我想补充一下@sickp给出的答案。

在他的Swift代码中有一个问题，任何多字节文本(例如表情符号)都会出现。NSRange和String在Swift中是不兼容的，所以委托类将它们结合在一起是令人沮丧的。技巧是简单地转换String对象到NSString正确的解决方案，基于什么@sickp写，实际上是这样的:

func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {

    let currentText = (textField.text as NSString?) ?? NSString()
    let currentCharacterCount = currentText.length
    if range.length + range.location > currentCharacterCount {
        return false
    }
    let newLength = currentText.replacingCharacters(in: range, with: string).characters.count
    return newLength <= 25
}

Swift 2.1+

func textField(textField: UITextField, shouldChangeCharactersInRange range: NSRange, replacementString string: String) -> Bool {

    if (range.length + range.location > textField.text!.characters.count )
    {
        return false;
    }

    let newLength = textField.text!.characters.count + string.characters.count - range.length
    return newLength <= 25
}

希望能有所帮助

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;
    }

}

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;
  }

你不能直接这样做- UITextField没有maxLength属性，但你可以设置UITextField的委托，然后使用:

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

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

            }         
        }