如何在UICollectionView中添加Header？

这里我的代码没有故事板

class ViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {

    override func viewDidLoad() {
        super.viewDidLoad()
        let flowLayout = UICollectionViewFlowLayout()
        flowLayout.itemSize = CGSize(width: 100, height: 100)
        flowLayout.scrollDirection = .vertical
        let collection = UICollectionView(frame: CGRect(x: 0, y: 0, width: self.view.frame.size.width, height: self.view.frame.size.height), collectionViewLayout: flowLayout)
        collection.backgroundColor = .white
        collection.delegate = self
        collection.dataSource = self

        self.view.addSubview(collection)

        collection.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "one")
        collection.register(UICollectionReusableView.self, forSupplementaryViewOfKind: UICollectionElementKindSectionHeader, withReuseIdentifier: "Header")
        collection.register(UICollectionReusableView.self, forSupplementaryViewOfKind: UICollectionElementKindSectionFooter, withReuseIdentifier: "Footer")
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        return CGSize(width: 100, height: 100)
    }
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> CGSize {
        return CGSize(width: self.view.frame.size.width, height: 50)
    }
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForFooterInSection section: Int) -> CGSize {
        return CGSize(width: self.view.frame.size.width, height: 50)
    }
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 2
    }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 10
    }

    func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
        switch kind {
        case UICollectionElementKindSectionHeader:
            let headerView = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: "Header", for: indexPath)

            headerView.backgroundColor = UIColor.blue
            return headerView

        case UICollectionElementKindSectionFooter:
            let footerView = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: "Footer", for: indexPath)
            footerView.backgroundColor = UIColor.green
            return footerView

        default:

            assert(false, "Unexpected element kind")
        }
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "one", for: indexPath)
        let title = UILabel(frame: CGRect(x: 0, y: 0, width: cell.bounds.size.width, height: 40))
        title.text = String(indexPath.row)
        cell.contentView.backgroundColor = .gray
        cell.contentView.addSubview(title)
        return cell
    }
}