frenzy-ios/Reader/CoreData/PersistentContainer.swift

//
//  PersistentContainer.swift
//  Reader
//
//  Created by Shadowfacts on 12/24/21.
//

import CoreData
import Fervor
import OSLog

class PersistentContainer: NSPersistentContainer {
    
    private static let managedObjectModel: NSManagedObjectModel = {
        let url = Bundle.main.url(forResource: "Reader", withExtension: "momd")!
        return NSManagedObjectModel(contentsOf: url)!
    }()
    
    private(set) lazy var backgroundContext: NSManagedObjectContext = {
        let context = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
        // todo: should the background context really be parented to the view context, or should they both be direct children of the PSC?
        context.parent = self.viewContext
        return context
    }()
    
    private weak var fervorController: FervorController?
    
    private let logger = Logger(subsystem: Bundle.main.bundleIdentifier!, category: "PersistentContainer")
    
    init(account: LocalData.Account, fervorController: FervorController) {
        self.fervorController = fervorController
        
        super.init(name: "\(account.id)", managedObjectModel: PersistentContainer.managedObjectModel)
        
        loadPersistentStores { description, error in
            if let error = error {
                fatalError("Unable to load persistent store: \(error)")
            }
        }
    }
    
    @MainActor
    private func saveViewContext() async throws {
        if viewContext.hasChanges {
            try viewContext.save()
        }
    }
    
    func lastSyncDate() async throws -> Date? {
        return try await backgroundContext.perform {
            let state = try self.backgroundContext.fetch(SyncState.fetchRequest()).first
            return state?.lastSync
        }
    }
    
    func updateLastSyncDate(_ date: Date) async throws {
        try await backgroundContext.perform {
            if let state = try self.backgroundContext.fetch(SyncState.fetchRequest()).first {
                state.lastSync = date
            } else {
                let state = SyncState(context: self.backgroundContext)
                state.lastSync = date
            }
            
            try self.backgroundContext.save()
        }
        try await self.saveViewContext()
    }
    
    func sync(serverGroups: [Fervor.Group], serverFeeds: [Fervor.Feed]) async throws {
        try await backgroundContext.perform {
            let existingGroups = try self.backgroundContext.fetch(Group.fetchRequest())
            for group in serverGroups {
                if let existing = existingGroups.first(where: { $0.id == group.id }) {
                    existing.updateFromServer(group)
                } else {
                    let mo = Group(context: self.backgroundContext)
                    mo.updateFromServer(group)
                }
            }
            for removed in existingGroups where !serverGroups.contains(where: { $0.id == removed.id }) {
                self.backgroundContext.delete(removed)
            }
            
            let existingFeeds = try self.backgroundContext.fetch(Feed.fetchRequest())
            for feed in serverFeeds {
                if let existing = existingFeeds.first(where: { $0.id == feed.id }) {
                    existing.updateFromServer(feed)
                } else {
                    let mo = Feed(context: self.backgroundContext)
                    mo.updateFromServer(feed)
                }
            }
            
            if self.backgroundContext.hasChanges {
                try self.backgroundContext.save()
            }
        }
        try await self.saveViewContext()
    }
    
    func syncItems(_ syncUpdate: ItemsSyncUpdate) async throws {
        try await backgroundContext.perform {
            self.logger.debug("syncItems: deleting \(syncUpdate.delete.count, privacy: .public) items")
            let deleteReq = Item.fetchRequest()
            deleteReq.predicate = NSPredicate(format: "id in %@", syncUpdate.delete)
            let delete = NSBatchDeleteRequest(fetchRequest: deleteReq as! NSFetchRequest<NSFetchRequestResult>)
            delete.resultType = .resultTypeObjectIDs
            let result = try self.backgroundContext.execute(delete)
            
            if let deleteResult = result as? NSBatchDeleteResult,
               let objectIDs = deleteResult.result as? [NSManagedObjectID] {
                NSManagedObjectContext.mergeChanges(fromRemoteContextSave: [NSDeletedObjectsKey: objectIDs], into: [self.backgroundContext])
                // todo: does the background/view contexts need to get saved then?
            }
            
            let req = Item.fetchRequest()
            req.predicate = NSPredicate(format: "id in %@", syncUpdate.upsert.map(\.id))
            let existing = try self.backgroundContext.fetch(req)
            self.logger.debug("syncItems: updating \(existing.count, privacy: .public) items, inserting \(syncUpdate.upsert.count - existing.count, privacy: .public)")
            // todo: this feels like it'll be slow when there are many items
            for item in syncUpdate.upsert {
                if let existing = existing.first(where: { $0.id == item.id }) {
                    existing.updateFromServer(item)
                } else {
                    let mo = Item(context: self.backgroundContext)
                    mo.updateFromServer(item)
                }
            }
            
            if self.backgroundContext.hasChanges {
                try self.backgroundContext.save()
            }
        }
        try await self.saveViewContext()
    }
    
}