项目中，经常需要用到九宫格布局的 CollectionView ,这时可能就需要给各个 Cell 添加边线，以与其他 Cell 隔开。

想到的有几种方案：

1. 在 Cell 的初始化方法里，添加以下代码

self.layer.borderColor = ...
self.layer.borderWidth = 1

缺点：不过这种方式会让相邻的 Cell 的边线为两倍的边线宽度，作为开发者来说，明显是不能接受的。

2.使用 xib 与 InterfaceBuilder 方式创建的 Cell，在 xib 里拉四个 view ，设置好约束，充当 Cell 的 subviews 来当作分割线使用，然后在通过拉 IBOutlet，控制四个 view 的 hidden 属性，来控制边线的显示

缺点：每次需要边线的 Cell， 都会做类似无聊的操作

3.定义一个能控制边线的 CollectionView Cell 的基类，需要添加边线就继承此基类，从而简化工作量。

struct LWLineSeparatorOptions: OptionSetType {
    let rawValue: Int
    init(rawValue: Int) {
        self.rawValue = rawValue
    }
    
    static let top = LWLineSeparatorOptions.init(rawValue: 1)
    static let bottom = LWLineSeparatorOptions.init(rawValue: 2)
    static let left = LWLineSeparatorOptions.init(rawValue: 4)
    static let right = LWLineSeparatorOptions.init(rawValue: 8)
}

class LWLineSeparatorCollectionViewCell: UICollectionViewCell {
    
    var linwSeparatorOptions: LWLineSeparatorOptions = [.top, .bottom, .left, .right] {
        didSet {
            let _ = contentView.layer.sublayers!.map {
                if $0.isKindOfClass(CAShapeLayer.self) {
                    $0.removeFromSuperlayer()
                }
            }
            setNeedsDisplay()
        }
    }
    var lineColor = UIColor.init(red: 224.0/255.0, green: 224.0/255.0, blue: 224.0/255.0, alpha: 0.7)
    
    override func drawRect(rect: CGRect) {
        super.drawRect(rect)
        
        if linwSeparatorOptions.contains(.top) {
            let layer = CAShapeLayer()
            layer.lineWidth = 1
            layer.borderWidth = 1
            layer.strokeColor = lineColor.CGColor
            
            let path = UIBezierPath.init()
            path.moveToPoint(CGPointMake(0, 0))
            path.addLineToPoint(CGPointMake(rect.size.width, 0))
            
            layer.path = path.CGPath
            contentView.layer.addSublayer(layer)
        }
        
        if linwSeparatorOptions.contains(.bottom) {
            let layer = CAShapeLayer()
            layer.lineWidth = 1
            layer.borderWidth = 1
            layer.strokeColor = lineColor.CGColor
            
            let path = UIBezierPath.init()
            path.moveToPoint(CGPointMake(0, rect.size.height))
            path.addLineToPoint(CGPointMake(rect.size.width, rect.size.height))
            
            layer.path = path.CGPath
            contentView.layer.addSublayer(layer)
        }
        
        if linwSeparatorOptions.contains(.left) {
            let layer = CAShapeLayer()
            layer.lineWidth = 1
            layer.borderWidth = 1
            layer.strokeColor = lineColor.CGColor
            
            let path = UIBezierPath.init()
            path.moveToPoint(CGPointMake(0, 0))
            path.addLineToPoint(CGPointMake(0, rect.size.height))
            
            layer.path = path.CGPath
            contentView.layer.addSublayer(layer)
        }
        
        if linwSeparatorOptions.contains(.right) {
            let layer = CAShapeLayer()
            layer.lineWidth = 1
            layer.borderWidth = 1
            layer.strokeColor = lineColor.CGColor
            
            let path = UIBezierPath.init()
            path.moveToPoint(CGPointMake(rect.size.width, 0))
            path.addLineToPoint(CGPointMake(rect.size.width, rect.size.height))
            
            layer.path = path.CGPath
            contentView.layer.addSublayer(layer)
        }
    }  
}

然后可以在delegate中这样使用：

func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCellWithReuseIdentifier(...)
        
        switch indexPath.item / 3 {
        case 0:
            cell.linwSeparatorOptions = [.top, .right]
        case 1:
            cell.linwSeparatorOptions = [.top, .right]
        case 2:
            cell.linwSeparatorOptions = [.top, .right, .bottom]
        default:
            {}()
        }
        ...
        return cell
}