title: Cells not appearing in UICollectionView
body: <p>I don't understand why my cells aren't showing: </p>
  
  <pre><code>class MenuBar: UIView, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
  
      let cellId = "pouet"
  
      lazy var customCollectionView : UICollectionView = {
          let layout = UICollectionViewLayout()
          let cv  = UICollectionView(frame: .zero, collectionViewLayout: layout)
          cv.backgroundColor = UIColor.gray
          cv.dataSource = self
          cv.delegate = self
  
          return cv
      }()
  
      override init(frame: CGRect) {
          super.init(frame: frame)
  
          customCollectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: cellId)
          addSubview(customCollectionView)
          addConstraintsWithFormat(format: "H:|[v0]|", views: customCollectionView)
          addConstraintsWithFormat(format: "V:|[v0]|", views: customCollectionView)
      }
  
      func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -&gt; Int {
          return 4
      }
  
      func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -&gt; UICollectionViewCell {
          let cell = customCollectionView.dequeueReusableCell(withReuseIdentifier: cellId, for: indexPath)
          print("test")
  
          return cell
      }
  
      required init?(coder aDecoder: NSCoder) {
          fatalError("init(coder:) has not been implemented")
      }
  }
  </code></pre>
  
  <p>Now my <code>print("test")</code> is not showing either, meaning that the method is actually not called, right? So what is wrong ?</p>