Swift 4

Set delegate of UITextField in view controller, field.delegate = self, and then:

extension ViewController: UITextFieldDelegate {
    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
        // don't force `endEditing` if you want to be asked for resigning
        // also return real flow value, not strict, like: true / false
        return textField.endEditing(false)
    }
}

Define this class and then set your text field to use the class and this automates the whole hiding keyboard when return is pressed automatically.

class TextFieldWithReturn: UITextField, UITextFieldDelegate
{

    required init?(coder aDecoder: NSCoder) 
    {
        super.init(coder: aDecoder)
        self.delegate = self
    }

    func textFieldShouldReturn(_ textField: UITextField) -> Bool
    {
        textField.resignFirstResponder()
        return true
    }

}

Then all you need to do in the storyboard is set the fields to use the class:

Ok, I think for a novice things might be a bit confusing. I think the correct answer is a mix of all the above, at least in Swift4.

Either create an extension or use the ViewController in which you'd like to use this but make sure to implement UITextFieldDelegate. For reusability's sake I found it easier to use an extension:

extension UIViewController : UITextFieldDelegate {
    ...
}

but the alternative works as well:

class MyViewController: UIViewController {
    ...
}

Add the method textFieldShouldReturn (depending on your previous option, either in the extension or in your ViewController)

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

In your viewDidLoad method, set the textfield's delegate to self

@IBOutlet weak var myTextField: UITextField!
...

override func viewDidLoad() {
    ..
    myTextField.delegte = self;
    ..
}

That should be all. Now, when you press return the textFieldShouldReturn should be called.

If you want to hide the keyboard for a particular keyboard use [self.view resignFirstResponder]; If you want to hide any keyboard from view use [self.view endEditing:true];