How to use a custom UIView in another View controller programmatically and handle constraints in Swift 4.

CustomUIView.Playground

//: A UIKit based Playground for presenting user interface
  
import UIKit
import PlaygroundSupport

class MyView: UIView {

    let label = UILabel()

    func updateText(_ text:String?) {
        guard let text = text else { return }
        label.text = text
    }

    override init(frame: CGRect){
        super.init(frame: frame)
        label.translatesAutoresizingMaskIntoConstraints = false
        label.text = "MyView!"
        label.numberOfLines = 0
        label.textColor = .red
        label.textAlignment = .center
        self.addSubview(label)
    }

    override func layoutSubviews() {
        super.layoutSubviews()
        label.frame = bounds
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
} // end MyView

class MyViewController : UIViewController {

    let aview = MyView()
    let myview = MyView()
    let parview = MyView()

    override func loadView() {
        let view = UIView()
        view.backgroundColor = .black
        self.view = view
    }

    override func viewDidLoad() {

        setupLabels()
        setupStack()
    }

    func setupLabels() {
        aview.updateText("A View")
        myview.updateText("Another View with more to say - this is a full bit of text")
        parview.updateText("Lorem ipsum dolor sit amet, consectetur adipiscing elit. Duis porttitor interdum ex dictum auctor. Proin nec imperdiet sem. Duis pulvinar tortor viverra elit convallis tempus. Maecenas egestas elit eu convallis mattis. Sed non hendrerit tortor. Phasellus lectus dolor, pretium sed dignissim scelerisque, vulputate id dolor. Sed ac rhoncus enim.")
        // can also access the label directly - but better to make a function if this will be changed often.
        myview.label.textColor = .white
    }

    func setupStack() {
        let subviews = [aview, myview, parview]
        let stackView = UIStackView()
        for view in subviews {
            stackView.addArrangedSubview(view)
        }
        stackView.axis = .vertical
        stackView.distribution = .fillEqually
        stackView.alignment = .fill
        stackView.spacing = 10
        stackView.translatesAutoresizingMaskIntoConstraints = false

        view.addSubview(stackView)

        //autolayout the stack view
        let sH = NSLayoutConstraint.constraints(withVisualFormat: "H:|-20-[stackView]-20-|", options: NSLayoutFormatOptions(rawValue: 0), metrics: nil, views: ["stackView":stackView])
        let sV = NSLayoutConstraint.constraints(withVisualFormat: "V:|-30-[stackView]-30-|", options: NSLayoutFormatOptions(rawValue:0), metrics: nil, views: ["stackView":stackView])
        view.addConstraints(sH)
        view.addConstraints(sV)
    }

}
// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()

But how to use reverse?

Means subview controller inside UIView.