取到当前的视图控制器

当前控制器
UINavigationController *nav = (UINavigationController *)self.window.rootViewController;
UIViewController *vc = nav.visibleViewController;


self.window.rootViewController.presentedViewController


presentedViewController和presentingViewController的区别
A presentViewController B 后，
a.presentedViewController就是b， 
b.presentingViewController就是a