UINavigationController结构:
$2 = 0x0801dbf0 <UIWindow: 0x713bba0; frame = (0 0; 320 568); layer = <UIWindowLayer: 0x713b4c0>>
| <UILayoutContainerView: 0x82213b0; frame = (0 0; 320 568); autoresize = W+H; layer = <CALayer: 0x8221470>>
| | <UINavigationTransitionView: 0x8223ce0; frame = (0 0; 320 568); clipsToBounds = YES; autoresize = W+H; layer = <CALayer: 0x8223db0>>
| | | <UIViewControllerWrapperView: 0x7537f20; frame = (0 64; 320 504); autoresize = W+H; layer = <CALayer: 0x7537fd0>>
| | | | <UIView: 0x7534310; frame = (0 0; 320 504); autoresize = W+H; layer = <CALayer: 0x7534370>>
| | | | | <UIRoundedRectButton: 0x7533b50; frame = (10 50; 80 30); opaque = NO; layer = <CALayer: 0x7533c20>>
| | | | | | <UIGroupTableViewCellBackground: 0x7534d30; frame = (0 0; 80 30); userInteractionEnabled = NO; layer = <CALayer: 0x7534e00>>
| | | | | | <UIImageView: 0x7535e70; frame = (1 1; 78 29); opaque = NO; userInteractionEnabled = NO; layer = <CALayer: 0x7536500>>
| | | | | | <UIButtonLabel: 0x7536b90; frame = (0 0; 0 0); clipsToBounds = YES; hidden = YES; opaque = NO; userInteractionEnabled = NO; layer = <CALayer: 0x7536c80>>
| | <UINavigationBar: 0x82215f0; frame = (0 20; 320 44); autoresize = W; gestureRecognizers = <NSArray: 0x8223620>; layer = <CALayer: 0x8221710>>
| | | <_UINavigationBarBackground: 0x8221300; frame = (0 0; 320 44); opaque = NO; autoresize = W; userInteractionEnabled = NO; layer = <CALayer: 0x8221a70>>
| | | | <UIImageView: 0x82221c0; frame = (0 44; 320 3); opaque = NO; autoresize = W+TM; userInteractionEnabled = NO; layer = <CALayer: 0x8222220>>
| | | <UINavigationItemView: 0x8224720; frame = (160 22; 0 0); opaque = NO; userInteractionEnabled = NO; layer = <CALayer: 0x82247f0>>