Handy methods on UIView to allow easier additions of subviews (and constraints via code)

UIViewFitInSuperView.swift

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

import UIKit
import PlaygroundSupport

extension NSLayoutConstraint {
    func applyInset(_ insets: UIEdgeInsets) -> NSLayoutConstraint {
        guard firstAttribute == secondAttribute else {
            return self
        }
        
        switch firstAttribute {
        case .top: constant = insets.top
        case .bottom: constant = -insets.bottom
        case .left: constant = insets.left
        case .right: constant = -insets.right
        default:
            break
        }
        
        return self
    }
}

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
        }
        
        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: 0).applyInset(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