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);
}

美国小妞Xamarin:

YourTextField.ShouldChangeCharacters = 
delegate(UITextField textField, NSRange range, string replacementString)
        {
            return (range.Location + replacementString.Length) <= 4; // MaxLength == 4
        };

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

            }         
        }

现在你想要多少字符，只要给值就行了

 - (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range   replacementString:(NSString *)string {
     NSUInteger newLength = [textField.text length] + [string length] - range.length;
     return (newLength > 25) ? NO : YES;
  }

稍微超出了回答原来的问题，并扩展了Frouo的答案，这里有扩展来修剪空白字符串和最大长度，并利用这些字符串扩展来修剪UITextField到最大长度:

// In String_Extensions.swift

extension String {

  func trimmedString() -> String {
    var trimmedString = self.stringByTrimmingCharactersInSet(NSCharacterSet.whitespaceAndNewlineCharacterSet())
    let components = trimmedString.componentsSeparatedByCharactersInSet(NSCharacterSet.whitespaceAndNewlineCharacterSet()).filter { count($0) > 0 }
    return " ".join(components)
  }

  func trimmedStringToMaxLength(maxLength: Int) -> String {
    return trimmedString().substringToIndex(advance(startIndex, min(count(self), maxLength))).trimmedString()
  }

}

// In UITextField_Extensions.swift

private var maxLengthDictionary = [UITextField : Int]()
private var textFieldMaxLength = 20

extension UITextField {

  @IBInspectable var maxLength: Int {
    get {
      if let maxLength = maxLengthDictionary[self] {
        return maxLength
      } else {
        return textFieldMaxLength
      }
    }
    set {
      maxLengthDictionary[self] = newValue < textFieldMaxLength + 1 ? newValue : textFieldMaxLength
    }
  }

  func trimAndLimitToMaxLength() {
    text = text.trimmedStringToMaxLength(maxLength)
  }

}

let someTextField = UITextField()
let someString = "   This   is   a   string   that   is longer than allowable for a text field.   "
someTextField.text = someString
someTextField.trimAndLimitToMaxLength()
println(someTextField.text) // Prints "This is a string tha"
let anotherTextField = UITextField()
anotherTextField.maxLength = 5
anotherTextField.text = someString
anotherTextField.trimAndLimitToMaxLength()
println(anotherTextField.text) // Prints "This"

trimAndLimitToMaxLength() could be used in UITextFieldDelegate's textFieldDidEndEditing(_:) so that a user could enter or paste in a longer than acceptable string and then shorten it vs. just cutting off the input at the max length. In doing this, I would also set attributed text styles to indicate any portion of the text that goes beyond the acceptable length (e.g., [NSBackgroundColorAttributeName : UIColor.redColor(), NSForegroundColorAttributeName : UIColor.whiteColor(), NSStrikethroughStyleAttributeName : NSNumber(int: 1)]

我在Swift中这样做是为了在使用数字垫时限制8个字符。

func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {
    return !(textField.text?.characters.count == MAX_LENGTH && string != "")
}

我必须测试string != ""以允许删除按钮在数字板上工作，否则它将不允许在达到最大值后删除文本字段中的字符。