adjustContentInset属性的计算方式

scrollView在iOS11新增的两个属性：adjustContentInset 和 contentInsetAdjustmentBehavior。

/* When contentInsetAdjustmentBehavior allows, UIScrollView may incorporate
 its safeAreaInsets into the adjustedContentInset.
 */
@property(nonatomic, readonly) UIEdgeInsets adjustedContentInset API_AVAILABLE(ios(11.0),tvos(11.0));

adjustContentInset表示contentView.frame.origin偏移了scrollview.frame.origin多少；是系统计算得来的，计算方式由contentInsetAdjustmentBehavior决定。有以下几种计算方式：

1. UIScrollViewContentInsetAdjustmentAutomatic：如果scrollview在一个automaticallyAdjustsScrollViewContentInset = YES的controller上，并且这个Controller包含在一个navigation controller中，这种情况下会设置在top & bottom上 adjustedContentInset = safeAreaInset + contentInset不管是否滚动。其他情况下与UIScrollViewContentInsetAdjustmentScrollableAxes相同

2. UIScrollViewContentInsetAdjustmentScrollableAxes: 在可滚动方向上adjustedContentInset = safeAreaInset + contentInset，在不可滚动方向上adjustedContentInset = contentInset；依赖于scrollEnabled和alwaysBounceHorizontal / vertical = YES，scrollEnabled默认为yes，所以大多数情况下，计算方式还是adjustedContentInset = safeAreaInset + contentInset

3. UIScrollViewContentInsetAdjustmentNever: adjustedContentInset = contentInset

4. UIScrollViewContentInsetAdjustmentAlways: adjustedContentInset = safeAreaInset + contentInset

当contentInsetAdjustmentBehavior设置为UIScrollViewContentInsetAdjustmentNever的时候，adjustContentInset值不受SafeAreaInset值的影响。