The behavior of the UICollectionViewFlowLayout is not defined because:
the item height must be less than the height of the UICollectionView minus the section insets top and bottom values, minus the content insets top and bottom values.
The relevant UICollectionViewFlowLayout instance is <UICollectionViewFlowLayout: 0x7f83304331d0>, and it is attached to <UICollectionView: 0x7f8331814200; frame = (0 0; 375 667); clipsToBounds = YES; gestureRecognizers = <NSArray: 0x60800005f740>; layer = <CALayer: 0x60800022bf60>; contentOffset: {0, -64}; contentSize: {375, 667}> collection view layout: <UICollectionViewFlowLayout: 0x7f83304331d0>.
Make a symbolic breakpoint at UICollectionViewFlowLayoutBreakForInvalidSizes to catch this in the debugger.
原因是：
UICollectionViewFlowLayout的itemSize大小超过了collectionView大小，因为有contentInset的影响，比如导航栏。
