Swift iOS-当没有'；CollectionView中没有任何单元格吗？

仅供参考，当用户向上滑动以查看更多单元格时，此问题用于手动滚动

我紧随其后，做出了一个黏糊糊的头球

当CollectionView中有足够的单元格填满整个屏幕（假设有50个或更多单元格）时，一切正常，粘性标题会滚动到导航栏下方的屏幕顶部

问题在于，如果CollectionView中没有任何单元格，或者单元格数量小于屏幕高度（即1或2个单元格），则粘性标题将不会滚动到导航栏下方的屏幕顶部。它保持不动。

如果导航栏中没有任何单元格，或者如果单元格数量小于屏幕高度，如何使粘性标题滚动到导航栏下方的场景顶部？

代码如下：

这适用于粘性标题上方的白色单元格，当粘性标题滚动到顶部时，该单元格将滚动到屏幕外：

class WhiteCellAboveStickyHeader: UICollectionViewCell{

    override init(frame: CGRect) {
        super.init(frame: frame)
        backgroundColor = .white

        // height is configured to 200 inside the collectionView's sizeForItemAt IndexPath
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

这适用于粘滞标题下方的单元格，当有足够的单元格时，粘滞标题将滚动至屏幕顶部：

class BlueCellBelowStickyHeader: UICollectionViewCell{

    override init(frame: CGRect) {
        super.init(frame: frame)
        backgroundColor = .blue

        // height is configured to 50 inside the collectionView's sizeForItemAt IndexPath
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

这是粘性标题的标题视图：

class StickyHeader: UICollectionReusableView {

    let header: UIView = {
        let header = UIView()
        header.translatesAutoresizingMaskIntoConstraints = false
        return header
    }()

    override init(frame: CGRect) {
        super.init(frame: frame)

        backgroundColor = .orange

        addSubview(header)
        header.leftAnchor.constraint(equalTo: self.leftAnchor, constant: 0).isActive = true
        header.rightAnchor.constraint(equalTo: self.rightAnchor, constant: 0).isActive = true
        header.topAnchor.constraint(equalTo: self.topAnchor, constant: 8).isActive = true
        header.bottomAnchor.constraint(equalTo: self.bottomAnchor, constant: 0).isActive = true
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

使用CollectionView初始化：

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {

var collectionView: UICollectionView!
let whiteCellAboveStickyHeader = "WhiteCellAboveStickyHeader"
let blueCellBelowStickyHeader = "BlueCellBelowStickyHeader"
let stickyHeader = "StickyHeader"

override func viewDidLoad() {
    super.viewDidLoad()

    view.backgroundColor = .white
    navigationItem.title = "Nav Bar"

    let layout = UICollectionViewFlowLayout()
    layout.sectionInset = UIEdgeInsetsMake(0, 0, 0, 0)

    collectionView = UICollectionView(frame: CGRect.zero, collectionViewLayout: layout)
    collectionView.translatesAutoresizingMaskIntoConstraints = false
    collectionView.dataSource = self
    collectionView.delegate = self
    collectionView.backgroundColor = .white
    collectionView.alwaysBounceVertical = true
    collectionView.showsVerticalScrollIndicator = false
    collectionView.register(WhiteCellAboveStickyHeader.self, forCellWithReuseIdentifier: whiteCellAboveStickyHeader)
    collectionView.register(BlueCellBelowStickyHeader.self, forCellWithReuseIdentifier: blueCellBelowStickyHeader)
    collectionView.register(StickyHeader.self, forSupplementaryViewOfKind: UICollectionElementKindSectionHeader, withReuseIdentifier: stickyHeader)
    view.addSubview(collectionView)

    collectionView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor).isActive = true
    collectionView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor).isActive = true
    collectionView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor).isActive = true
    collectionView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor).isActive = true

    // These 2 lines are what makes the sticky header sticky
    let stickyHeaderLayout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout
    stickyHeaderLayout?.sectionHeadersPinToVisibleBounds = true
}

func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 2
}

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

        // this is for the white cell above the sticky header
        if section == 0 {
            return 1
        }

        /*
         -this is for the blue cell below the sticky header
         -since there aren't any cells underneath the sticky header it won't scroll to the top of the screen
         -if this number is 50 it will scroll to the top
        */
        return 0
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

        // this is for the white cell above the sticky header
        if indexPath.section == 0 {

            let whiteCell = collectionView.dequeueReusableCell(withReuseIdentifier: whiteCellAboveStickyHeader, for: indexPath) as! WhiteCellAboveStickyHeader
            return whiteCell
        }

        //  this is for the blue cell below the sticky header
        let blueCell = collectionView.dequeueReusableCell(withReuseIdentifier: blueCellBelowStickyHeader, for: indexPath)  as! BlueCellBelowStickyHeader
        return blueCell
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {

    // this is for the white cell above the sticky header
    if indexPath.section == 0{
        return CGSize(width: view.frame.size.width, height: 200)
    }

    // this is for the blue cells underneath the sticky header
    return CGSize(width: view.frame.size.width, height: 50)
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {

    // this is for the cell above the sticky header
    if section == 0{
        return 0
    }

    // this is for the spacing between blue cells underneath the sticky header
    return 10
}

func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView{

        // since only 1 section will have a header in it then only return this

        var header: UICollectionReusableView?

        if kind == UICollectionElementKindSectionHeader{

            header = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: self.stickyHeader, for: indexPath) as! StickyHeader
        }

        return header!
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> CGSize {

        // if section is above sticky header make its height 0 so there won't be a header
        if section == 0 {
            return CGSize(width: 0, height: 0)
        }

        // for section header which is the StickyHeader configure it's height
        return CGSize(width: collectionView.frame.width, height: 100)
}

}

AppDelegate：

class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?


    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

        window = UIWindow(frame: UIScreen.main.bounds)
        window?.makeKeyAndVisible()

        UINavigationBar.appearance().barTintColor = UIColor.lightGray
        UINavigationBar.appearance().tintColor = UIColor.black
        UINavigationBar.appearance().titleTextAttributes = [NSAttributedStringKey.foregroundColor: UIColor.black]

        let navVC = UINavigationController(rootViewController: ViewController())
        window?.rootViewController = navVC

        return true
    }

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