UICollectionViewFlowLayout懒加载

- (UICollectionViewFlowLayout *)flowLayout {
    if (!_flowLayout) {
        _flowLayout = [[UICollectionViewFlowLayout alloc] init];
        //每个item大小
        _flowLayout.itemSize = CGSizeMake(165, 250);
        //间距
//        CGFloat spacing = (kScreenWidth - 300) / 4.0;
        //最小行间距
        _flowLayout.minimumLineSpacing = 15;
        //最小列间距
        _flowLayout.minimumInteritemSpacing = 30;
        //上下左右四个边缘的间距
        //每个区的上下左右
        _flowLayout.sectionInset = UIEdgeInsetsMake(15, 15, 15, 15);
        
    }
    return _flowLayout;
}

UICollectionView懒加载

- (UICollectionView *)collectionView {
    if (!_collectionView) {
        _collectionView = [[UICollectionView alloc] initWithFrame:self.view.bounds collectionViewLayout:self.flowLayout];
        _collectionView.backgroundColor = [UIColor clearColor];
        //代理
        _collectionView.dataSource = self;
        _collectionView.delegate = self;
    }
    return _collectionView;
}

注册item

    //注册item
    [self.collectionView registerClass:[customCell class] forCellWithReuseIdentifier:@"Cell"];

代理方法
签订协议<UICollectionViewDataSource, UICollectionViewDelegate>

//item个数
- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
    return self.dataSource.count;
}
//分区数
- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView {
    return 2;
}
//item
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    customCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
    cell.backgroundColor = [UIColor yellowColor];
    //获取cell对应的图片
    NSDictionary *URLDic = self.dataSource[indexPath.item];
    //获取字典内的图片网址
    NSString *imageURL = URLDic[@"thumbURL"];
    NSString *imageWidth = [NSString stringWithFormat:@"%@", URLDic[@"width"]];
    cell.widthLabel.text = imageWidth;
    //居中
    cell.widthLabel.textAlignment = NSTextAlignmentCenter;
    NSURL *imageurl = [NSURL URLWithString:imageURL];
    //获取图片
    [cell.imageView sd_setImageWithURL:imageurl placeholderImage:[UIImage imageNamed:@"placeholder"]];
    //图片失真解决方法
    cell.imageView.contentMode = UIViewContentModeScaleAspectFit;
    return cell;
}
//item点击方法
- (void)collectionView:(UICollectionView *)collectionView didSelectItemAtIndexPath:(NSIndexPath *)indexPath {
        NSLog(@"点击");
}

注册分区头和分区尾

    //注册分区头
    [self.collectionView registerClass:[UICollectionViewCell class] forSupplementaryViewOfKind:UICollectionElementKindSectionHeader withReuseIdentifier:@"header"];
    //注册分区尾
    [self.collectionView registerClass:[UICollectionViewCell class] forSupplementaryViewOfKind:UICollectionElementKindSectionFooter withReuseIdentifier:@"footer"];

分区头分区尾代理方法

//分区尾大小
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout referenceSizeForFooterInSection:(NSInteger)section {
    return CGSizeMake(0, 30);
}
//分区头大小
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout referenceSizeForHeaderInSection:(NSInteger)section {
    return CGSizeMake(0, 30);
}
//分区头部尾部协议方法
//分区头部尾部共用一个协议方法
- (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath {
    if ([kind isEqualToString:UICollectionElementKindSectionHeader]) {
        //获取分区头是从重用队列去取的，而不是alloc init 的的
        UICollectionReusableView *headerView = [collectionView dequeueReusableSupplementaryViewOfKind:UICollectionElementKindSectionHeader withReuseIdentifier:@"header" forIndexPath:indexPath];
        headerView.backgroundColor = [UIColor lightGrayColor];
        UILabel *headerLabel = [[UILabel  alloc] init];
        headerLabel.frame = CGRectMake(0, 0, 150, 30);
        headerLabel.text = @"header--美女";
        headerLabel.backgroundColor = [UIColor redColor];
        [headerView addSubview:headerLabel];
        
        return headerView;
    }else {
        UICollectionReusableView *footerView = [collectionView dequeueReusableSupplementaryViewOfKind:UICollectionElementKindSectionFooter withReuseIdentifier:@"footer" forIndexPath:indexPath];
        footerView.backgroundColor = [UIColor lightGrayColor];
        UILabel *footerLabel = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 150, 30)];
        footerLabel.text = @"footer--漂亮";
        footerLabel.backgroundColor = [UIColor greenColor];
        [footerView addSubview:footerLabel];
        
        return footerView;
    }
    
}