ConstraintBuilder.swift

public struct ConstraintBuilder: ConstraintBuilding {
    public let anchorable: Anchorable
    private let superviewBlock: (NSLayoutConstraint.Relation) -> [NSLayoutConstraint]

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

    public func equalToSuperview() -> [NSLayoutConstraint] {
        superviewBlock(.equal)
    }

    public func combine(with other: ConstraintBuilder) -> ConstraintBuilder {
        assert(anchorable === other.anchorable)
        return ConstraintBuilder(
            anchorable: anchorable,
            superviewBlock: {
                superviewBlock($0) + other.superviewBlock($0)
            }
        )
    }
}

extension ConstraintBuilder {
    public init(
        anchor lhs: Anchor,
        constant: CGFloat = 0,
        multiplier: CGFloat = 1
    ) {
        var uiView: UIView? { lhs.anchorable as? UIView }
        var superview: UIView? { uiView?.superview }
        superviewBlock = { relation in
            superview.map { rhs in
                [
                    lhs.constraint(
                        relation,
                        to: .init(anchorable: rhs, attribute: lhs.attribute),
                        constant: constant,
                        multiplier: multiplier
                    )
                ]
            } ?? []
        }
        self.anchorable = lhs.anchorable
        uiView?.translatesAutoresizingMaskIntoConstraints = false
    }
}