DidBecomeActive.swift

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

final class MyViewController: UIViewController {
    
    override func loadView() {
        let view = UIView()
        view.backgroundColor = .white

        let label = UILabel()
        label.frame = CGRect(x: 150, y: 200, width: 200, height: 20)
        label.text = "Hello World!"
        label.textColor = .black
        
        view.addSubview(label)
        self.view = view
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Setup self as oberver for UIApplication.didBecomeActiveNotification notification name
        NotificationCenter.default.addObserver(self,
                                               selector: #selector(appDidBecomeActive),
                                               name: UIApplication.didBecomeActiveNotification, object: nil)
    }
    
    // That function runs when app will be active
    @objc private func appDidBecomeActive() {
        print("UIApplication is active now")
    }
    
}
// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()