- (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:@"超过最大字数不能输入了" delegate:nil cancelButtonTitle:@"Ok" otherButtonTitles:nil, nil] autorelease];  
      [alert show];  
      return NO;  
    }  
  }  
  return YES;  
}