constrain(_:)

Declaration

func constrain(_ container: inout Self.Attributes)

Discussion

A function that transforms the 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 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 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.