Did select tababr

DidSelectTabBar.m

- (void)tabBarController:(UITabBarController *)tabBarController didSelectViewController:(UIViewController *)viewController {
    id nc = viewController;
    id vc = [[nc viewControllers] objectAtIndex:0];
    
    static UIViewController *previousController = nil;
    if (previousController == viewController) {
        if ([vc isKindOfClass:[TopicListViewController class]]) {

        }
    }
    previousController = viewController;
}