[ios] Find the current top view controller for your iOS application

- (UIViewController *)topViewController{
  return [self topViewController:[UIApplication sharedApplication].keyWindow.rootViewController];
}
 
- (UIViewController *)topViewController:(UIViewController *)rootViewController
{
  if (rootViewController.presentedViewController == nil) {
    return rootViewController;
  }
  
  if ([rootViewController.presentedViewController isMemberOfClass:[UINavigationController class]]) {
    UINavigationController *navigationController = (UINavigationController *)rootViewController.presentedViewController;
    UIViewController *lastViewController = [[navigationController viewControllers] lastObject];
    return [self topViewController:lastViewController];
  }
  
  UIViewController *presentedViewController = (UIViewController *)rootViewController.presentedViewController;
  return [self topViewController:presentedViewController];
}