UICollectionViewFlowLayout类是一个具体的布局对象，它将一个个部件组织成一个可分组的（每个分组都有可选的页眉和页脚视图）网格；这些部件在集合视图中从一行流向下一行或者一列流向下一列（根据滚动方向），每个单元都可以是相同的尺寸或者不同的尺寸；
一个流布局是通过集合视图的委托来决定每个分组的部件，头部以及底部的尺寸大小的，这个委托对象必须执行UICollectionViewDelegateFlowLayout协议，通过这个协议可以让你动态的适应布局信息，如果你没有提供一个委托对象，那流布局将会使用你所设置的本类的以下属性的默认值。
流布局在一个方向上是用一个固定的距离来布局他们的内容视图，而在其他方向上是一个可滚动的距离，例如，在一个垂直滚动网格，当内容视图的高度动态适应网格中的分组和单元格的数目 网格的内容视图的宽度将被约束到相应的集合视图的宽度。布局配置为默认垂直滚动，你也可以使用scrolldirection属性来配置的滚动方向。
流布局的每一个分组都有它自己的头部和底部，如果你想获取头部和底部视图，你必须配置头部和底部的尺寸为非零值，你可以通过实现相应地委托方法来配置，或者你也可以通过设置相应地属性值来配置（headerReferenceSize 和 footerReferenceSize），如果尺寸为0，那么相应的头部和底部视图将不会被添加到集合视图中去。
//列间距
@property (nonatomic) CGFloat minimumLineSpacing;
// 行间距
@property (nonatomic) CGFloat minimumInteritemSpacing;
//cell尺寸
@property (nonatomic) CGSize itemSize;
//预计cell尺寸
@property (nonatomic) CGSize estimatedItemSize NS_AVAILABLE_IOS(8_0); // defaults to CGSizeZero - setting a non-zero size enables cells that self-size via -perferredLayoutAttributesFittingAttributes:
//滚动方向（水平或者垂直）
@property (nonatomic) UICollectionViewScrollDirection scrollDirection; // default is UICollectionViewScrollDirectionVertical
//头部尺寸
@property (nonatomic) CGSize headerReferenceSize;
//底部尺寸
@property (nonatomic) CGSize footerReferenceSize;
//组的边缘尺寸
@property (nonatomic) UIEdgeInsets sectionInset;
UICollectionViewDelegateFlowLayout协议
//每个cell的尺寸
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath;
//每个分组的边缘尺寸
- (UIEdgeInsets)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout insetForSectionAtIndex:(NSInteger)section;
/每个分组的/列间距
- (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout minimumLineSpacingForSectionAtIndex:(NSInteger)section;
//每个分组的行间距
- (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout minimumInteritemSpacingForSectionAtIndex:(NSInteger)section;
//每个分组的头部尺寸
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout referenceSizeForHeaderInSection:(NSInteger)section;
//每个分组的底部尺寸
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout referenceSizeForFooterInSection:(NSInteger)section;
创建UICollectionView，通过- (instancetype)initWithFrame:(CGRect)frame collectionViewLayout:(UICollectionViewLayout *)layout;方法来创建集合视图，layout不能为空；
UICollectionViewFlowLayout*flowLayout = [[UICollectionViewFlowLayoutalloc]init];
flowLayout.scrollDirection= UICollectionViewScrollDirectionVertical;
// 设置headview
flowLayout.headerReferenceSize= CGSizeMake(self.frame.size.width,30);
CGRect rect =self.bounds;
rect.origin.x= rect.size.width* count;
UICollectionView*collectionView = [[UICollectionViewalloc]initWithFrame:rectcollectionViewLayout:flowLayout];
collectionView.backgroundColor= [UIColorwhiteColor];
[collectionViewregisterClass:[LaunchCollectionViewCellclass]forCellWithReuseIdentifier:CellIdentifier];
// 设置headview
[collectionViewregisterClass:[UICollectionReusableViewclass]forSupplementaryViewOfKind:UICollectionElementKindSectionHeaderwithReuseIdentifier:@"ReusableView"];
collectionView.showsHorizontalScrollIndicator=NO;
collectionView.showsVerticalScrollIndicator=NO;
collectionView.pagingEnabled=NO;
collectionView.scrollEnabled=NO;
collectionView.dataSource=self;
collectionView.delegate=self;
[scrollViewaddSubview:collectionView];
和UITableView类似，实现两个协议UICollectionViewDelegate和UICollectionViewDataSource
UICollectionViewDataSource
//每组的单元格数量
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section;
//创建单元格视图，
//这里的cell必须通过 -dequeueReusableCellWithReuseIdentifier:forIndexPath:方法来检索可重用的单元格，且可重用标示符必须和创建UICollectionView时通过- registerClass:forCellWithReuseIdentifier:或者registerNib: forCellWithReuseIdentifier:方法注册的标示符要一致
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath;
//分组数
- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView;
//创建头部或者底部视图
// 这里的视图必须通过 -dequeueReusableSupplementaryViewOfKind:withReuseIdentifier:forIndexPath:方法来检索可重用的视图，且可重用标示符必须和创建UICollectionView时通过registerClass: forSupplementaryViewOfKind:withReuseIdentifier:或者registerNib: forSupplementaryViewOfKind: withReuseIdentifier:方法注册的标示符要一致
- (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath;
UICollectionViewDelegate
- (BOOL)collectionView:(UICollectionView *)collectionView shouldHighlightItemAtIndexPath:(NSIndexPath *)indexPath;
- (void)collectionView:(UICollectionView *)collectionView didHighlightItemAtIndexPath:(NSIndexPath *)indexPath;
- (void)collectionView:(UICollectionView *)collectionView didUnhighlightItemAtIndexPath:(NSIndexPath *)indexPath;
- (BOOL)collectionView:(UICollectionView *)collectionView shouldSelectItemAtIndexPath:(NSIndexPath *)indexPath;
- (BOOL)collectionView:(UICollectionView *)collectionView shouldDeselectItemAtIndexPath:(NSIndexPath *)indexPath; // called when the user taps on an already-selected item in multi-select mode
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath;
- (void)collectionView:(UICollectionView *)collectionView didDeselectItemAtIndexPath:(NSIndexPath *)indexPath;
- (void)collectionView:(UICollectionView *)collectionView willDisplayCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath NS_AVAILABLE_IOS(8_0);
- (void)collectionView:(UICollectionView *)collectionView willDisplaySupplementaryView:(UICollectionReusableView *)view forElementKind:(NSString *)elementKind atIndexPath:(NSIndexPath *)indexPath NS_AVAILABLE_IOS(8_0);
- (void)collectionView:(UICollectionView *)collectionView didEndDisplayingCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath;
- (void)collectionView:(UICollectionView *)collectionView didEndDisplayingSupplementaryView:(UICollectionReusableView *)view forElementOfKind:(NSString *)elementKind atIndexPath:(NSIndexPath *)indexPath;
// These methods provide support for copy/paste actions on cells.
// All three should be implemented if any are.
- (BOOL)collectionView:(UICollectionView *)collectionView shouldShowMenuForItemAtIndexPath:(NSIndexPath *)indexPath;
- (BOOL)collectionView:(UICollectionView *)collectionView canPerformAction:(SEL)action forItemAtIndexPath:(NSIndexPath *)indexPath withSender:(id)sender;
- (void)collectionView:(UICollectionView *)collectionView performAction:(SEL)action forItemAtIndexPath:(NSIndexPath *)indexPath withSender:(id)sender;
// support for custom transition layout
- (UICollectionViewTransitionLayout *)collectionView:(UICollectionView *)collectionView transitionLayoutForOldLayout:(UICollectionViewLayout *)fromLayout newLayout:(UICollectionViewLayout *)toLayout;
此外UICollectionView还能通过以下方法实现简单的修改
//滚动到某分组的某单元
- (void)scrollToItemAtIndexPath:(NSIndexPath *)indexPath atScrollPosition:(UICollectionViewScrollPosition)scrollPosition animated:(BOOL)animated;
// These methods allow dynamic modification of the current set of items in the collection view
//插入某些组
- (void)insertSections:(NSIndexSet *)sections;
//删除某些组
- (void)deleteSections:(NSIndexSet *)sections;
//重新加载某些组
- (void)reloadSections:(NSIndexSet *)sections;
//移动组
- (void)moveSection:(NSInteger)section toSection:(NSInteger)newSection;
//添加一些单元格
- (void)insertItemsAtIndexPaths:(NSArray *)indexPaths;
//删除一些单元格
- (void)deleteItemsAtIndexPaths:(NSArray *)indexPaths;
//创新加载一些单元格
- (void)reloadItemsAtIndexPaths:(NSArray *)indexPaths;
//移动单元格
- (void)moveItemAtIndexPath:(NSIndexPath *)indexPath toIndexPath:(NSIndexPath *)newIndexPath;
