在项目解bug过程中，偶然发现自己的知识盲区。
一个跟控制器tabbarVC下面包含两个tab,在其中一个tab中一直push到第三层。例如,从首页-->商品页-->商品详情页。此时如果pop回来的话，只会走三个nav的viewWillAppear方法，与tabbarVC没有关系。
NSMutableArray *array = [NSMutableArray array];
LocalViewController *localVC = [[LocalViewController alloc] init];
localVC.view.backgroundColor = [UIColor whiteColor];
localVC.tabBarItem.title = @"本地";
localVC.tabBarItem.image = [[UIImage imageNamed:@"tab_mall_normal"] imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal];
localVC.tabBarItem.selectedImage = [[UIImage imageNamed:@"tab_mall_select"] imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal];
UINavigationController *localNav = [[UINavigationController alloc] initWithRootViewController:localVC];
[array addObject:localNav];
MineViewController *mineVC = [[MineViewController alloc] init];
mineVC.view.backgroundColor = [UIColor whiteColor];
mineVC.tabBarItem.title = @"我的";
mineVC.tabBarItem.image = [[UIImage imageNamed:@"tab_mine_normal"] imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal];
mineVC.tabBarItem.selectedImage = [[UIImage imageNamed:@"tab_mine_select"] imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal];
UINavigationController *mineNav = [[UINavigationController alloc] initWithRootViewController:mineVC];
[array addObject:mineNav];
self.selectedIndex = 0;
self.viewControllers = array;
此时若从商品页present到商品详情页，从商品详情页dismiss回商品页，会依次调用tabbarVC的viewWillAppear--->商品页VC的viewWillAppear--->tabbarVC的viewDidAppear--->商品页VC的viewDidAppear。（原以为不会走tabbar方法，经过调试才知道自己想当然了。）
