UIButton Selector Error with Swift 4: Argument of '#selector' refers to instance method that is not exposed to Objective-C

Swift 4 (Xcode 9.0) does not work with the below Swift 3 code while drawing a UIButton:

The error message says that:

Argument of '#selector' refers to instance method 'click()' that is not exposed to Objective-C

Add '@objc' to expose this instance method to Objective-C

By clicking the "Fix" button, Xcode automatically fix this error by placing '@objc' in front of func click() as below:

The complete code to draw a button that prints a message while touching it:

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {

        super.viewDidLoad()

        

        let button = UIButton(frame: CGRect(x: 0, y: 0, width: 100, height: 30))

        button.center = view.center

        button.setTitle("Press", for: .normal)

        button.setTitleColor(.blue, for: .normal)

        button.setTitleColor(.cyan, for: .highlighted)

        button.addTarget(self, action: #selector(click), for: UIControlEvents.touchUpInside)

        view.addSubview(button)

    }

    @objc func click() {

        print("Pressed!")

    }

    override func didReceiveMemoryWarning() {

        super.didReceiveMemoryWarning()

        // Dispose of any resources that can be recreated.

    }

}