textfield 限制字数

- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string;
{
    if ([string isEqualToString:@"\n"])  
    {
        return YES;
    }
    NSString * toBeString = [textField.text stringByReplacingCharactersInRange:range withString:string]; //关键代码。将新输入的内容与原内容合并
     
    if (self.myTextField == textField)  
    {
        if ([toBeString length] > 20) {
            textField.text = [toBeString substringToIndex:20]; //总内容超过限制字数20，则截取前20个字符
            return NO;
        }
    }
  return YES;
}