为了使它与任何长度的字符串剪切和粘贴工作，我建议将函数更改为如下内容:

#define MAX_LENGTH 20

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
    {
        NSInteger insertDelta = string.length - range.length;

        if (textField.text.length + insertDelta > MAX_LENGTH)
        {
           return NO; // the new string would be longer than MAX_LENGTH
        }
        else {
            return YES;
        }
    }

使用接口构建器，您可以在任何函数中链接并获得“编辑更改”的事件。 现在你可以检查长度

- (IBAction)onValueChange:(id)sender 
{
    NSString *text = nil;
    int MAX_LENGTH = 20;
    switch ([sender tag] ) 
    {
        case 1: 
        {
            text = myEditField.text;
            if (MAX_LENGTH < [text length]) {
                myEditField.text = [text substringToIndex:MAX_LENGTH];
            }
        }
            break;
        default:
            break;
    }

}

你也可以在Swift 4中使用NotificationCenter来做到这一点

NotificationCenter.default.addObserver(self, selector: #selector(self.handleTextChange(recognizer:)), name: NSNotification.Name.UITextFieldTextDidChange, object: yourTextField)

    @objc func handleTextChange(recognizer: NSNotification) {
            //max length is 50 charater max
            let textField = recognizer.object as! UITextField

            if((textField.text?.count)! > 50) {
                let newString: String? = (textField.text as NSString?)?.substring(to: 50)
                textField.text = newString

            }         
        }

Swift 4.2和UITextFieldDelegate方法

这适用于我和限制文本字段有一个最大输入8个字符。希望NSRange最终会改变为Range，但现在我很高兴使用NSString从NSRange创建一个范围涉及到处理另一个可选。

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    let text = textField.text ?? ""
    let nsString = text as NSString
    let newText = nsString.replacingCharacters(in: range, with: string)
    return newText.count <= 8
}

Swift 2.1+

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

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

    let newLength = textField.text!.characters.count + string.characters.count - range.length
    return newLength <= 25
}

希望能有所帮助

适用于Swift 3.1或更高版本

首先添加协议UITextFieldDelegate

如:-

class PinCodeViewController: UIViewController, UITextFieldDelegate { 
.....
.....
.....

}

之后创建你的UITextField并设置委托

完成经验:-

import UIKit

class PinCodeViewController: UIViewController, UITextFieldDelegate {

let pinCodetextField: UITextField = {
    let tf = UITextField()
    tf.placeholder = "please enter your pincode"
    tf.font = UIFont.systemFont(ofSize: 15)
    tf.borderStyle = UITextBorderStyle.roundedRect
    tf.autocorrectionType = UITextAutocorrectionType.no
    tf.keyboardType = UIKeyboardType.numberPad
    tf.clearButtonMode = UITextFieldViewMode.whileEditing;
    tf.contentVerticalAlignment = UIControlContentVerticalAlignment.center
 return tf
  }()


 override func viewDidLoad() {
   super.viewDidLoad()
   view.addSubview(pinCodetextField)
    //----- setup your textfield anchor or position where you want to show it----- 


    // after that 
pinCodetextField.delegate = self // setting the delegate

 }
func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    return !(textField.text?.characters.count == 6 && string != "")
     } // this is return the maximum characters in textfield 
    }