PropertyObservers.swift

import UIKit
import PlaygroundSupport

class CustomView: UIView {

    var message: String = "" {
        willSet {
            print("newValue: \(newValue)")
            updateUI(with: newValue)
        }
    }

    func updateUI(with message: String) {
        messageLabel.text = "Message: \(message)"
    }
    
    lazy var messageLabel: UILabel = {
        let label = UILabel()
        label.textColor = .black
        label.text = "Message: \(message)"
        label.translatesAutoresizingMaskIntoConstraints = false
        return label
    }()

    override init(frame: CGRect) {
        super.init(frame: frame)
        self.backgroundColor = .white

        self.addSubview(messageLabel)

        NSLayoutConstraint.activate([
            messageLabel.centerXAnchor.constraint(equalTo: self.centerXAnchor),
            messageLabel.centerYAnchor.constraint(equalTo: self.centerYAnchor),
        ])
        
        DispatchQueue.main.asyncAfter(deadline: .now() + 3) {
            self.message = "Hello World!"
        }
    }

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

let view = CustomView(
    frame: CGRect(x: 0,
                  y: 0,
                  width: 300,
                  height: 300)
)

PlaygroundPage.current.liveView = view