How to clear UITextView upon editing?

Looks like you're not setting your UITextView's delegate.

class ViewController: UIViewController, UITextViewDelegate {

@IBOutlet weak var myTextView: UITextView!

override func viewDidLoad() {
    super.viewDidLoad()
    myTextView.delegate = self
}

func textViewDidBeginEditing(textView: UITextView) {
    myTextView.text = String()
}

UITextViewDelegate Protocol Reference