- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string;  {    if ([string isEqualToString:@"/n"])     {      return YES;    }    NSString * toBeString = [textField.text stringByReplacingCharactersInRange:range withString:string];        if (self.myTextField == textField)     {      if ([toBeString length] > 20) {        textField.text = [toBeString substringToIndex:20];        UIAlertView *alert = [[[UIAlertView alloc] initWithTitle:nil message:@"超過最大字數不能輸入了" delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil, nil] autorelease];        [alert show];        return NO;      }    }    return YES;  }