FKTabBarController *tabBarController = [[FKTabBarController alloc]initWithNibName:nil bundle:nil];
NSMutableArray *viewControllers = [@[] mutableCopy];
NSMutableArray *items = @[].mutableCopy;
for (int i=0; i<4; i++) {
    UINavigationController *nc = [[UINavigationController alloc] initWithRootViewController:[[DemoViewController alloc]initWithNibName:nil bundle:nil]];
    [viewControllers addObject:nc];
    UIImage *icon = [UIImage imageWithColor:[UIColor clearColor]];
    [items addObject:[[FKTabBarItem alloc] initWithIcon:icon
                                          selectedColor:[UIColor blackColor]
                                        unselectedColor:[UIColor grayColor]]];
}
tabBarController.viewController = viewController;
tabBarController.tabBar.items = items;

UITabBarController *tabBarController = self.navigationController.tabBarController;

UITabBarItem *tabBarItem = tabBarController.tabBar.items[tabBarController.selectedIndex];
tabBarItem.badgeValue = [@([self.navigationController.viewControllers count]) stringValue];