`setCollectionViewLayout` 动画在更改集合视图框架时被破坏

Question

我正在尝试在水平胶片布局和垂直堆栈布局之间转换我的收藏视图。

水平膜条	垂直/扩展堆栈

这就是我目前正在做的事情：

1. 使用setCollectionViewLayout(_:animated:completion:)水平和垂直滚动方向之间的过渡
2. 更改集合视图的高度约束，随着UIView.animate和self.view.layoutIfNeeded()

从胶片到垂直堆栈的动画很好。但是，从垂直堆栈到胶片的动画被破坏了。这是它的样子：

如果我collectionViewHeightConstraint默认设置为 300，并删除这些行：

collectionViewHeightConstraint.constant = 300
collectionViewHeightConstraint.constant = 50

...过渡动画两种方式都很好。但是，有多余的间距，我希望胶片布局仅在 1 行中。

如何使动画双向流畅？这是我的代码（链接到演示项目）：

class ViewController: UIViewController {

    var isExpanded = false
    var verticalFlowLayout = UICollectionViewFlowLayout()
    var horizontalFlowLayout = UICollectionViewFlowLayout()
    
    @IBOutlet weak var collectionView: UICollectionView!
    @IBOutlet weak var collectionViewHeightConstraint: NSLayoutConstraint!
    @IBAction func toggleExpandPressed(_ sender: Any) {
        
        isExpanded.toggle()
        if isExpanded {
            collectionView.setCollectionViewLayout(verticalFlowLayout, animated: true) /// set vertical scroll
            collectionViewHeightConstraint.constant = 300 /// make collection view height taller
        } else {
            collectionView.setCollectionViewLayout(horizontalFlowLayout, animated: true) /// set horizontal scroll
            collectionViewHeightConstraint.constant = 50 /// make collection view height shorter
        }
        
        /// animate the collection view's height
        UIView.animate(withDuration: 1) {
            self.view.layoutIfNeeded()
        }
        
        /// Bonus points:
        /// This makes the animation way more worse, but I would like to be able to scroll to a specific IndexPath during the transition.
        //  collectionView.scrollToItem(at: IndexPath(item: 9, section: 0), at: .bottom, animated: true)
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        verticalFlowLayout.scrollDirection = .vertical
        horizontalFlowLayout.scrollDirection = .horizontal
        
        collectionView.collectionViewLayout = horizontalFlowLayout
        collectionView.dataSource = self
        collectionView.delegate = self
    }
}

extension ViewController: UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        
        /// if expanded, cells should be full-width
        /// if not expanded, cells should have a width of 50
        return isExpanded ? CGSize(width: collectionView.frame.width, height: 50) : CGSize(width: 100, height: 50)
    }
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
        return 0 /// no spacing needed for now
    }
}

/// sample data source
extension ViewController: UICollectionViewDataSource {
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 10
    }
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "ID", for: indexPath)
        cell.contentView.layer.borderWidth = 5
        cell.contentView.layer.borderColor = UIColor.red.cgColor
        return cell
    }
}

Answer 1

UICollectionViewFlowLayout我通过使用自定义类（链接到演示存储库）使其工作。中心单元甚至在两种布局中保持居中（这实际上是我问题中“奖励积分”部分的确切目的）！

这是我的视图控制器。UICollectionViewDelegateFlowLayout我现在使用自定义闭包sizeForListItemAt和，而不是遵守sizeForStripItemAt。

class ViewController: UIViewController {

    var isExpanded = false
    lazy var listLayout = FlowLayout(layoutType: .list)
    lazy var stripLayout = FlowLayout(layoutType: .strip)
    
    @IBOutlet weak var collectionView: UICollectionView!
    @IBOutlet weak var collectionViewHeightConstraint: NSLayoutConstraint!
    @IBAction func toggleExpandPressed(_ sender: Any) {
        
        isExpanded.toggle()
        if isExpanded {
            collectionView.setCollectionViewLayout(listLayout, animated: true)
            collectionViewHeightConstraint.constant = 300
        } else {
            collectionView.setCollectionViewLayout(stripLayout, animated: true)
            collectionViewHeightConstraint.constant = 60
        }
        UIView.animate(withDuration: 0.6) {
            self.view.layoutIfNeeded()
        }
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        collectionView.collectionViewLayout = stripLayout
        collectionView.dataSource = self
        
        /// use these instead of `UICollectionViewDelegateFlowLayout`
        listLayout.sizeForListItemAt = { [weak self] indexPath in
            return CGSize(width: self?.collectionView.frame.width ?? 100, height: 50)
        }
        stripLayout.sizeForStripItemAt = { indexPath in
            return CGSize(width: 100, height: 50)
        }
    }
}

/// sample data source
extension ViewController: UICollectionViewDataSource {
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 10
    }
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "ID", for: indexPath)
        cell.contentView.layer.borderWidth = 5
        cell.contentView.layer.borderColor = UIColor.red.cgColor
        return cell
    }
}

然后，这是我的自定义UICollectionViewFlowLayout课程。在里面prepare，我手动计算并设置每个单元格的框架。我不太确定为什么会这样，但系统现在能够找出哪些单元格与哪些单元格相等，甚至跨多个FlowLayouts （listLayout和stripLayout.）。

enum LayoutType {
    case list
    case strip
}

class FlowLayout: UICollectionViewFlowLayout {

    var layoutType: LayoutType
    var sizeForListItemAt: ((IndexPath) -> CGSize)? /// get size for list item
    var sizeForStripItemAt: ((IndexPath) -> CGSize)? /// get size for strip item
    
    var layoutAttributes = [UICollectionViewLayoutAttributes]() /// store the frame of each item
    var contentSize = CGSize.zero /// the scrollable content size of the collection view
    override var collectionViewContentSize: CGSize { return contentSize } /// pass scrollable content size back to the collection view
    
    override func prepare() { /// configure the cells' frames
        super.prepare()
        
        guard let collectionView = collectionView else { return }
        let itemCount = collectionView.numberOfItems(inSection: 0) /// I only have 1 section
        
        if layoutType == .list {
            var y: CGFloat = 0 /// y position of each cell, start at 0
            for itemIndex in 0..<itemCount {
                let indexPath = IndexPath(item: itemIndex, section: 0)
                let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
                attributes.frame = CGRect(
                    x: 0,
                    y: y,
                    width: sizeForListItemAt?(indexPath).width ?? 0,
                    height: sizeForListItemAt?(indexPath).height ?? 0
                )
                layoutAttributes.append(attributes)
                y += attributes.frame.height /// add height to y position, so next cell becomes offset
            }                           /// use first item's width
            contentSize = CGSize(width: sizeForStripItemAt?(IndexPath(item: 0, section: 0)).width ?? 0, height: y)
        } else {
            var x: CGFloat = 0 /// z position of each cell, start at 0
            for itemIndex in 0..<itemCount {
                let indexPath = IndexPath(item: itemIndex, section: 0)
                let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
                attributes.frame = CGRect(
                    x: x,
                    y: 0,
                    width: sizeForStripItemAt?(indexPath).width ?? 0,
                    height: sizeForStripItemAt?(indexPath).height ?? 0
                )
                layoutAttributes.append(attributes)
                x += attributes.frame.width /// add width to z position, so next cell becomes offset
            }                              /// use first item's height
            contentSize = CGSize(width: x, height: sizeForStripItemAt?(IndexPath(item: 0, section: 0)).height ?? 0)
        }
    }
    
    /// pass attributes to the collection view flow layout
    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return layoutAttributes[indexPath.item]
    }
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        return layoutAttributes.filter { rect.intersects($0.frame) }
    }

    /// initialize with a layout
    init(layoutType: LayoutType) {
        self.layoutType = layoutType
        super.init()
    }
    
    /// boilerplate code
    required init?(coder aDecoder: NSCoder) { fatalError("init(coder:) has not been implemented") }
    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool { return true }
    override func invalidationContext(forBoundsChange newBounds: CGRect) -> UICollectionViewLayoutInvalidationContext {
        let context = super.invalidationContext(forBoundsChange: newBounds) as! UICollectionViewFlowLayoutInvalidationContext
        context.invalidateFlowLayoutDelegateMetrics = newBounds.size != collectionView?.bounds.size
        return context
    }
}

感谢这篇精彩的文章提供的巨大帮助。