各系统版本导航栏的层级

iOS8、iOS9：

[NavigationController.navigationBar subviews]: 
<__NSArrayM 0x7f968ed39d90>(
<_UINavigationBarBackground: 0x7f968f0bdf00; frame = (0 -20; 375 64); autoresize = W; userInteractionEnabled = NO; layer = <CALayer: 0x7f968c489850>>,
<UINavigationButton: 0x7f968ed02bf0; frame = (8 8; 39 30); opaque = NO; layer = <CALayer: 0x7f968f17c180>>,
<UINavigationItemView: 0x7f968f16e460; frame = (163.5 8; 48.5 27); opaque = NO; userInteractionEnabled = NO; layer = <CALayer: 0x7f968ed39a60>>,
<_UINavigationBarBackIndicatorView: 0x7f968f0810b0; frame = (8 11.5; 13 21); alpha = 0; opaque = NO; userInteractionEnabled = NO; layer = <CALayer: 0x7f968c4a0c20>>
)

iOS8和9中_UINavigationBarBackground的subviews也是一致的：

<__NSArrayM 0x7f968ed1d820>(
<_UIBackdropView: 0x7f968f1c58a0; frame = (0 0; 375 64); opaque = NO; autoresize = W+H; userInteractionEnabled = NO; layer = <_UIBackdropViewLayer: 0x7f968ed89710>>,
<UIImageView: 0x7f968f07bea0; frame = (0 64; 375 0.5); userInteractionEnabled = NO; layer = <CALayer: 0x7f968f0cbc60>>
)

iOS10：

[NavigationController.navigationBar subviews]: 
<__NSArrayM 0x60000005d1f0>(
<_UIBarBackground: 0x7fe15e45aed0; frame = (0 -20; 375 64); userInteractionEnabled = NO; layer = <CALayer: 0x60800042fa80>>,
<UINavigationButton: 0x7fe15e59d710; frame = (16 7; 38 30); opaque = NO; layer = <CALayer: 0x600000626d80>>,
<<UINavigationItemView: 0x7fe15e751b50; frame = (162.5 8; 50 27); opaque = NO; userInteractionEnabled = NO; layer = <CALayer: 0x600000431960>>: item=<<UINavigationItem: 0x6000001dae50>: title:'Wallet'> title=Wallet>,
<_UINavigationBarBackIndicatorView: 0x7fe15e423ba0; frame = (8 11.5; 13 21); alpha = 0; opaque = NO; userInteractionEnabled = NO; layer = <CALayer: 0x608000224c60>>
)

iOS10中_UIBarBackground.subviews :

<__NSArrayM 0x60000005dd90>(
<UIImageView: 0x7fe15e463260; frame = (0 64; 375 0.5); userInteractionEnabled = NO; layer = <CALayer: 0x60800023cda0>>,
<UIVisualEffectView: 0x7fe15e4d2200; frame = (0 0; 375 64); layer = <CALayer: 0x60800023e500>>
)

iOS11：

[NavigationController.navigationBar subviews]: 
<__NSArrayM 0x11215a0c0>(
<_UIBarBackground: 0x1123a1c40; frame = (0 -20; 320 64); userInteractionEnabled = NO; layer = <CALayer: 0x11260d150>>,
<_UINavigationBarLargeTitleView: 0x1123aad20; frame = (0 44; 320 49); clipsToBounds = YES; hidden = YES; layer = <CALayer: 0x1123a8800>>,
<_UINavigationBarContentView: 0x1123acd70; frame = (0 0; 320 44); layer = <CALayer: 0x1123ad030>>,
<_UINavigationBarModernPromptView: 0x111113540; frame = (0 0; 0 0); alpha = 0; hidden = YES; layer = <CALayer: 0x111114e30>>
)

iOS11中_UIBarBackground.subviews 和iOS10一样，另附其他子视图：

_UINavigationBarLargeTitleView.subviews:
<__NSArrayM 0x1123a8af0>(
<UILabel: 0x1123c19a0; frame = (15.8563 26.5; 37.2874 0); transform = [1.065354246692275, 0, 0, 1.065354246692275, 0, 0]; userInteractionEnabled = NO; layer = <_UILabelLayer: 0x1123c20b0>>
)

_UINavigationBarContentView.subviews:
<__NSArrayM 0x1123fa870>(
<_UIButtonBarStackView: 0x112619d40; frame = (8 0; 46 44); layer = <CALayer: 0x1123bc230>>,
<UILabel: 0x11261eea0; frame = (142.5 10.5; 35 23.5); text = '钱包'; userInteractionEnabled = NO; layer = <_UILabelLayer: 0x11260b710>>
)

_UINavigationBarModernPromptView.subviews:
<__NSArrayM 0x1123a9800>(
<UILabel: 0x112197750; frame = (0 27; 0 0); text = ''; userInteractionEnabled = NO; layer = <_UILabelLayer: 0x112197a30>>
)