UICollectilnViewFlowLayout

//38行
@protocol UICollectionViewDelegateFlowLayout <UICollectionViewDelegate>
//39行
@optional

//item的大小          41行
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath;

//页边距          42行
//参数1: collectionView
//参数2: 布局对象
//参数3: 索引号
//返回: 页边距
- (UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout insetForSectionAtIndex:(NSInteger)section;

//布局对象          50行
@interface UICollectionViewFlowLayout : UICollectionViewLayout

//最小行间距          52行
@property (nonatomic) CGFloat minimumLineSpacing;

//最小列间距          53行
@property (nonatomic) CGFloat minimumInteritemSpacing;

//滑动方向          56行
@property (nonatomic) UICollectionViewScrollDirection scrollDirection; // default is UICollectionViewScrollDirectionVertical