How to hide keyboard in swift on pressing return key?

You can make the app dismiss the keyboard using the following function

func textFieldShouldReturn(_ textField: UITextField) -> Bool {
    self.view.endEditing(true)
    return false
}

Here is a full example to better illustrate that:

//
//  ViewController.swift
//
//

import UIKit

class ViewController: UIViewController, UITextFieldDelegate {

    @IBOutlet var myTextField : UITextField

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        self.myTextField.delegate = self
    }

    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
        self.view.endEditing(true)
        return false
    }
}

Code source: http://www.snip2code.com/Snippet/85930/swift-delegate-sample