簡單說明：
UITextField限制限制字符串長度：例如要求字符串長度為11位，則textField.text.length > 10
代碼如下：

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
    if (textField == self.userName) {
        if (textField.text.length > 10 && string.length > 0) return NO;
    }
    
    return YES;
}

Swift

  func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        
  
            let proposeLength = (textField.text?.lengthOfBytes(using: String.Encoding.utf8))! - range.length + string.lengthOfBytes(using: String.Encoding.utf8)
            if proposeLength > 20 { return false }

        return true
        
    }

UITextField限制限制字符串對(duì)應(yīng)的int數(shù)值：例如要求字符串>100

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

    if (string.length == 0)
    {
        
    }
    else
    {
        if ([textField.text integerValue] * 10 + [string integerValue]>100)
        {
            return NO;
        }
        
    }
    return YES;
}