     }
 
     func shelfView(shelfView: Shelf.View, numberOfItemsInSection section: Int) -> Int {
-        return 20
+        if show1stSection {
+            return 20
+        }
+        return 0
     }
 
     func shelfView(shelfView: Shelf.View, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {

class ViewController: UIViewController {

    var shelfView: Shelf.View?

    override func viewDidLoad() {
        super.viewDidLoad()


        self.shelfView = shelfView

        let barButtonItem = UIBarButtonItem(title: "toggle", style: .Plain, target: self, action: "toggle")
        navigationItem.rightBarButtonItem = barButtonItem
    }

    func toggle() {
        if let shelfView = shelfView {
            shelfView.beginUpdates()
            if show1stSection {
                shelfView.deleteContentsInSections([0,1], withRowAnimation: .Automatic)
            } else {
                shelfView.insertContentsInSections([0,1], withRowAnimation: .Automatic)
            }
            show1stSection = !show1stSection
            shelfView.endUpdates()
        }
    }
