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

            }         
        }

斯威夫特4

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
    guard let text = textField.text else { return true }
    let newLength = text.count + string.count - range.length
    return newLength <= 10
}

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)]

其他答案不处理用户可以从剪贴板粘贴长字符串的情况。如果我粘贴一个很长的字符串，它应该被截断，但显示出来。 在你的委托中使用这个:

static const NSUInteger maxNoOfCharacters = 5;

-(IBAction)textdidChange:(UITextField * )textField
{
NSString * text = textField.text;

if(text.length > maxNoOfCharacters)
{
    text = [text substringWithRange:NSMakeRange(0, maxNoOfCharacters)];
    textField.text = text;
}

// use 'text'

}

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);
}

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;
        }
    }