iOS UICollectionView：具有交替网格对齐的圆形视图的单元格

Question

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。如何将对齐方式更改为：顶部圆圈和其下方的两个圆圈形成等边三角形（顶部圆圈和底部圆圈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

Answer 1

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

• collectionViewContentSize，即集合视图的完整可滚动contentSize 的大小；

• layoutAttributesForItem(at indexPath:)，即特定单元格的关键属性（即center和size）是什么；和

• layoutAttributesForElements(in rect:)，即属于该特定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可见的单元格以及这些单元格的属性；这基本上是上一个方法中单元格的属性数组，过滤到仅在 rect 中的那些。

因此，在 Swift 3 中，您可以执行以下操作：

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：

显然，根据您的需要自定义它，但它说明了基本思想。

---

在下面的原始答案中，我假设您想在一个圆圈中看到这些单元格，如 WWDC 2012 视频 Advanced Collection Views and Building Custom Layouts 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：

---

有关示例，请参阅https://github.com/robertmryan/CircularCollectionView。

---

注意，您提到您希望“圆圈之间没有间距”，因此只需相应地调整 radius 和/或 itemSize 以获得所需的布局。