UICollectionView无法自动滚动

如何解决UICollectionView无法自动滚动

因此，我有一个用于自动滚动UICollectionView的代码（水平启用分页）。它正在工作。现在，我更新了Xcode和iOS，突然之间，相同的代码不再起作用。我真的没有改变其他任何东西。还有其他人也遇到过这个问题吗？ 该集合视图位于标头可重用视图的内部。

func setTimer() {
        let _ = Timer.scheduledTimer(timeInterval: 3.0,target: self,selector: #selector(self.autoScroll),userInfo: nil,repeats: true)
    }
    
    @objc func autoScroll() {
        print("x is",self.x,self.allBanners.count)
        if self.x < self.allBanners.count {
            self.x = self.x + 1
        } else {
            self.x = 1
        }
//        DispatchQueue.main.async {
        self.resuableView.featuredCollectionView?.scrollToItem(at: IndexPath(item: self.x,section: 0),at: .centeredHorizontally,animated: true)
            self.resuableView.featuredCollectionView?.setNeedsLayout()
//        }
    }
    
    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        let xPosition = scrollView.contentOffset.x
        let w = scrollView.bounds.size.width
        self.x = Int(ceil(xPosition/w))
        print("dec",self.x)
    }

如果我打印x的值，它将正确打印。也就是说，每隔3秒，它会增加1，但是，scrollToItem无法正常工作。 另外，如果我禁用分页，那么它也可以工作，但是，我确实确实需要分页。 即使我手动将集合视图滚动到任何项目，只要调用 @objc func autoScroll（）函数，它都会将其还原回第一项。

有人知道为什么会这样吗？ 附言我已经尝试过不带异步，带异步，不带setNeedsLayout的情况。 我有Xcode版本12.0.1（12A7300）

解决方法

我注意到了一些怪癖，尤其是在iOS 14上...

如果单元格中断自动布局，scrollToItemAt以及手动滚动都会失败。

这是一个可行的完整示例：

---

简单单元格类

class MyTestCell: UICollectionViewCell {
    var cView: UIView = UIView()
    var theLabel: UILabel = UILabel()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }
    func commonInit() -> Void {

        // stylize the cell a little with a bordered-rounded-corner view inset by 8-pts
        cView.backgroundColor = .cyan
        cView.layer.borderWidth = 1
        cView.layer.borderColor = UIColor.blue.cgColor
        cView.layer.cornerRadius = 8

        cView.translatesAutoresizingMaskIntoConstraints = false
        contentView.addSubview(cView)
        
        theLabel.translatesAutoresizingMaskIntoConstraints = false
        cView.addSubview(theLabel)
        
        let g = contentView.layoutMarginsGuide
        
        NSLayoutConstraint.activate([
            
            cView.topAnchor.constraint(equalTo: g.topAnchor,constant: 0.0),cView.leadingAnchor.constraint(equalTo: g.leadingAnchor,cView.trailingAnchor.constraint(equalTo: g.trailingAnchor,cView.bottomAnchor.constraint(equalTo: g.bottomAnchor,theLabel.topAnchor.constraint(equalTo: cView.topAnchor,constant: 8.0),theLabel.leadingAnchor.constraint(equalTo: cView.leadingAnchor,theLabel.trailingAnchor.constraint(equalTo: cView.trailingAnchor,constant: -8.0),theLabel.bottomAnchor.constraint(equalTo: cView.bottomAnchor,])
        
        theLabel.textAlignment = .center
        theLabel.numberOfLines = 0
    }
}

---

视图控制器类

class AutoScrollViewController: UIViewController {
    
    var collectionView: UICollectionView!

    var myData: [String] = []
    
    var x: Int = 0

    override func viewDidLoad() {
        super.viewDidLoad()
        
        // create 30 strings
        myData = (0..<30).map { "Cell \($0)" }

        // couple cells with longer text for variety
        myData[3] = "More text in cell 3"
        myData[7] = "The text in cell 7 is long enough to need to wrap onto multiple lines."

        // create a flow layout
        let f = setupFlowLayout()
        
        // create the collection view
        collectionView = UICollectionView(frame: .zero,collectionViewLayout: f)
        
        // we want to use paging
        collectionView.isPagingEnabled = true
        
        // register the cell
        collectionView.register(MyTestCell.self,forCellWithReuseIdentifier: "cell")
        
        // data source and delgate
        collectionView.dataSource = self
        collectionView.delegate = self
        
        // add a "Tap instruction" label
        let instLabel = UILabel()
        instLabel.numberOfLines = 0
        instLabel.textAlignment = .center
        instLabel.text = "Cells will auto-advance\nevery 3 seconds.\n\nYou can also Tap here\nto jump to Cell 23"
        instLabel.translatesAutoresizingMaskIntoConstraints = false

        // add UI elements
        view.addSubview(collectionView)
        view.addSubview(instLabel)

        collectionView.translatesAutoresizingMaskIntoConstraints = false
        instLabel.translatesAutoresizingMaskIntoConstraints = false
            
        // respect safe area
        let g = view.safeAreaLayoutGuide
        
        NSLayoutConstraint.activate([
            
            // constrain collection view Top / Leading / Trailing + 40-pts
            collectionView.topAnchor.constraint(equalTo: g.topAnchor,constant: 40.0),collectionView.leadingAnchor.constraint(equalTo: g.leadingAnchor,collectionView.trailingAnchor.constraint(equalTo: g.trailingAnchor,constant: -40.0),// collection view Height: 160
            collectionView.heightAnchor.constraint(equalToConstant: 160.0),// constrain instruction label below and centered to the collection view
            instLabel.topAnchor.constraint(equalTo: collectionView.bottomAnchor,instLabel.centerXAnchor.constraint(equalTo: collectionView.centerXAnchor),instLabel.widthAnchor.constraint(equalTo: collectionView.widthAnchor),])
        
        // for testing,add a tap recognizer to the view
        let t = UITapGestureRecognizer(target: self,action: #selector(self.tapScroll))
        view.addGestureRecognizer(t)

    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        // set collection view item size here
        if let f = collectionView.collectionViewLayout as? UICollectionViewFlowLayout {
            f.itemSize = CGSize(width: collectionView.frame.width,height: collectionView.frame.height)
        }
        setTimer()
    }

    func setTimer() {
        let _ = Timer.scheduledTimer(timeInterval: 3.0,target: self,selector: #selector(self.autoScroll),userInfo: nil,repeats: true)
    }
    
    @objc func tapScroll() {
        self.x = 23
        collectionView.scrollToItem(at: IndexPath(item: self.x,section: 0),at: .left,animated: true)
    }
    
    @objc func autoScroll() {
        print("x is",self.x,"Cell Count is",self.myData.count)
        self.x = self.x + 1
        if self.x >= self.myData.count {
            self.x = 0
        }
        collectionView.scrollToItem(at: IndexPath(item: self.x,animated: true)
    }

}

---

具有数据源和代理实施的扩展

extension AutoScrollViewController: UICollectionViewDataSource,UICollectionViewDelegate {
    
    private func setupFlowLayout() -> UICollectionViewFlowLayout {
        let flowLayout = UICollectionViewFlowLayout()
        flowLayout.scrollDirection = .horizontal
        flowLayout.sectionInset = .zero
        flowLayout.minimumLineSpacing = 0
        flowLayout.minimumInteritemSpacing = 1
        return flowLayout
    }
    
    func collectionView(_ collectionView: UICollectionView,numberOfItemsInSection section: Int) -> Int {
        return myData.count
    }
    
    func collectionView(_ collectionView: UICollectionView,cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let c = collectionView.dequeueReusableCell(withReuseIdentifier: "cell",for: indexPath) as! MyTestCell
        // alternate cell background color so we can see it
        c.contentView.backgroundColor = indexPath.item % 2 == 0 ? .orange : .yellow
        c.theLabel.text = myData[indexPath.item]
        return c
    }

    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
        let xPosition = scrollView.contentOffset.x
        let w = scrollView.bounds.size.width
        self.x = Int(ceil(xPosition/w))
        print("dec",xPosition,w,self.x)
    }

}

---

没有@IBOutlet或@IBAction连接，因此只需为UIViewController的自定义类分配默认的AutoScrollViewController（根据上面的代码即可）。