Very simple key path based Auto Layout helpers

AutoLayoutHelpers.swift

import UIKit

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

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

    @discardableResult
    func attach<Axis, Anchor>(_ keyPath: KeyPath<UIView, Anchor>,
                              to: KeyPath<UIView, Anchor>,
                              of secondView: UIView,
                              constant: CGFloat = 0,
                              relation: NSLayoutConstraint.Relation) -> NSLayoutConstraint where Anchor: NSLayoutAnchor<Axis> {
        let builder = attach(keyPath, to, constant: constant, relation: relation)
        let constraint = builder(self, secondView)
        constraint.isActive = true

        return constraint
    }
}

func attach<Axis, Anchor>(_ keyPath: KeyPath<UIView, Anchor>,
                          _ to: KeyPath<UIView, Anchor>,
                          constant: CGFloat = 0,
                          relation: NSLayoutConstraint.Relation) -> ConstraintBuilder where Anchor: NSLayoutAnchor<Axis> {
    return { view, parent in
        switch relation {
        case .equal:
            return view[keyPath: keyPath].constraint(equalTo: parent[keyPath: to], constant: constant)
        case .greaterThanOrEqual:
            return view[keyPath: keyPath].constraint(greaterThanOrEqualTo: parent[keyPath: to], constant: constant)
        case .lessThanOrEqual:
            return view[keyPath: keyPath].constraint(lessThanOrEqualTo: parent[keyPath: to], constant: constant)
        @unknown default:
            fatalError("Unexpected NSLayoutConstraint.Relation case: \(relation)")
        }
    }
}

func attach<Axis, Anchor>(_ keyPath: KeyPath<UIView, Anchor>,
                          constant: CGFloat = 0,
                          relation: NSLayoutConstraint.Relation) -> ConstraintBuilder where Anchor: NSLayoutAnchor<Axis> {
    return attach(keyPath, keyPath, constant: constant, relation: relation)
}

func attach<Anchor>(_ keyPath: KeyPath<UIView, Anchor>,
                    constant: CGFloat,
                    relation: NSLayoutConstraint.Relation) -> ConstraintBuilder where Anchor: NSLayoutDimension {
    return { view1, _ in
        switch relation {
        case .equal:
            return view1[keyPath: keyPath].constraint(equalToConstant: constant)
        case .lessThanOrEqual:
            return view1[keyPath: keyPath].constraint(lessThanOrEqualToConstant: constant)
        case .greaterThanOrEqual:
            return view1[keyPath: keyPath].constraint(greaterThanOrEqualToConstant: constant)
        @unknown default:
            fatalError("Unexpected NSLayoutConstraint.Relation case: \(relation)")
        }
    }
}


let v1 = UIView()
v1.backgroundColor = .red
v1.frame = CGRect(x: 0, y: 0, width: 200, height: 200)
let v2 = UIView()
v2.backgroundColor = .green

v1.addSubview(v2, constraints: [
    attach(\.leadingAnchor, constant: 10, relation: .equal),
    attach(\.trailingAnchor, constant: -10, relation: .equal),
    attach(\.heightAnchor, constant: 100, relation: .equal)
])

v2.attach(\.topAnchor, to: \.bottomAnchor, of: v1, relation: .equal)