cjnevin

addSubview(redView) {
   $0.edges(20).equalToSuperview()
}

cjnevin

extension ConstraintBuilding {
  public func edges(_ constant: CGFloat = 0) -> ConstraintBuilder {
      top(constant).leading(constant).trailing(-constant).bottom(-constant)
  }
}

cjnevin

addSubview(redView) {
  $0.top(20).leading(20).trailing(-20).bottom(-20).equalToSuperview()
}

cjnevin

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)

cjnevin

public protocol ConstraintBuilding {
    var anchorable: Anchorable { get }
    func build(
        _ anchor: Anchor,
        constant: CGFloat,
        multiplier: CGFloat
    ) -> ConstraintBuilder
}

extension ConstraintBuilding {

cjnevin

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) }

cjnevin

addSubview(redView) {
   $0.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 20)
   $0.trailingAnchor.constraint(equalTo: trailingAnchor, constant: -20)
   $0.topAnchor.constraint(equalTo: topAnchor, constant: 20)
   $0.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -20)
}

cjnevin

@resultBuilder
public enum AutoLayoutBuilder {
    public static func buildBlock(_ components: Constrainable...) -> [NSLayoutConstraint] {
        components.flatMap(\.constraints)
    }
}

extension UIView {
    @discardableResult public func addSubview<View: UIView>(_ view: View, @AutoLayoutBuilder with makeConstraints: (View) -> [Constrainable]) -> [NSLayoutConstraint] {

cjnevin

public protocol Constrainable {
    var constraints: [NSLayoutConstraint] { get }
}

extension NSLayoutConstraint: Constrainable {
    public var constraints: [NSLayoutConstraint] { [self] }
}

extension Array: Constrainable where Element == NSLayoutConstraint {
    public var constraints: [NSLayoutConstraint] { self }

cjnevin

addSubviews(leftView, middleView, rightView) { leftView, middleView, rightView in
  leftView.verticalEdges.leading == Superview()

  middleView.verticalEdges == Superview()
  middleView.leading(20) >= leftView.trailing
  middleView.trailing(-20) <= rightView.leading

  rightView.verticalEdges.trailing == Superview()
}