我正在尝试为圆形的自定义单元格实现 UICollectionView。现在默认情况下，圆圈的对齐方式与普通方形单元格相同:顶部圆圈和底部圆圈位于同一垂直线上。我怎样才能改变对齐方式:顶部圆和它下面的两个圆形成一个等边三角形(顶部圆和底部圆的位置按半径长度移动)？如下:

from OOO
     OOO
     OOO

to   O O O
    O O O (no spacing among the circles)
     O O O

最佳答案

基本思想是创建一个自定义的 UICollectionViewLayout 来实现:

• collectionViewContentSize，即 Collection View 的完整可滚动 contentSize 的大小；

• layoutAttributesForItem(at indexPath:)，即特定的关键属性(即 center 和 size)是什么细胞;和

• layoutAttributesForElements(in rect:)，即属于这个特定 rect 的单元格的关键属性是什么......这将用于识别在任何给定时间点哪些单元格是可见的，以及这些单元格的属性；这基本上是前一个方法中单元格属性的数组，过滤后仅包含 rect 中的单元格。

因此，在 Swift 3 中你可以这样做:

class AlternatingGridLayout: UICollectionViewLayout {

    private var itemSize: CGSize!
    private var numberOfItems: Int!
    private var itemsPerRow: Int!
    private var rows: Int!
    private var circleViewCenterOffset: CGPoint!
    private var radiusOfCircleViews: CGFloat!
    private var insets: UIEdgeInsets!

    override func prepare() {
        super.prepare()

        guard let collectionView = collectionView else { return }

        radiusOfCircleViews = CGFloat(40.0)
        itemSize = CGSize(width: radiusOfCircleViews * 2, height: radiusOfCircleViews * 2)
        circleViewCenterOffset = CGPoint(x: 2 * radiusOfCircleViews * cos(.pi / 3),
                                         y: 2 * radiusOfCircleViews * sin(.pi / 3))
        numberOfItems = collectionView.numberOfItems(inSection: 0)
        itemsPerRow = Int(floor((collectionView.bounds.width - radiusOfCircleViews) / CGFloat(2 * radiusOfCircleViews)) + 0.5)
        rows = (numberOfItems - 1) / itemsPerRow + 1
        let excess = collectionView.bounds.width - (CGFloat(itemsPerRow) * radiusOfCircleViews * 2 + circleViewCenterOffset.x)
        insets = UIEdgeInsets(top: 10, left: excess / 2, bottom: 10, right: excess / 2)
    }

    override var collectionViewContentSize: CGSize {
        return CGSize(width: collectionView!.bounds.width,
                      height: 2 * radiusOfCircleViews + CGFloat(rows - 1) * circleViewCenterOffset.y + insets.top + insets.bottom)
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)

        attributes.center = centerForItem(at: indexPath)
        attributes.size = itemSize

        return attributes
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        return (0 ..< collectionView!.numberOfItems(inSection: 0)).map { IndexPath(item: $0, section: 0) }
            .filter { rect.intersects(rectForItem(at: $0)) }
            .compactMap { self.layoutAttributesForItem(at: $0) }  // `flatMap` in Xcode versions before 9.3
    }

    private func centerForItem(at indexPath: IndexPath) -> CGPoint {
        let row = indexPath.item / itemsPerRow
        let col = indexPath.item - row * itemsPerRow

        var x: CGFloat = radiusOfCircleViews + CGFloat(col) * (radiusOfCircleViews * 2)
        let y: CGFloat = radiusOfCircleViews + CGFloat(row) * (circleViewCenterOffset.y)

        if row % 2 == 0 {
            x += circleViewCenterOffset.x
        }

        return CGPoint(x: x + insets.left, y: y + insets.top)
    }

    private func rectForItem(at indexPath: IndexPath) -> CGRect {
        let center = centerForItem(at: indexPath)

        return CGRect(x: center.x - radiusOfCircleViews, y: center.y - radiusOfCircleViews, width: radiusOfCircleViews * 2, height: radiusOfCircleViews * 2)
    }        
}

产生:

很明显，您可以根据需要自定义它，但它说明了基本思想。

在下面我的原始回答中，我假设您希望看到这些单元格呈圆形，如 WWDC 2012 视频 Advanced Collection Views and Building Custom Layouts 中所示。 (视频大约 40 多分钟)。见下文。

例如，在 Swift 3 中:

class CircleLayout: UICollectionViewLayout {

    private var center: CGPoint!
    private var itemSize: CGSize!
    private var radius: CGFloat!
    private var numberOfItems: Int!

    override func prepare() {
        super.prepare()

        guard let collectionView = collectionView else { return }

        center = CGPoint(x: collectionView.bounds.midX, y: collectionView.bounds.midY)
        let shortestAxisLength = min(collectionView.bounds.width, collectionView.bounds.height)
        itemSize = CGSize(width: shortestAxisLength * 0.1, height: shortestAxisLength * 0.1)
        radius = shortestAxisLength * 0.4
        numberOfItems = collectionView.numberOfItems(inSection: 0)
    }

    override var collectionViewContentSize: CGSize {
        return collectionView!.bounds.size
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)

        let angle = 2 * .pi * CGFloat(indexPath.item) / CGFloat(numberOfItems)

        attributes.center = CGPoint(x: center.x + radius * cos(angle), y: center.y + radius * sin(angle))
        attributes.size = itemSize

        return attributes
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        return (0 ..< collectionView!.numberOfItems(inSection: 0))
            .compactMap { item -> UICollectionViewLayoutAttributes? in    // `flatMap` in Xcode versions prior to 9.3
                self.layoutAttributesForItem(at: IndexPath(item: item, section: 0))
        }
    }
}

然后您可以简单地设置collectionViewLayout，然后实现标准的UICollectionViewDataSource方法。

class ViewController: UICollectionViewController {

    var numberOfCells = 10

    override func viewDidLoad() {
        super.viewDidLoad()

        collectionView?.collectionViewLayout = CircleLayout()

        // just for giggles and grins, let's show the insertion of a cell

        DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
            self.collectionView?.performBatchUpdates({
                self.numberOfCells += 1
                self.collectionView?.insertItems(at: [IndexPath(item: 0, section: 0)])
            }, completion: nil)
        }
    }

}

// MARK: UICollectionViewDataSource

extension ViewController {
    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return numberOfCells
    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "CircleCell", for: indexPath)
        return cell
    }
}

产生:

参见 https://github.com/robertmryan/CircularCollectionView sample 。

请注意，您提到您想要“圆圈之间没有间距”，因此只需相应地调整 radius 和/或 itemSize 以获得您想要的布局。

关于iOS UICollectionView : Cells with circular view in alternating grid alignment，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/41837170/