给collectionView每个section加背景

先看效果图，这里放上淘宝上的例子：

WechatIMG209.jpeg

大概就是这样的东西，collection的每个section都是一个整体（这里也可以强行把section头试图和cell拼起来，当然我觉得挺麻烦的所以这里说的是另一种方法）
我们下面说的是另一种方式：使用重新Collection layout的方式来试下。

1. 首先自定义layout类继承于 UICollectionViewFlowLayout
2. 重写 - (void)prepareLayout；- (NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect;
   具体代码如下(懒人模式开启)：

/**
 自定义section背景view 注意继承于UICollectionReusableView
 */
@interface KFZShopCatorySectionWhiteBgView : UICollectionReusableView

@end

@implementation KFZShopCatorySectionWhiteBgView

- (void)applyLayoutAttributes:(UICollectionViewLayoutAttributes *)layoutAttributes{
    [super applyLayoutAttributes:layoutAttributes];
    
    self.backgroundColor = [UIColor whiteColor];
    self.layer.cornerRadius = 8;
    self.layer.masksToBounds = YES;
}

@end

//下面是自定义的layout
@interface KFZShopCatoryFlowLayput  ()
//存放新的layouttAttributes
@property (nonatomic, strong) NSMutableArray<UICollectionViewLayoutAttributes *> *decorationViewAttrs;

@end

@implementation KFZShopCatoryFlowLayput

- (void)prepareLayout{
    [super prepareLayout];
    
    NSInteger sections = [self.collectionView numberOfSections];
    if (sections == 0) {//没有内容直接返回
        return;
    }
    
    id<UICollectionViewDelegateFlowLayout> delegate  = (id<UICollectionViewDelegateFlowLayout>) self.collectionView.delegate;
    
    //1.初始化 注册背景view类
    [self registerClass:[KFZShopCatorySectionWhiteBgView class] forDecorationViewOfKind:@"KFZShopCatorySectionWhiteBgView"];
    [self.decorationViewAttrs removeAllObjects];
    
    
    
    for (NSInteger section = 0; section < sections; section++) {
       NSInteger numberOfItems = [self.collectionView numberOfItemsInSection:section];
            if (numberOfItems > 0) {
                UICollectionViewLayoutAttributes *firstAttr = [self layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:section]];
                UICollectionViewLayoutAttributes *lastAttr = [self layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForRow:(numberOfItems - 1) inSection:section]];
                
                UIEdgeInsets sectionInset = self.sectionInset;
                if ([delegate respondsToSelector:@selector(collectionView:layout:insetForSectionAtIndex:)]) {
                    UIEdgeInsets inset = [delegate collectionView:self.collectionView layout:self insetForSectionAtIndex:section];
                    if (!UIEdgeInsetsEqualToEdgeInsets(inset, sectionInset)) {
                        sectionInset = inset;
                    }
                }
                
                CGSize sectionHeaderSize;
                if ([delegate respondsToSelector:@selector(collectionView:layout:referenceSizeForHeaderInSection:)]) {
                    
                    sectionHeaderSize = [delegate collectionView:self.collectionView layout:self referenceSizeForHeaderInSection:section];
                }
                
                if (section == 0) {
                    sectionHeaderSize.height = 0;
                }
                
                CGRect sectionFrame = CGRectUnion(firstAttr.frame, lastAttr.frame);
                sectionFrame.origin.x -= 15;
                sectionFrame.origin.y -= sectionHeaderSize.height;
                sectionFrame.size.width = sectionFrame.size.width + 30;
                sectionFrame.size.height = sectionFrame.size.height + sectionHeaderSize.height + 15;
                
                //2. 定义
                UICollectionViewLayoutAttributes *attr = [UICollectionViewLayoutAttributes layoutAttributesForDecorationViewOfKind:@"KFZShopCatorySectionWhiteBgView" withIndexPath:[NSIndexPath indexPathForRow:0 inSection:section]];
                attr.frame = sectionFrame;
                attr.zIndex = -1;
                [self.decorationViewAttrs addObject:attr];
            }else{
                continue ;
            }
    }
    
}

- (NSArray<UICollectionViewLayoutAttributes *> *)layoutAttributesForElementsInRect:(CGRect)rect{
    
    NSMutableArray * attrs = [[super layoutAttributesForElementsInRect:rect] mutableCopy];
    for (UICollectionViewLayoutAttributes *attr in self.decorationViewAttrs) {
        if (CGRectIntersectsRect(rect, attr.frame)) {
            [attrs addObject:attr];
        }
    }
    return [attrs copy];
}

- (NSMutableArray<UICollectionViewLayoutAttributes *> *)decorationViewAttrs{
    if (!_decorationViewAttrs) {
        _decorationViewAttrs = [NSMutableArray array];
    }
    return _decorationViewAttrs;
}

@end