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)]

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);
}

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 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
    }
}

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;
    }

}

美国小妞Xamarin:

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