# AttributedTextValueConstraint A protocol for defining a constraint on the value of a certain attribute. ``` protocol AttributedTextValueConstraint : Hashable, Sendable, AttributedTextFormattingDefinition ``` ## Overview Used as an [`AttributedTextFormattingDefinition`](/documentation/SwiftUI/AttributedTextFormattingDefinition), this constrains the [`AttributeKey`](/documentation/SwiftUI/AttributedTextValueConstraint/AttributeKey)’s value using the `constrain(_:)-(Attributes)` function. Given value constraints can read other attribute values, it is crucial to avoid mixing value constraints in a way where they create cyclic dependencies with undefined behavior. Thus, it is recommended to think about value constraints in the context of the [`AttributedTextFormattingDefinition`](/documentation/SwiftUI/AttributedTextFormattingDefinition) they will be used in: A simple constraint only accesses a single attribute. It can be made generic over the attribute scope so it can be reused in different [`AttributedTextFormattingDefinition`](/documentation/SwiftUI/AttributedTextFormattingDefinition)s. ``` struct NoBlackOrWhiteForeground: AttributedTextValueConstraint { typealias AttributeKey = AttributeScopes.SwiftUIAttributes.ForegroundColorAttribute func constrain( _ container: inout Attributes ) { if container.foregroundColor == .white || container.foregroundColor == .black { container.foregroundColor = .primary } } } ``` When the constraint needs to access other attribute values, it is recommended to define it on a specific attribute scope that is used for a single [`AttributedTextFormattingDefinition`](/documentation/SwiftUI/AttributedTextFormattingDefinition). ``` extension MyTextFormattingDefinition { struct Scope: AttributeScope { /* ... */ let foregroundColor: AttributeScopes.SwiftUIAttributes.ForegroundColorAttribute let backgroundColor: AttributeScopes.SwiftUIAttributes.BackgroundColorAttribute } } 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 } } } ``` Constraints that access multiple attributes and are generic over the scope should document their dependencies so that the dependencies can be considered for the ordering of constraints in the [`body`](/documentation/SwiftUI/AttributedTextFormattingDefinition/body-1b01t). ``` /// Makes the background color for all Genmoji blue. /// /// - Note: This constraint depends on a valid adaptiveImageGlyph value. struct BlueGenmojiBackgroundConstraint: AttributedTextValueConstraint { typealias AttributeKey = AttributeScopes.SwiftUIAttributes .BackgroundColorAttribute func constrain( _ container: inout Attributes ) { if container[ AttributeScopes.SwiftUIAttributes.AdaptiveImageGlyphAttribute.self ] != nil { container.backgroundColor = .blue } } } ``` ## Topics ### Associated Types [`associatedtype AttributeKey : AttributedStringKey`](/documentation/SwiftUI/AttributedTextValueConstraint/AttributeKey) The attribute constrained by this constraint. ### Instance Methods [`func constrain(inout Self.Attributes)`](/documentation/SwiftUI/AttributedTextValueConstraint/constrain(_:)) Enforce constraints on the attribute. ### Type Aliases [`typealias Attributes`](/documentation/SwiftUI/AttributedTextValueConstraint/Attributes) A proxy type for a container of partially constrained attributes. ## Relationships ### Inherits From [`AttributedTextFormattingDefinition`](/documentation/SwiftUI/AttributedTextFormattingDefinition) [`Hashable`](/documentation/Swift/Hashable) [`SendableMetatype`](/documentation/Swift/SendableMetatype) [`Equatable`](/documentation/Swift/Equatable) [`Sendable`](/documentation/Swift/Sendable) ### Conforming Types [`ValueConstraint`](/documentation/SwiftUI/AttributedTextFormatting/ValueConstraint) --- 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)