AttributedTextValueConstraint

protocol · iOS 26.0+ · SDK 27.1

protocol AttributedTextValueConstraint : Hashable, Sendable, AttributedTextFormattingDefinition

A protocol for defining a constraint on the value of a certain attribute. Used as an ``AttributedTextFormattingDefinition``, this constrains the ``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`` 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``s. struct NoBlackOrWhiteForeground<Scope: AttributeScope>: 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``. 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 ``AttributedTextFormattingDefinition/body-swift.property``. /// Makes the background color for all Genmoji blue. /// /// - Note: This constraint depends on a valid adaptiveImageGlyph value. struct BlueGenmojiBackgroundConstraint<Scope: AttributeScope>: AttributedTextValueConstraint { typealias AttributeKey = AttributeScopes.SwiftUIAttributes .BackgroundColorAttribute func constrain( _ container: inout Attributes ) { if container[ AttributeScopes.SwiftUIAttributes.AdaptiveImageGlyphAttribute.self ] != nil { container.backgroundColor = .blue } } }

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