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")
   }

Swift 4 + 2。

通过实现UITextFieldDelegate方法

ViewController: class MyViewController: UIViewController { let MAX_LENGTH = 256 @IBOutlet weak var myTextField: UITextField! override viewDidLoad() { self.myTextField.delegate = self } } Delegate: extension MyViewController: UITextFieldDelegate { func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool { let userText = textView.text ?? "" var newText = "" if range.length > 0 { let txt = NSString(string: userText) if txt.length > 0 { newText = txt.replacingCharacters(in: range, with: text) } } else { newText = userText + text } return newText.count <= MAX_LENGTH } }

我根据@Frouo给出了补充答案。我认为他的回答是最美丽的方式。因为这是一个我们可以重用的通用控件。

private var kAssociationKeyMaxLength: Int = 0

extension UITextField {

    @IBInspectable var maxLength: Int {
        get {
            if let length = objc_getAssociatedObject(self, &kAssociationKeyMaxLength) as? Int {
                return length
            } else {
                return Int.max
            }
        }
        set {
            objc_setAssociatedObject(self, &kAssociationKeyMaxLength, newValue, .OBJC_ASSOCIATION_RETAIN)
            self.addTarget(self, action: #selector(checkMaxLength), for: .editingChanged)
        }
    }

    func checkMaxLength(textField: UITextField) {

        guard !self.isInputMethod(), let prospectiveText = self.text,
            prospectiveText.count > maxLength
            else {
                return
        }

        let selection = selectedTextRange
        let maxCharIndex = prospectiveText.index(prospectiveText.startIndex, offsetBy: maxLength)
        text = prospectiveText.substring(to: maxCharIndex)
        selectedTextRange = selection
    }

    //The method is used to cancel the check when use Chinese Pinyin input method.
    //Becuase the alphabet also appears in the textfield when inputting, we should cancel the check.
    func isInputMethod() -> Bool {
        if let positionRange = self.markedTextRange {
            if let _ = self.position(from: positionRange.start, offset: 0) {
                return true
            }
        }
        return false
    }

}

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;
    }

}

为了完成八月的回答，建议函数的一个可能实现(参见UITextField的委托)。

我没有测试domness代码，但我的代码不会卡住，如果用户达到了限制，它与一个新的字符串兼容，取代一个更小或等于一个。

-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    //limit the size :
    int limit = 20;
    return !([textField.text length]>limit && [string length] > range.length);
}

最好的方法是设置文本更改通知。在视图控制器方法的-awakeFromNib中，你需要:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(limitTextField:) name:@"UITextFieldTextDidChangeNotification" object:myTextField];

然后在同一个类中添加:

- (void)limitTextField:(NSNotification *)note {
    int limit = 20;
    if ([[myTextField stringValue] length] > limit) {
        [myTextField setStringValue:[[myTextField stringValue] substringToIndex:limit]];
    }
}

然后链接出口myTextField到你的UITextField，它将不让你添加任何字符后，你达到限制。一定要把这个添加到你的dealloc方法中:

[[NSNotificationCenter defaultCenter] removeObserver:self name:@"UITextFieldTextDidChangeNotification" object:myTextField];