UICollectionViewLayout 的

shouldInvalidateLayoutForBoundsChange: 
//方法

要吃透一个框架， 不仅仅是 改改 变量

// 很强大的结构体 
// ?
struct {
        unsigned int pagerViewDidScroll   :1;
        unsigned int didScrollFromIndexToNewIndex   :1;
        unsigned int initializeTransformAttributes   :1;
        unsigned int applyTransformToAttributes   :1;
    }_delegateFlags;
    struct {
        unsigned int cellForItemAtIndex   :1;
        unsigned int layoutForPagerView   :1;
    }_dataSourceFlags;