Anchorable.swift

public protocol Anchorable: AnyObject, ConstraintBuilding {}

extension Anchorable {
    func anchor(_ attribute: NSLayoutConstraint.Attribute) -> Anchor {
        .init(anchorable: self, attribute: attribute)
    }
    public var leading: Anchor { anchor(.leading) }
    public var trailing: Anchor { anchor(.trailing) }
    public var top: Anchor { anchor(.top) }
    public var bottom: Anchor { anchor(.bottom) }
}

public struct Anchor {
    let anchorable: Anchorable
    let attribute: NSLayoutConstraint.Attribute

    func constraint(
        _ relation: NSLayoutConstraint.Relation,
        to anchor: Anchor,
        constant: CGFloat = 0,
        multiplier: CGFloat = 1
    ) -> NSLayoutConstraint {
        return NSLayoutConstraint(
            item: anchorable,
            attribute: attribute,
            relatedBy: relation,
            toItem: anchor.attribute == .notAnAttribute ? nil : anchor.anchorable,
            attribute: anchor.attribute,
            multiplier: multiplier,
            constant: constant
        )
    }
}

extension UIView: Anchorable {
    public var anchorable: Anchorable { self }

    public func build(_ anchor: Anchor, constant: CGFloat, multiplier: CGFloat) -> ConstraintBuilder {
        .init(anchor: anchor, constant: constant, multiplier: multiplier)
    }
}