collectionView初始化

collectionView初始化时一定要加layout。不然会报错：

UICollectionView must be initialized with a non-nil layout parameter.

        let layout = UICollectionViewFlowLayout()
        layout.scrollDirection = .horizontal
        layout.minimumInteritemSpacing = 5
        layout.itemSize = CGSize( 80, height: StoreDetailsViewController.headerHeight)

        self.collectionView = UICollectionView(frame: CGRect(x: 0, y: NavigationBarBaseController.barHeight,  self.view.frame.width, height: StoreDetailsViewController.headerHeight), collectionViewLayout: layout)