Handy methods on UIView to allow easier additions of subviews (and constraints via code), only in one UIView extension

UIViewFitInSuperview2.swift

//: Playground - noun: a place where people can play

import UIKit
import PlaygroundSupport

extension UIView {
    func addAndFitSubview(_ subview: UIView, insets: UIEdgeInsets = .zero) {
        addSubview(subview)
        subview.fitInSuperview(with: insets)
    }
    
    func fitInSuperview(with insets: UIEdgeInsets = .zero) {
        guard let superview = superview else {
            assertionFailure("fitInSuperview was called but view was not in a view hierarchy.")
            return
        }
        
        let applyInset: (NSLayoutAttribute, UIEdgeInsets) -> CGFloat = {
            switch $0 {
            case .top: return $1.top
            case .bottom: return -$1.bottom
            case .left: return $1.left
            case .right: return -$1.right
            default:
                return 0
            }
        }
        
        translatesAutoresizingMaskIntoConstraints = false
        
        let attributes = [NSLayoutAttribute.top, .left, .right, .bottom]
        superview.addConstraints(attributes.map {
            return NSLayoutConstraint(item: self,
                                      attribute: $0,
                                      relatedBy: .equal,
                                      toItem: superview,
                                      attribute: $0,
                                      multiplier: 1,
                                      constant: applyInset($0, insets))
        })
    }
}

let myView = UIView(frame: CGRect(x: 0, y: 0, width: 320, height: 480))
myView.backgroundColor = .white

let label = UILabel()
label.backgroundColor = .red
label.text = "Hello, Public Extensions"
label.textAlignment = .center
label.numberOfLines = 0
label.font = UIFont(name: "Helvetica", size: 30)

//the insets parameter has a default value, so you don't need to pass insets.
myView.addAndFitSubview(label, insets: UIEdgeInsets(top: 20, left: 20, bottom: 20, right: 20))

PlaygroundPage.current.liveView = myView