ViewController

import UIKit
import DataSources

class ViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
    
    var collectionView: UICollectionView = {
        let layout = UICollectionViewFlowLayout()
        layout.sectionInset = .zero
        layout.minimumLineSpacing = 0
        layout.minimumInteritemSpacing = 0
        
        let collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
        collectionView.register(Cell.self, forCellWithReuseIdentifier: "Cell")
        collectionView.register(Header.self, forSupplementaryViewOfKind: UICollectionElementKindSectionHeader, withReuseIdentifier: "Header")
        collectionView.backgroundColor = .white
        return collectionView
    }()
    
    private lazy var _dataSource: DataController<CollectionViewAdapter> = .init(adapter: .init(collectionView: self.collectionView))
    
    private let section0 = Section(ModelA.self, isEqual: { $0.identity == $1.identity })
    
    var section0Models: [ModelA] = [] {
        didSet {
            _dataSource.update(
                in: section0,
                items: section0Models,
                updateMode: .partial(animated: true),
                completion: {
                    
            })
        }
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        collectionView.delegate = self
        collectionView.dataSource = self
        
        _dataSource.add(section: section0)
        
        for _ in 0..<20 {
            section0Models.append(ModelA(identity: UUID().uuidString, title: String.randomEmoji()))
        }
        view.addSubview(collectionView)
        
        //This return zero items even though section0Models has 20 items
        _dataSource.numberOfItems(in: section)
        
    }
    
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return _dataSource.numberOfSections()
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return _dataSource.numberOfItems(in: section)
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath) as! Cell
        let m = _dataSource.item(at: indexPath, in: section0)
        cell.label.text = m.title
        return cell
    }
    
    
    func collectionView(_ collectionView: UICollectionView, viewForSupplementaryElementOfKind kind: String, at indexPath: IndexPath) -> UICollectionReusableView {
        if kind == UICollectionElementKindSectionHeader {
            let view = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: "Header", for: indexPath) as! Header
            view.label.text = "Section " + indexPath.section.description
            return view
        }
        fatalError()
    }
    
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> CGSize {
        
        return CGSize(width: collectionView.bounds.width, height: 50)
    }
    
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        
        return CGSize(width: collectionView.bounds.width / 6, height: 50)
    }
}