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

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

import UIKit
import CoreData
import SafariServices

protocol ItemsViewControllerDelegate: AnyObject {
    func showReadItem(_ item: Item)
}

class ItemsViewController: UIViewController {
    
    weak var delegate: ItemsViewControllerDelegate?
    
    let fervorController: FervorController
    let fetchRequest: NSFetchRequest<NSManagedObjectID>
    
    private var collectionView: UICollectionView!
    private var dataSource: UICollectionViewDiffableDataSource<Section, NSManagedObjectID>!
    
    private var batchUpdates: [() -> Void] = []
    
    init(fetchRequest: NSFetchRequest<NSManagedObjectID>, fervorController: FervorController) {
        precondition(fetchRequest.entityName == "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()
        
        if UIDevice.current.userInterfaceIdiom != .mac {
            view.backgroundColor = .appBackground
        }
            
        var configuration = UICollectionLayoutListConfiguration(appearance: .plain)
        configuration.backgroundColor = .clear
        let layout = UICollectionViewCompositionalLayout.list(using: configuration)
        collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: layout)
        collectionView.delegate = self
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(collectionView)
        
        dataSource = createDataSource()
        
        NSLayoutConstraint.activate([
            collectionView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor),
            collectionView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor),
            collectionView.topAnchor.constraint(equalTo: view.topAnchor),
            collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
        ])
        
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "published", ascending: false)]
        do {
            let ids = try fervorController.persistentContainer.viewContext.fetch(fetchRequest)
            
            var snapshot = NSDiffableDataSourceSnapshot<Section, NSManagedObjectID>()
            snapshot.appendSections([.items])
            snapshot.appendItems(ids, toSection: .items)
            dataSource.apply(snapshot, animatingDifferences: false)
            
            NotificationCenter.default.addObserver(self, selector: #selector(managedObjectsDidChange), name: .NSManagedObjectContextObjectsDidChange, object: fervorController.persistentContainer.viewContext)
            
        } catch {
            let alert = UIAlertController(title: "Error fetching items", message: error.localizedDescription, preferredStyle: .alert)
            alert.addAction(UIAlertAction(title: "Ok", style: .default, handler: nil))
            present(alert, animated: true)
        }
    }
    
    private func createDataSource() -> UICollectionViewDiffableDataSource<Section, NSManagedObjectID> {
        let itemCell = UICollectionView.CellRegistration<ItemCollectionViewCell, NSManagedObjectID> { [unowned self] cell, indexPath, itemIdentifier in
            cell.delegate = self
            cell.scrollView = self.collectionView
            let item = self.fervorController.persistentContainer.viewContext.object(with: itemIdentifier) as! Item
            cell.updateUI(item: item)
        }
        return UICollectionViewDiffableDataSource(collectionView: collectionView) { collectionView, indexPath, itemIdentifier in
            return collectionView.dequeueConfiguredReusableCell(using: itemCell, for: indexPath, item: itemIdentifier)
        }
    }
    
    @objc private func managedObjectsDidChange(_ notification: Notification) {
        let inserted = notification.userInfo?[NSInsertedObjectsKey] as? NSSet
        let updated = notification.userInfo?[NSUpdatedObjectsKey] as? NSSet
        let deleted = notification.userInfo?[NSDeletedObjectsKey] as? NSSet
        
        // managed objectss from the notification are tied to the thread it was delivered on
        // so get the published dates and evaluate the predicate here
        let insertedItems = inserted?.compactMap { $0 as? Item }.filter { fetchRequest.predicate?.evaluate(with: $0) ?? true }.map { ($0, $0.published) }
        
        var snapshot = self.dataSource.snapshot()
        
        if let updated = updated {
            snapshot.reconfigureItems(updated.compactMap { ($0 as? Item)?.objectID })
        }
        if let deleted = deleted {
            snapshot.deleteItems(deleted.compactMap { ($0 as? Item)?.objectID })
        }
        
        if let insertedItems = insertedItems {
            self.fervorController.persistentContainer.performBackgroundTask { ctx in
                
                // for newly inserted items, the ctx doesn't have the published date so we check the data we got from the notification
                func publishedForItem(_ id: NSManagedObjectID) -> Date? {
                    if let (_, pub) = insertedItems.first(where: { $0.0.objectID == id }) {
                        return pub
                    } else {
                        return (ctx.object(with: id) as! Item).published
                    }
                }
                
                // todo: this feels inefficient
                for (inserted, insertedPublished) in insertedItems {
                    // todo: uhh, what does sql do if the published date is nil?
                    guard let insertedPublished = insertedPublished else {
                        snapshot.insertItems([inserted.objectID], beforeItem: snapshot.itemIdentifiers.first!)
                        continue
                    }
                    
                    var index = 0
                    while index < snapshot.itemIdentifiers.count,
                          let pub = publishedForItem(snapshot.itemIdentifiers[index]),
                          insertedPublished < pub {
                        index += 1
                    }
                    
                    // index is the index of the first item which the inserted one was published after
                    // (i.e., the item that should appear immediately after inserted in the list)
                    
                    if index == snapshot.itemIdentifiers.count {
                        snapshot.appendItems([inserted.objectID], toSection: .items)
                    } else {
                        snapshot.insertItems([inserted.objectID], beforeItem: snapshot.itemIdentifiers[index])
                    }
                }
                
                DispatchQueue.main.async {
                    self.dataSource.apply(snapshot, animatingDifferences: false)
                }
            }
        } else {
            DispatchQueue.main.async {
                self.dataSource.apply(snapshot, animatingDifferences: false)
            }
        }
    }

}

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

extension ItemsViewController: UICollectionViewDelegate {
    func collectionView(_ collectionView: UICollectionView, contextMenuConfigurationForItemAt indexPath: IndexPath, point: CGPoint) -> UIContextMenuConfiguration? {
        guard let id = dataSource.itemIdentifier(for: indexPath) else {
            return nil
        }
        let item = fervorController.persistentContainer.viewContext.object(with: id) as! Item
        return UIContextMenuConfiguration(identifier: nil, previewProvider: {
            ReadViewController(item: item, fervorController: self.fervorController)
        }, actionProvider: { _ in
            var children: [UIMenuElement] = []
            if let url = item.url {
                children.append(UIAction(title: "Open in Safari", image: UIImage(systemName: "safari"), handler: { [weak self] _ in
                    let vc = SFSafariViewController(url: url)
                    vc.preferredControlTintColor = .appTintColor
                    self?.present(vc, animated: true)
                }))
                #if targetEnvironment(macCatalyst)
                self.activityItemsConfiguration = UIActivityItemsConfiguration(objects: [url as NSURL])
                children.append(UICommand(title: "Share…", action: Selector(("unused")), propertyList: UICommandTagShare))
                #else
                children.append(UIAction(title: "Share", image: UIImage(systemName: "square.and.arrow.up"), handler: { [weak self] _ in
                    self?.present(UIActivityViewController(activityItems: [url], applicationActivities: nil), animated: true)
                }))
                #endif
            }
            if item.read {
                children.append(UIAction(title: "Mark as Unread", image: UIImage(systemName: "checkmark.circle"), handler: { [unowned self] _ in
                    Task {
                        await self.fervorController.markItem(item, read: false)
                    }
                }))
            } else {
                children.append(UIAction(title: "Mark as Read", image: UIImage(systemName: "checkmark.circle.fill"), handler: { [unowned self] _ in
                    Task {
                        await self.fervorController.markItem(item, read: true)
                    }
                }))
            }
            return UIMenu(children: children)
        })
    }
    
    func collectionView(_ collectionView: UICollectionView, willPerformPreviewActionForMenuWith configuration: UIContextMenuConfiguration, animator: UIContextMenuInteractionCommitAnimating) {
        guard let vc = animator.previewViewController else {
            return
        }
        animator.preferredCommitStyle = .pop
        animator.addCompletion {
            self.show(vc, sender: nil)
        }
    }
}

extension ItemsViewController: ItemCollectionViewCellDelegate {
    func itemCellSelected(cell: ItemCollectionViewCell, item: Item) {
        cell.setRead(true, animated: true)
        if let delegate = delegate {
            delegate.showReadItem(item)
        } else {
            show(ReadViewController(item: item, fervorController: fervorController), sender: nil)
        }
    }
}