yet another way to sync core data with paging api

sync_coredata_from_paging_data.swift

import Foundation
import CoreData

struct RemoteUser: Decodable {
    var userId: Int
    var name: String
}

class User: NSManagedObject {
    @NSManaged public var userId: Int
    @NSManaged public var name: String
    
    func importFrom(_ object: RemoteUser) {
        //get rid of context.hasChanges is true even set same value
        if userId != object.userId { userId = object.userId }
        if name != object.name { name = object.name }
    }
}

class PagingDataSyncEngine {
    
    var container: NSPersistentContainer!
    
    private lazy var backgroundContext: NSManagedObjectContext = {
        container.newBackgroundContext()
    }()
    
    func sync(from remoteUsers: [RemoteUser], offset: Int, isFullFilled: Bool, completion: @escaping (Error?) -> ()) {
        let moc = backgroundContext
        
        moc.perform {
            var err: Error?
            do {
                if remoteUsers.isEmpty {
                    try self.batchDeleteUsers { fetchRequest in
                        fetchRequest.fetchOffset = offset
                    }
                }else {
                    try self.sync(from: remoteUsers, isFullFilled: isFullFilled)
                    
                    if moc.hasChanges {
                        try moc.save()
                    }
                }
            }catch {
                err = error
            }
            
            DispatchQueue.main.async {
                completion(err)
            }
        }
    }
    
    private func sync(from remoteUsers: [RemoteUser], isFullFilled: Bool) throws {
        guard !remoteUsers.isEmpty else { fatalError() }
        
        let ids = remoteUsers.map { $0.userId }
        
        // batch delete
        var predicates = [NSPredicate]()
        
        if !isFullFilled {
            predicates.append(NSPredicate(format: "%K > %d", #keyPath(User.userId), ids.last!))
        }
        
        if remoteUsers.count > 1 {
            let (min, max) = (ids.first!, ids.last!)
            let key = #keyPath(User.userId)
            predicates.append(NSPredicate(format: "%K > %d AND %K < %d AND NOT %K IN %@", key, min, key, max, key, ids))
        }
        
        if !predicates.isEmpty {
            try batchDeleteUsers { fetchRequest in
                fetchRequest.predicate = NSCompoundPredicate(orPredicateWithSubpredicates: predicates)
            }
        }
        
        // find-or-create
        let fetchRequest = User.fetchRequest()
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: #keyPath(User.userId), ascending: true)]
        fetchRequest.predicate = NSPredicate(format: "%K IN %@", #keyPath(User.userId), ids)
        let existedUsers = try backgroundContext.fetch(fetchRequest) as! [User]
        
        let idUserMapping = existedUsers.reduce(into: [:]) { $0[$1.userId] = $1 }
        
        remoteUsers.forEach { item in
            if let user = idUserMapping[item.userId] {
                // update
                user.importFrom(item)
            }else {
                // create
                let user = NSEntityDescription.insertNewObject(forEntityName: "User", into: backgroundContext) as! User
                user.importFrom(item)
            }
        }
    }
    
    private func batchDeleteUsers(decoration: (NSFetchRequest<NSFetchRequestResult>) -> ()) throws {
        let fetchRequest = User.fetchRequest()
        fetchRequest.sortDescriptors = [NSSortDescriptor(key: #keyPath(User.userId), ascending: true)]
        
        decoration(fetchRequest)
        
        let batchDeleteRequest = NSBatchDeleteRequest(fetchRequest: fetchRequest)
        batchDeleteRequest.resultType = .resultTypeObjectIDs
        
        let result = try backgroundContext.execute(batchDeleteRequest) as? NSBatchDeleteResult
        
        if let ids = result?.result as? [NSManagedObjectID], !ids.isEmpty {
            NSManagedObjectContext.mergeChanges(fromRemoteContextSave: [NSDeletedObjectsKey: ids], into: [container.viewContext])
        }
    }
}