Example of a basic ViewModel

simpleViewModel

protocol ViewModelDelegate: class {
    func viewModelDidStartLoad()
    func viewModelDidLoad()
    func viewModelDidFail()
}

class ViewModel: NSObject {
    
    // MARK: - Properties
    private weak var delegate: ViewModelDelegate?
    var items = [CellItem]()
   
    // MARK: - Initialization
    init(delegate: ViewModelDelegate) {
        self.delegate = delegate
    }
    
    // MARK: - Configuration
    func load() {
        self.delegate?.viewModelDidStartLoad()
        self.addItems()
        self.delegate?.viewModelDidLoad()
    }
    
}

// MARK: - UICollectionViewDataSource
extension ViewModel: UICollectionViewDataSource {
    func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return items.count
    }
    
    func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
        let item = self.items[indexPath.row]
        let cell = collectionView.dequeueReusableCellWithReuseIdentifier(item.reuseIdentifier(), forIndexPath: indexPath)
        item.configureCell(cell)
        return cell
    }
}