https://stackoverflow.com/questions/56762547/how-do-i-check-for-the-same-event-on-different-textfields-in-swift

ButtonEnabling.swift

class ViewController: UIViewController {

    @IBOutlet weak var textOTP1: UITextField!
    @IBOutlet weak var textOTP2: UITextField!
    @IBOutlet weak var textOTP3: UITextField!
    @IBOutlet weak var textOTP4: UITextField!
    @IBOutlet weak var textOTP5: UITextField!
    @IBOutlet weak var textOTP6: UITextField!

    @IBOutlet weak var verifyButton: UIButton!

    // in addition to outlets above, I'd also add all of the text fields to this outlet collection

    @IBOutlet var textFields: [UITextField]!

    override func viewDidLoad() {
        super.viewDidLoad()

        enableButton(false)

        // if you wanted to add the targets programmatically, you could do a loop
        // like this, though I'd just hook up these actions right in IB
        //
        // for textField in textFields {
        //     textField.addTarget(self, action: #selector(textChanges(_:)), for: .editingChanged)
        // }
    }

    @IBAction func didEditingChanged(_ textField: UITextField) {
        checkIfWritten()
    }
}

private extension ViewController {
    func checkIfWritten() {
        // rather than if statement with all of the text fields, I'd just iterate through
        // the IBOutlet collection we created in IB, and if any of them are empty, then
        // disable the button and exit

        for textField in textFields where textField.text!.isEmpty {
            enableButton(false)
            return
        }

        // if we've gotten here, we know that all the text fields must not be empty, and we can just enable the button

        enableButton(true)
    }

    // since you're calling this from at least two different places, I'd keep it DRY
    // and move this into its own method.

    func enableButton(_ isEnabled: Bool) {
        if isEnabled {
            verifyButton.alpha = 1
            verifyButton.isEnabled = true
        } else {
            verifyButton.alpha = 0.2
            verifyButton.isEnabled = false
        }
    }
}