这个是对automaticallyAdjustsScrollViewInsets和edgesForExtendedLayout的总结
iOS grabs the first subview in your viewcontroller's view, so the one at index 0, and if it's a subclass of UIScrollView then applies the explained properties to it.
就是说控制器的View第一个子视图是UIScrollView 或者UIScrollView的子类，比如UITableView,上面的属性将会起到应用，且navigationBar不隐藏的情况下。
