Simulator Screen Shot Apr 17, 2017, 7.14.28 PM.png

同时根据不同数量的数据能够自动生成正确size的圆：

Simulator Screen Shot Apr 17, 2017, 7.18.16 PM.png

基本思路还是利用CollectionViewLayout来客制化CollectionView的外观，既然如此，那么我们先创建一个Layout的subclass：

import UIKit

protocol CircularDelegateLayout {
    func collectionView(collectionView:UICollectionView, radiusForItemAtIndexPath indexPath:IndexPath) -> CGFloat
}

class CircularLayout: UICollectionViewLayout {
    //Well, I am not using delegate since I am control freak, however, you can customize radius if you want through protocol above.
    var delegate:CircularDelegateLayout!
    fileprivate var viewCenter:CGPoint!
    fileprivate var numberOfItems = 0
    fileprivate var centerCircleRadius:CGFloat!
    fileprivate var itemRadius:CGFloat!
    fileprivate var itemSize:CGSize!
    fileprivate var centerItemSize:CGSize!
    
    override var collectionViewContentSize: CGSize {
        return collectionView!.bounds.size
    }
    
    override func prepare() {
        super.prepare()
        guard let collectionView = collectionView else {return}
        //You only have one section, yes, only one
        numberOfItems = collectionView.numberOfItems(inSection: 0)
        viewCenter = CGPoint(x: collectionView.bounds.midX, y: collectionView.bounds.midY)
        let shortest = min(collectionView.bounds.width, collectionView.bounds.height)
        centerCircleRadius = shortest / 3.4
        
        //Why 7 is the magic number? Well, to be frankly, it's not, but I like 7, so I put it here.
        if numberOfItems > 7 {
            /*switch reader {
                case .EastAsian:
                    print("Ask your high school teacher to figure out why.")
                default:
                    print("Never mind, it's Voodoo, just use it.")
             }
             */
            itemRadius = CGFloat(2 * Double(centerCircleRadius) * sin(.pi / Double(numberOfItems - 1)))
        }else {
            itemRadius = centerCircleRadius
        }
        
        itemSize = CGSize(width: itemRadius, height: itemRadius)
        centerItemSize = CGSize(width: centerCircleRadius, height: centerCircleRadius)
    }
    
    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
        let angle = 2 * .pi * CGFloat(indexPath.item) / CGFloat(numberOfItems - 1)
        
        switch indexPath.item {
        case 0:
            attributes.center = viewCenter
            attributes.size = centerItemSize
        default:
            /*switch reader {
             case .EastAsian:
                print("Ask your high school teacher to figure out why.")
             default:
                print("Never mind, it's Voodoo, just use it.")
             }
             */
            attributes.center = CGPoint(x: viewCenter.x + centerCircleRadius * cos(angle), y: viewCenter.y + centerCircleRadius * sin(angle))
            attributes.size = itemSize
        }
        return attributes
    }
    
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        guard let collectionView = collectionView else {return nil}
        return (0..<collectionView.numberOfItems(inSection: 0)).flatMap{ item -> UICollectionViewLayoutAttributes? in
            self.layoutAttributesForItem(at: IndexPath(item: item, section: 0))
        }
    }
    
}

Documentation是写给公司老外看的，相信在座的都是中国人，这种简单的几何不需要解释了。如果对Layout有疑问的，可以参考前一篇文章：http://www.jianshu.com/p/47fca57fd30b

下面我们来看一下如何使用这个东西，首先你需要在你的StoryBoard里面把CollectionView对应的Layout改掉：

Screen Shot 2017-04-17 at 7.25.16 PM.png

然后给这个Layout在对应的ViewController里面创建一个属性，lay back and enjoy：

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var circularCollectionView: UICollectionView!
    @IBOutlet weak var circularLayout: CircularLayout!
    
    fileprivate var data = ["Zero","First","Second","Third","Forth","Fifth","Sixth","Seventh","Eighth","Ninth","Tenth","One", "Two", "Three"]
    fileprivate var realData:[String] = []
    fileprivate var count = 0
    
    override func viewDidLoad() {
        super.viewDidLoad()
        circularCollectionView.register(UINib(nibName: "CircularCellCollectionViewCell", bundle: nil), forCellWithReuseIdentifier: "CircularCell")
        circularCollectionView.delegate = CollectionFactory.shared
        circularCollectionView.dataSource = CollectionFactory.shared
        CollectionFactory.shared.delegate = self
    }
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        let vm = CollectionFactory.shared.registerViewModel(vm:CircularModel()) as! CircularModel
        circularLayout.delegate = vm
        Timer.scheduledTimer(withTimeInterval: 0.5, repeats: true) { (timer) in
            if self.count < self.data.count {
                self.realData.append(self.data[self.count])
                self.count += 1
                DispatchQueue.main.async {
                    self.circularCollectionView.reloadData()
                }
            }else {
                self.count = 0
                timer.invalidate()
            }
            
        }
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
}

extension ViewController:FactoryDataSource {
  
    var dataContainer:[Any]{return realData}
}

完整的project可以在这里找到：https://github.com/LHLL/CircularCollectionView