swift – NSCollectionView dataSource无法正常工作

在WWDC视频之后,我创建了一个NSCollectionViewFlowLayout的子类来确定集合视图的布局.

class Layout : NSCollectionViewFlowLayout {

  override func prepareLayout() {

    super.prepareLayout()

    self.minimumInteritemSpacing = 0
    self.minimumLineSpacing = 0

    let cheight = self.collectionView!.bounds.height
    let cwidth = self.collectionView!.bounds.width

    self.itemSize = CGSizeMake(cwidth / 2.0,cheight / 6.0)

  }

}

之后,我创建了一个NSObject子类作为数据源.

class DataSource : NSObject,NSCollectionViewDataSource {

  func collectionView(collectionView: NSCollectionView,numberOfItemsInSection section: Int) -> Int {
    return 5
  }

  func collectionView(collectionView: NSCollectionView,itemForRepresentedObjectAtIndexPath indexPath: NSIndexPath) -> NSCollectionViewItem {

    /* DOESN'T GET CALLED! */

    let item = collectionView.makeItemWithIdentifier("cell",forIndexPath: indexPath)

    item.view.wantsLayer = true
    item.view.layer?.backgroundColor = NSColor.redColor().CGColor

    return item

  }

}

问题是collectionView：itemForRepresentedObjectAtIndexPath：永远不会被调用.

这是我初始化集合视图的方式：

let collectionView = NSCollectionView(frame: view.bounds)

let dataSource = DataSource()
let layout = Layout()

collectionView.collectionViewLayout = layout
collectionView.registerClass(NSCollectionViewItem.self,forItemWithIdentifier: "cell")
collectionView.dataSource = dataSource

collectionView.backgroundColors = [.blackColor()]

我可以在superview中清楚地看到集合视图,但是没有单元格.

此外,如果在委托之外调用此行(但在注册单元类之后)会导致应用程序崩溃！

let item = collectionView.makeItemWithIdentifier("cell",forIndexPath: /*any index path*/)

我做错了什么或NSCollectionView新API坏了吗？

谢谢.