     let collectionView: UICollectionView = {
         let collectionView = UICollectionView(frame: .zero, collectionViewLayout: GridCollectionViewFlowLayout())
         
-        collectionView.register(UICollectionViewCell.classForCoder(), forCellWithReuseIdentifier: "cell")
+        collectionView.register(GridCollectionViewCell.classForCoder(), forCellWithReuseIdentifier: GridCollectionViewCell.gridReuseIdentifier)
         
         return collectionView
     }()

//

import UIKit

class GridCollectionViewFlowLayout: UICollectionViewFlowLayout {
    

    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: GridCollectionViewCell.gridReuseIdentifier, for: indexPath) as! GridCollectionViewCell
        
        cell.backgroundColor = cellColors[indexPath.item]
        cell.label.text = "Tap to highlight"
        
        return cell
    }
