How to disable pasting in a TextField in Swift?

For Swift 3 it's changed to:

override public func canPerformAction(_ action: Selector, withSender sender: Any?) -> Bool {
    if action == #selector(copy(_:)) || action == #selector(paste(_:)) {
        return false
    }

    return true
}

I agree with Leonardo Savio Dabus, if I were you I'd use string checking and just give out a warning, it makes things easier. BUT, if disabling paste option is a fancy feature you really want to put into your app, then you need to do more work. I'll provide the steps below.

Step 1: You need to create another class which extends the UITextField. In this example, I made my CustomUITextField.

import Foundation
import UIKit  //Don't forget this

class CustomUITextField: UITextField {
   override func canPerformAction(_ action: Selector, withSender sender: Any?) -> Bool {
        if action == #selector(UIResponderStandardEditActions.paste(_:)) {
            return false
        }
        return super.canPerformAction(action, withSender: sender)
   }
}

Step 2: Wire the storyboard with your ViewController. You need to declare an IBOutlet as in normal case:

@IBOutlet var textFieldA: CustomUITextField?

Wire the circle next to the @IBOutlet to the TextField in the storyboard. THEN, this is important and easy to be ignored:

• Go to your storyboard
• Click the target TextField
• Select Identity Inspector (the 3rd one)
• Change the class to CustomUITextField

Quick snapshot is provided below.

That's it, hope this works.

Credit:

Main reference

If you want to know more about the behavior of canPerformAction method, though it's an Objective-C version, the concepts are shared here.

For Swift 5

UIResponder​Standard​Edit​Actions has been added recently (iOS 10.0+) through which we can safely check if action is "paste" or not.

import UIKit

class NMTextField: UITextField {
    override func canPerformAction(_ action: Selector, withSender sender: Any?) -> Bool {
        if action == #selector(UIResponderStandardEditActions.paste(_:)) {
            return false
        }
        return super.canPerformAction(action, withSender: sender)
    }
}