基本思路
如果界面已经有了UITabBarController，则可以先获取当前tab对应的UINavigationController，再获取它的最顶层vc
UINavigationController *navi = [self.tabBarController selectedViewController];
UIViewController *topVC = navi.topViewController;
那么topVC就是最前VC了么？topVC其实还有可能是UITabBarController。
if([topVC isKindOfClass:[UITabBarController class]]){
UITabBarController *tabVC = (UITabBarController *)topVC;
currentVC= tabVC.selectedViewController;
}
else{
if(currentVC.presentedViewController){
currentVC = currentVC.presentedViewController;
}
}
if([currentVC isKindOfClass:[UINavigationController class]]){
UINavigationController *naviVC = (UINavigationController *)currentVC;
currentVC = naviVC.topViewController;
}
return NSStringFromClass([currentVC class]);
NavigationController可能不止一个
