Eidhof's Autolayout DSL with a couple additions

autolayoutDSL

typealias Constraint = (_ child: UIView, _ parent: UIView) -> NSLayoutConstraint

func equal<Axis, Anchor>(_ keyPath: KeyPath<UIView, Anchor>, _ to: KeyPath<UIView, Anchor>, _ constant: CGFloat = 0) -> Constraint where Anchor: NSLayoutAnchor<Axis> {
    return { $0[keyPath: keyPath].constraint(equalTo: $1[keyPath: to], constant: constant) }
}

func equal<Axis, Anchor>(_ keyPath: KeyPath<UIView, Anchor>, _ constant: CGFloat = 0) -> Constraint where Anchor: NSLayoutAnchor<Axis> {
    return equal(keyPath, keyPath, constant)
}

func ==<Axis, Anchor>(_ left: KeyPath<UIView, Anchor>, _ right: KeyPath<UIView, Anchor>) -> Constraint where Anchor: NSLayoutAnchor<Axis> {
    return equal(left, right)
}


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extension UIView {
    func addSubview(_ child: UIView, constraints: [Constraint]) {
        addSubview(child)
        child.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate(constraints.map { $0(child, self) })
    }
}