UIView

• 默认值：
  View Controller 的 View 的 safeAreaInsets (在viewDidAppear 查看)：

  1. 不在Navigation Controller中：{20, 0, 0, 0 }
  2. 在Navigation Controller中：{64, 0, 0, 0 }
  3. View Controller 的 additionalSafeAreaInsets : {0, 0, 0, 0 }

• 修改后：
  修改View Controller 的 additionalSafeAreaInsets 后：
  view.safeAreaInsets = view.safeAreaInsets + vc. additionalSafeAreaInsets

UIScrollView

由 contentInsetAdjustmentBehavior 决定：

• never: adjustedContentInset = safeAreaInsets
• always: adjustedContentInset = contentInset + safeAreaInsets
• scrollableAxes:
  可滚动方向： adjustedContentInset = contentInset + safeAreaInsets
  可不滚动方向：adjustedContentInset = contentInset
• automatic:
  automaticallyAdjustsScrollViewInsets = YES： adjustedContentInset = contentInset + safeAreaInsets
  automaticallyAdjustsScrollViewInsets = NO：与scrollableAxes 相同。

参考资料：

• iOS 11 安全区域适配总结
• Positioning Content Relative to the Safe Area