frenzy-ios/Reader/Screens/Items/ItemsViewController.swift

//
//  ItemsViewController.swift
//  Reader
//
//  Created by Shadowfacts on 1/9/22.
//

import UIKit
import CoreData

class ItemsViewController: UIViewController {
    
    let fervorController: FervorController
    let fetchRequest: NSFetchRequest<Item>
    
    private var collectionView: UICollectionView!
    private var dataSource: UICollectionViewDiffableDataSource<Section, Item>!
    private var resultsController: NSFetchedResultsController<Item>!
    
    init(fervorController: FervorController, fetchRequest: NSFetchRequest<Item>) {
        self.fervorController = fervorController
        self.fetchRequest = fetchRequest
        
        super.init(nibName: nil, bundle: nil)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        let configuration = UICollectionLayoutListConfiguration(appearance: .plain)
        let layout = UICollectionViewCompositionalLayout.list(using: configuration)
        collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: layout)
        collectionView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        collectionView.register(ItemCollectionViewCell.self, forCellWithReuseIdentifier: "itemCell")
        view.addSubview(collectionView)
        
        dataSource = createDataSource()
        
        var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
        snapshot.appendSections([.items])
        dataSource.apply(snapshot, animatingDifferences: false)
        
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "published", ascending: false)]
        resultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: fervorController.persistentContainer.viewContext, sectionNameKeyPath: nil, cacheName: nil)
        resultsController.delegate = self
        try! resultsController.performFetch()
    }
    
    private func createDataSource() -> UICollectionViewDiffableDataSource<Section, Item> {
        let dataSource = UICollectionViewDiffableDataSource<Section, Item>(collectionView: collectionView) { collectionView, indexPath, item in
            let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "itemCell", for: indexPath) as! ItemCollectionViewCell
            cell.updateUI(item: item)
            return cell
        }
        return dataSource
    }

}

extension ItemsViewController {
    enum Section: Hashable {
        case items
    }
}

extension ItemsViewController: NSFetchedResultsControllerDelegate {
    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChangeContentWith snapshot: NSDiffableDataSourceSnapshotReference) {
        var snapshot = self.dataSource.snapshot()
        snapshot.deleteItems(snapshot.itemIdentifiers(inSection: .items))
        // use resultsController here instead of controller so we don't have to cast
        snapshot.appendItems(resultsController.fetchedObjects!, toSection: .items)
        self.dataSource.apply(snapshot, animatingDifferences: false)
    }
}