count(textField.text)在SWIFT 2.0中已弃用

public func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {

        if let textField = textField as? UITextField {

            if (range.length + range.location > textField.text!.characters.count) {
                return false;
            }

            let newLength = textField.text!.characters.count + string.characters.count - range.length;
            switch(textField.tag) { //In case you want to handle multiple textfields
                case Constants.TAG1:
                    return newLength <= 20;
                case Constants.TAG2:
                    return newLength <= 30;
                default:
                    return newLength <= 15;
            }
        }
        return true;
    }

将textfield委派方法中的条件写成：-

func textField(textField: UITextField!, shouldChangeCharactersInRange range: NSRange, replacementString string: String!) -> Bool {

    if (count(textField.text) > 4  && range.length == 0)

   {
        return false // return NO to not change text
    }
    else

    {

     }

将所有代码部分写入其他部分。

委派方法或NSFormatter，例如NSNumberFormatter。格式化程序通常是最合适的，因为它也提供当地语系化支援。

我知道它有点太晚了，但我仍然想分享它太，我发现了一种方法，这是更容易设置一个限制字符的文本字段在快速发展。
下面是代码：-

import UIKit

private var maxLengths = [UITextField: Int]()

extension UITextField {

    @IBInspectable var maxLength: Int {
        get {
            guard let length = maxLengths[self] else {
                return Int.max
            }

            return length
        }
        set {
            maxLengths[self] = newValue
            addTarget(self, action: #selector(limitLength), for: .editingChanged)
        }
    }

    @objc func limitLength(textField: UITextField) {
        guard let prospectiveText = textField.text, prospectiveText.count > maxLength else {
            return
        }

        let selection = selectedTextRange
        let maxCharIndex = prospectiveText.index(prospectiveText.startIndex, offsetBy: maxLength)

        #if swift(>=4.0)
            text = String(prospectiveText[..<maxCharIndex])
        #else
            text = prospectiveText.substring(to: maxCharIndex)
        #endif

        selectedTextRange = selection
    }

}

然后通过面板设置限制。
图片：x1c 0d1x

只要尝试这个限制TF的长度

编辑已更改的TF操作出口

@IBAction func otpTF2EditingChnaged(_ sender: UITextField) {
        if (sender.text?.count == 1) {
            otpTF3.becomeFirstResponder()
        }
        checkMaxLength(textField: sender , maxLength: 1)
    }

将限制长度的函数

private func checkMaxLength(textField: UITextField!, maxLength: Int) {
        if (textField.text!.count > maxLength) {
            textField.deleteBackward()
        }
    }