UIWebView * webView = [[UIWebView alloc] initWithFrame:CGRectZero];
  NSString * oldAgent = [webView stringByEvaluatingJavaScriptFromString:@"navigator.userAgent"];
  NSLog(@"old agent :%@", oldAgent);

  //add my info to the new agent
  NSString * newAgent = [oldAgent stringByAppendingString:@" SuGrand/2.4.7 ch_appstore"];

  // or updata my info to the new agent
//  NSString * newAgent = [NSString stringWithFormat:@"Mozilla/5.0 (iPhone; CPU iPhone OS 8_0 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) Mobile/12H141"];

  NSLog(@"new agent :%@", newAgent);

  //regist the new agent
  NSDictionary * dic = [[NSDictionary alloc] initWithObjectsAndKeys:newAgent, @"UserAgent", nil];
  [[NSUserDefaults standardUserDefaults] registerDefaults:dic];

  __weak typeof(self) weakSelf = self;

  [self.webView evaluateJavaScript:@"navigator.userAgent" completionHandler:^(id result, NSError *error) {
    __strong typeof(weakSelf) strongSelf = weakSelf;

    NSLog(@"old agent :%@", result);

    NSString *userAgent = result;
    NSString *newUserAgent = [userAgent stringByAppendingString:@" Appended Custom User Agent"];

    NSDictionary *dictionary = [NSDictionary dictionaryWithObjectsAndKeys:newUserAgent, @"UserAgent", nil];
    [[NSUserDefaults standardUserDefaults] registerDefaults:dictionary];

    strongSelf.webView = [[WKWebView alloc] initWithFrame:strongSelf.view.bounds];

    // After this point the web view will use a custom appended user agent
    [strongSelf.webView evaluateJavaScript:@"navigator.userAgent" completionHandler:^(id result, NSError *error) {
      NSLog(@"new agent :%@", result);
    }];
  }];