
var priority: NSLayoutConstraint.Priority { get set }
```

## Discussion

Discussion By default, all constraints are required; this property is set to required in macOS or UILayoutPriorityRequired in iOS. If a constraint’s priority level is less than 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 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 in macOS or UILayoutPriorityRequired in iOS.

## See Also

### Getting the layout priority

- [NSLayoutConstraint.Priority](appkit/nslayoutconstraint/priority-swift.struct.md)