UsingPlainAPI.swift

let backgroundView = UIView.mock(backgroundColor: .neon(.violet))
let box1 = UIView.mock(backgroundColor: .neon(.red), preferredSize: .largeSquare)
let box2 = UIView.mock(backgroundColor: .neon(.yellow), preferredSize: .largeSquare)

view.addSubview(backgroundView)
view.addSubview(box1)
view.addSubview(box2)

backgroundView.translatesAutoresizingMaskIntoConstraints = false
box1.translatesAutoresizingMaskIntoConstraints = false
box2.translatesAutoresizingMaskIntoConstraints = false

NSLayoutConstraint.activate([

  backgroundView.topAnchor.constraint(equalTo: view.topAnchor),
  backgroundView.leftAnchor.constraint(equalTo: view.leftAnchor),
  backgroundView.rightAnchor.constraint(equalTo: view.rightAnchor),
  backgroundView.bottomAnchor.constraint(equalTo: view.bottomAnchor),

  box1.topAnchor.constraint(equalTo: view.topAnchor, constant: 10),
  box1.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 10),
  box1.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -10),

  box2.topAnchor.constraint(equalTo: view.topAnchor, constant: 10),
  box2.leftAnchor.constraint(equalTo: box1.rightAnchor, constant: 10),
  box2.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -10),
  box2.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -10)
])