Simple Closure example

// Start a new project
//  Add a UILabel and UIButton to the default view controller
//  connect the label to countLabel @IBOutlet
//  connect the button to doFetch @IBAction
//
// each tap of the button will call fetch() in LoginManager.shared
//  and use a 1-second delay to simulate fetching data from an API

class ViewController: UIViewController {

    @IBOutlet var countLabel: UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()

        // set the "callback" closure in LoginManager
        LoginManager.shared.callback = { [weak self] str in
            // safely unwrap self
            guard let self = self else { return }
            // we have to update UI elements on the main thread
            DispatchQueue.main.async {
                // set the returned str to countLabel's text
                self.countLabel.text = str
            }
        }
    }
    @IBAction func doFetch(_ sender: Any) {
        countLabel.text = "Calling API..."
        LoginManager.shared.fetch()
    }

}

class LoginManager {

    static let shared = LoginManager()

    // counter var
    var myCounter: Int = 0

    // closure for communication
    var callback: ((String) -> ())?

    func fetch() -> Void {
        // increment the counter
        myCounter += 1
        // 1-second delay to simulate getting data from API
        DispatchQueue.global(qos: .userInitiated).asyncAfter(deadline: .now() + 1, execute: {
            // get a random number from 1 to 9999
            let n = Int.random(in: 1...9999)
            // use the closure to send it back as a string
            self.callback?("API Count: \(self.myCounter) Random: \(n)")
        })
    }

}