UICollectionView - 在单元格之间画一条线

如何在 UICollectionView 中的单元格之间绘制一条跨越空间的线？ 预期的输出是这样的：

我所做的最好的事情就是在每个单元格内添加行。如何连接穿过空间的线？

我做了一个扩展，你可以像这样使用：

collectionView.drawLineFrom(indexPathA, to: indexPathB, color: UIColor.greenColor())

这是扩展：

extension UICollectionView {

    func drawLineFrom(
        from: NSIndexPath,
        to: NSIndexPath,
        lineWidth: CGFloat = 2,
        strokeColor: UIColor = UIColor.blueColor()
    ) {
        guard
            let fromPoint = cellForItemAtIndexPath(from)?.center,
            let toPoint = cellForItemAtIndexPath(to)?.center
        else {
            return
        }

        let path = UIBezierPath()

        path.moveToPoint(convertPoint(fromPoint, toView: self))
        path.addLineToPoint(convertPoint(toPoint, toView: self))

        let layer = CAShapeLayer()

        layer.path = path.CGPath
        layer.lineWidth = lineWidth
        layer.strokeColor = strokeColor.CGColor

        self.layer.addSublayer(layer)
    }
}

结果如下：