iOSUITextField限制输入字数

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string{    if (!string.length)    {        return YES;    }       if (textField.keyboardType == UIKeyboardTypeNumberPad)    {        if ([string rangeOfCharacterFromSet:[[NSCharacterSet decimalDigitCharacterSet] invertedSet]].location != NSNotFound)        {            return NO;        }    }        NSString *updatedText = [textField.text stringByReplacingCharactersInRange:range withString:string];        if (updatedText.length > 11) // 4 was chosen for SSN verification    {        if (string.length > 1)        {            // BasicAlert(@"", @"This field accepts a maximum of 4 characters.");        }                return NO;    }        return YES;}