Constraints.swift

// viewA is INSIDE OF or a SIBLING OF viewB
let viewA: UIView = ...
let viewB: UIView = ...

// Create constraint like...
let constraint = NSLayoutConstraint(item: viewA, attribute: .height, relatedBy: .equal, toItem: viewB, attribute: .height, multiplier: 1, constant: 0)

// or
let constraints = NSLayoutConstraint.constraints(withVisualFormat: "V:|-(margin)-[view]-(margin)-|", options: [], metrics: ["margin": 15.0], views: ["view": viewA])

// The visual version above ^^ would break down to essentially this:
[
  NSLayoutConstraint(item: viewA, attribute: .top, relatedBy: .equal, toItem: viewB, attribute: .top, multiplier: 1, constant: 15.0)
  NSLayoutConstraint(item: viewA, attribute: .bottom, relatedBy: .equal, toItem: viewB, attribute: .bottom, multiplier: 1, constant: 15.0)
]

// Optional: Set priority over similar constraints
constraint.priority = .defaultHigh // or .required, or ...

// Add to some view
aSuperview.addConstraint(constraint)

// If viewB is a superview
viewB.addConstraint(constraint)

// Or
aSuperview.addConstraints(constraints)

// Or let it find out the "superview" for each constraint for you
NSLayoutConstraint.activate([constraint])

// Or ...
NSLayoutConstraint.activate(constraints)