# constrain(_:) Enforce constraints on the attribute. ``` func constrain(_ container: inout Self.Attributes) ``` ## Discussion A function that transforms the [`AttributeKey`](/documentation/SwiftUI/AttributedTextValueConstraint/AttributeKey) on the given `container` so it represents a formatting that the conforming type defines to be valid. This function can generally read any attribute on `container` and it will produce a value that has been constrained by all [`AttributedTextValueConstraint`](/documentation/SwiftUI/AttributedTextValueConstraint) listed in the associated text formatting definition above the constraint reading the attribute. Consider the following example: ``` struct NoEqualForegroundAndBackground: AttributedTextValueConstraint { typealias Scope = MyTextFormattingDefinition.Scope typealias AttributeKey = AttributeScopes.SwiftUIAttributes.BackgroundColorAttribute func constrain( _ container: inout Attributes ) { if let color = container.foregroundColor, container.backgroundColor == color { container.backgroundColor = nil } } } ``` When this constrain function accesses `container.foregroundColor`, the system establishes that the background color depends on the foreground color. At that time, it checks if the [`AttributedTextFormattingDefinition`](/documentation/SwiftUI/AttributedTextFormattingDefinition) this constraint is part of defines constraints on the foreground color *above* `NoEqualForegroundAndBackground` and applies them. Thus, when the access to `container.foregroundColor` returns, this function reads the constrained value. --- 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)