dismiss keyboard with a uiTextView

Easiest and best way to do this using UITextView Extension.
Credit: http://www.swiftdevcenter.com/uitextview-dismiss-keyboard-swift/
Your ViewController Class

class ViewController: UIViewController {

    @IBOutlet weak var myTextView: UITextView!

    override func viewDidLoad() {
        super.viewDidLoad()
        // 1
        self.myTextView.addDoneButton(title: "Done", target: self, selector: #selector(tapDone(sender:)))
    }
    // 2
    @objc func tapDone(sender: Any) {
        self.view.endEditing(true)
    }
}

Add UITextView Extension

extension UITextView {

    func addDoneButton(title: String, target: Any, selector: Selector) {

        let toolBar = UIToolbar(frame: CGRect(x: 0.0,
                                              y: 0.0,
                                              width: UIScreen.main.bounds.size.width,
                                              height: 44.0))//1
        let flexible = UIBarButtonItem(barButtonSystemItem: .flexibleSpace, target: nil, action: nil)//2
        let barButton = UIBarButtonItem(title: title, style: .plain, target: target, action: selector)//3
        toolBar.setItems([flexible, barButton], animated: false)//4
        self.inputAccessoryView = toolBar//5
    }
}

For more detail: visit full documentation

This works for me:

import UIKit

class ViewController: UIViewController, UITextViewDelegate {


    @IBOutlet weak var textView: UITextView!

    override func viewDidLoad() {
        super.viewDidLoad()

        textView.delegate = self
    }

    /* Updated for Swift 4 */
    func textView(_ textView: UITextView, shouldChangeTextIn range: NSRange, replacementText text: String) -> Bool {
        if(text == "\n") {
            textView.resignFirstResponder()
            return false
        }
        return true
    }

    /* Older versions of Swift */
    func textView(textView: UITextView, shouldChangeTextInRange range: NSRange, replacementText text: String) -> Bool {
        if(text == "\n") {
            textView.resignFirstResponder()
            return false
        }
        return true
    }

}

Add UITextViewDelegate to your class and then set your delegate for your textView or your textField in viewDidLoad. Should look something like this:

// in viewDidLoad
textField.delegate = self
textView.delegate = self

Swift 3

// hides text views
func textView(_ textView: UITextView, shouldChangeTextIn range: NSRange, replacementText text: String) -> Bool {
    if (text == "\n") {
        textView.resignFirstResponder()
        return false
    }
    return true
}
// hides text fields
func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    if (string == "\n") {
        textField.resignFirstResponder()
        return false
    }
    return true
}

Swift 2.0

The below syntax has been tested for Swift 1.2 & Swift 2.0

func textView(textView: UITextView, shouldChangeTextInRange range: NSRange, replacementText text: String) -> Bool {
    if(text == "\n") {
        textView.resignFirstResponder()
        return false
    }
    return true
}

Below code will dismissing the keyboard when click return/done key on UITextView.

In Swift 3.0

import UIKit

class ViewController: UIViewController, UITextViewDelegate {

@IBOutlet var textView: UITextView!

override func viewDidLoad() {
    super.viewDidLoad()

    textView.delegate = self
}

func textView(_ textView: UITextView, shouldChangeTextIn range: NSRange, replacementText text: String) -> Bool
{
    if(text == "\n")
    {
        view.endEditing(true)
        return false
    }
    else
    {
        return true
    }
}

In Swift 2.2

func textView(textView: UITextView, shouldChangeTextIn range: NSRange, replacementText text: String) -> Bool
{
    if text == "\n"
    {
        view.endEditing(true)
        return false
    }
    else
    {
        return true
    }
}