爲什麼sizeForItemAtIndexPath永遠不會被調用？

Question

在定義爲我的自定義視圖控制器如下：

class Wall : UIViewController, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout, WallLayoutDelegate { 

    override func viewDidLoad() { 
     super.viewDidLoad() 

     let layout = WallLayout() 
     layout.delegate = self 


     let collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: layout) 
     collectionView.registerClass(WallCell.self, forCellWithReuseIdentifier: Wall.reuseIdentifier) 
     collectionView.delegate = self 
     collectionView.dataSource = self 
     collectionView.backgroundColor = UIColor.blackColor() 

     self.view.addSubview(collectionView) 

     self.collectionView = collectionView 
    } 
    // CODE 

    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize { 
     return CGSizeMake(50, 100) 
    } 

}  

功能sizeForItemAtIndexPath不會被調用。它是如何來的？

Answer 1

我發現這是因爲佈局取決於我使用UICollectionViewDelegateFlowLayout時無法由集合視圖推演出來。