我模拟实际的字符串替换，计算未来字符串的长度:

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {

    NSString *newString = [textField.text stringByReplacingCharactersInRange:range withString:string];

    if([newString length] > maxLength)
       return NO;

    return YES;
}

这应该足以解决问题(用你想要的极限替换4)。只要确保在IB中添加委托。

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
     NSString *newString = [textField.text stringByReplacingCharactersInRange:range withString:string];
     return (newString.length<=4);
}

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    if (textField.text.length >= 50) {
        return NO;
    }
    return YES;
}

Swift 5

添加UITextfield的扩展，然后选择一个文本字段并检查属性检查器

private var __maxLengths = [UITextField: Int]()

extension UITextField {
    @IBInspectable var maxLength: Int {
        get {
            guard let l = __maxLengths[self] else {
               return 150 // (default int limit)
            }
            return l
        }
        set {
            __maxLengths[self] = newValue
            addTarget(self, action: #selector(setMaxLength), for: .editingChanged)
        }
    }

    @objc func setMaxLength(textField: UITextField) {
        let t = textField.text
        textField.text = t?.prefix(maxLength).description
    }
}

在Swift 5.2中工作:

 class AngListVC: UIViewController, UITextFieldDelegate {

    @IBOutlet weak var angTextField: UITextField!

    override func viewDidLoad() {
        super.viewDidLoad()
        angTextField.delegate = self
        angTextField.addTarget(self, action: #selector(textFieldDidChange(_:)), for: .editingChanged)
      
    }

   func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
           let userText =  angTextField.text ?? ""
           var newText = ""
           if range.length > 0 {
               let txt = NSString(string: userText)
               if txt.length > 0 {
                   newText = txt.replacingCharacters(in: range, with: "")
               }
           } else {
               newText = userText + ""
           }
           return newText.count <= 3
       }

@objc func textFieldDidChange(_ textField: UITextField) {
        print("textFieldDidChange")
   }

我发现这个方法又快又简单

- (IBAction)backgroundClick:(id)sender {
    if (mytext.length <= 7) {
        [mytext resignFirstResponder];
    } else {
        UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Too Big" 
                                                        message:@"Please Shorten Name"
                                                       delegate:nil 
                                              cancelButtonTitle:@"Cancel"
                                              otherButtonTitles:nil];
        [alert show];
        [alert release];
    }
}