对于那些寻找答案的人，给你。

let sectionBackgroundDecoration = NSCollectionLayoutDecorationItem.background(
            elementKind: FollowingCollectionViewController.sectionBackgroundDecorationElementKind)
        section.decorationItems = [sectionBackgroundDecoration]

        let layout = UICollectionViewCompositionalLayout(section: section)
        layout.register(
            SectionBackgroundDecorationView.self,
            forDecorationViewOfKind: FollowingCollectionViewController.sectionBackgroundDecorationElementKind)

对于我来说，我想给整个部分添加一个背景视图，除了页眉。我没有在网上找到任何关于这个问题的东西。一个解决方案是给backgroundDecorationView添加contentInsets，除了supplementaryView的高度/宽度：

let itemSize = NSCollectionLayoutSize(
    widthDimension: .fractionalWidth(1),
    heightDimension: .fractionalHeight(1))

let item = NSCollectionLayoutItem(layoutSize: itemSize)

// Group

let groupSize = NSCollectionLayoutSize(
    widthDimension: .fractionalWidth(1),
    heightDimension: .absolute(90))

let group = NSCollectionLayoutGroup.vertical(
    layoutSize: groupSize,
    subitem: item,
    count: 1)

// Header

let headerContentHeight: CGFloat = 15
let headerTopBottomInset: CGFloat = 7

let headerHeight = headerContentHeight+(headerTopBottomInset*2)

let headerSize = NSCollectionLayoutSize(
    widthDimension: .fractionalWidth(1.0),
    heightDimension: .absolute(headerHeight))
let sectionHeader = NSCollectionLayoutBoundarySupplementaryItem(
    layoutSize: headerSize,
    elementKind: SectionHeaderView.identifier,
    alignment: .top)
sectionHeader.contentInsets = NSDirectionalEdgeInsets(top: headerTopBottomInset, leading: 40, bottom: headerTopBottomInset, trailing: 40)

// Background

let backgroundItem = NSCollectionLayoutDecorationItem.background(elementKind: RoundedBGDecoratorView.identifier)

let sectionInsets = NSDirectionalEdgeInsets(top: 10, leading: 20, bottom: 10, trailing: 20)

// we are only adding top insets because background views get drawn starting from the top directly without taking the top inset value in calculations
backgroundItem.contentInsets = NSDirectionalEdgeInsets(
    top: headerContentHeight+headerTopBottomInset+sectionInsets.top,
    leading: sectionInsets.leading,
    bottom: sectionInsets.bottom,
    trailing: sectionInsets.trailing)

let section = NSCollectionLayoutSection(group: group)
section.contentInsets = sectionInsets

section.boundarySupplementaryItems = [sectionHeader]
section.decorationItems = [backgroundItem]