MULTITHREADING, CORE DATA AND REALM Poznań 28.02.2019 Łukasz Niedźwiedź Let It Swift #1 

MULTITHREADING, CORE DATA AND REALM AGENDA ▸ Problem ▸ Core Data ▸ Realm ▸ Multithreading ▸ Common mistakes ▸ Solution 

PROBLEM 

MULTITHREADING, CORE DATA AND REALM SCENARIO ▸ We have been working all day long ▸ We are hungry ▸ It’s late ▸ We are ordering a delicious pizza right before it closes 

MULTITHREADING, CORE DATA AND REALM RESULT ▸ We are waiting for pizza ▸ Pizzeria is closed ▸ We can’t contact with a delivery man or a restaurant ▸ We are starving ▸ We are unhappy ▸ We are not able to write a good quality code because we didn’t eat last night 

MULTITHREADING, CORE DATA AND REALM WHAT HAPPENED? ▸ Application received push notification ▸ Application fetches latest orders from API with timestamp ▸ Core Data/Realm failed to save due to threading issue ▸ Delivery man doesn’t see our order ▸ It’s late so he is going home with our pizza 

CORE DATA 

MULTITHREADING, CORE DATA AND REALM CORE DATA ▸ ORM (object-relational mapping) ▸ Developed by Apple ▸ Objective-C based ▸ Uses SQLite 

MULTITHREADING, CORE DATA AND REALM HOW CORE DATA WORKS ▸ Xcode editor tool that can generate model classes ▸ Core Data Stack ▸ Two concurrency types - Main and Private 

MULTITHREADING, CORE DATA AND REALM MODEL CLASS EXAMPLE class Pizza: NSManagedObject { @NSManaged var name: String @NSManaged var price: Double @NSManaged var address: String @NSManaged var comment: String? } // Generated by XCode @nonobjc public class func fetchRequest() -> NSFetchRequest { return NSFetchRequest(entityName: "Pizza") } @NSManaged public var address: String? @NSManaged public var comment: String? @NSManaged public var name: String? @NSManaged public var price: Double 

MULTITHREADING, CORE DATA AND REALM CORE DATA STACK MANAGED OBJECT MODEL MANAGED OBJECT CONTEXT PERSISTENT STORE COORDINATOR STORES DESCRIPTIONS 

MULTITHREADING, CORE DATA AND REALM CORE DATA STACK - PERSISTENT STORE CONTAINER init(completionClosure: @escaping () -> ()) { persistentContainer = NSPersistentContainer(name: "DataModel") persistentContainer.loadPersistentStores() { _, error in if let error = error { // Data migration/ No space } completionClosure() } } var privateContext: NSManagedObjectContext { return persistentContainer.newBackgroundContext() } var viewContext: NSManagedObjectContext { return persistentContainer.viewContext } 

MULTITHREADING, CORE DATA AND REALM CORE DATA STACK - PERSISTENT STORE COORDINATOR // [1] let bundle = Bundle(for: CoreDataStack.self) let url = bundle.url(forResource: "Model", withExtension: "momd")! let model = NSManagedObjectModel(contentsOf: url)! // [2] storeCoordinator = NSPersistentStoreCoordinator(managedObjectModel: model) context = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType) context.persistentStoreCoordinator = storeCoordinator // [3] let storeUrl = URL(fileURLWithPath: "StoreUrl.sqlit") try! storeCoordinator .addPersistentStore(ofType: NSSQLiteStoreType, configurationName: nil, at: storeUrl, options: nil) 

MULTITHREADING, CORE DATA AND REALM CONCURRENCY TYPES ▸ Main - associates context with Main Queue ▸ Private - associates context with Private Queue 

REALM 

MULTITHREADING, CORE DATA AND REALM REALM ▸ Embedded Database ▸ Developed by Realm ▸ Objective-C based ▸ Cross-platform support 

MULTITHREADING, CORE DATA AND REALM HOW REALM WORKS? ▸ Each model subclass from Object ▸ Auto syncing data ▸ Thread-confined ▸ Thread safe reference 

MULTITHREADING, CORE DATA AND REALM MODEL CLASS EXAMPLE final class RLMPizza: Object { @objc dynamic var name: String = "" @objc dynamic var price: Double = 0 @objc dynamic var address: String = "" @objc dynamic var comment: String? = nil /* Required inits */ } 

MULTITHREADING, CORE DATA AND REALM REALM CONFIGURATION let realm = try! Realm() 

MULTITHREADING, CORE DATA AND REALM AUTO-SYNCING DATA ▸ Refresh instances in every run-loop ▸ Posting notification about data update ▸ Notification are delivered asynchronously ▸ Convient implementation for observing 

MULTITHREADING, CORE DATA AND REALM THREAD-CONFINEND ▸ Each Realm belongs to the thread on which was created ▸ Each Object belongs to realm thread ▸ Unmanaged Object behaves like NSObject 

MULTITHREADING, CORE DATA AND REALM THREAD SAFE REFERENCE ▸ Must be initialised with thread-confined object ▸ Should be short lived ▸ Resolved at most once 

MULTITHREADING 

MULTITHREADING, CORE DATA AND REALM THREADING IN CORE DATA JSON CORE DATA 

MULTITHREADING, CORE DATA AND REALM THREADING IN CORE DATA JSON MANAGED OBJECT CORE DATA 

MULTITHREADING, CORE DATA AND REALM THREADING IN CORE DATA JSON MANAGED OBJECT PRIVATE CONTEXT Saving Contexts 

MULTITHREADING, CORE DATA AND REALM THREADING IN CORE DATA JSON MANAGED OBJECT Context Perform Block PRIVATE CONTEXT Saving Contexts 

MULTITHREADING, CORE DATA AND REALM THREADING IN CORE DATA JSON MANAGED OBJECT Context Perform Block … PRIVATE CONTEXT Saving Contexts 

MULTITHREADING, CORE DATA AND REALM THREADING IN CORE DATA JSON MANAGED OBJECT Context Perform Block … PRIVATE CONTEXT Saving Contexts Main context Perform And Wait Block MAIN CONTEXT 

MULTITHREADING, CORE DATA AND REALM THREADING IN CORE DATA - PERSISTANCE CONTAINER JSON MANAGED OBJECT Persistsnce Store Perform Background Task PRIVATE CONTEXT Saving Contexts 

MULTITHREADING, CORE DATA AND REALM THREADING IN CORE DATA - PASSING OBJECT JSON MANAGED OBJECT Persistsnce Store Perform Background Task PRIVATE CONTEXT Saving Contexts MANAGED OBJECT ID 

MULTITHREADING, CORE DATA AND REALM THREADING IN REALM JSON REALM 

MULTITHREADING, CORE DATA AND REALM THREADING IN REALM JSON OBJECT REALM 

MULTITHREADING, CORE DATA AND REALM THREADING IN REALM JSON OBJECT REALM Realm write 

MULTITHREADING, CORE DATA AND REALM THREADING IN REALM JSON OBJECT Realm Thread REALM Realm write 

MULTITHREADING, CORE DATA AND REALM THREADING IN REALM - THREAD SAFE REFERENCE JSON OBJECT Realm Thread REALM Realm write THREAD SAFE REFERENCE 

MISTAKES 

MULTITHREADING, CORE DATA AND REALM COMMON MISTAKES ▸ Accessing realm object on wrong thread ▸ Accessing realm/managed object context on wrong thread ▸ Passing realm/managed object context among thread ▸ Wrongly configured Core Data Stack ▸ Race condition ▸ Third party frameworks 

SOLUTION 

MULTITHREADING, CORE DATA AND REALM DOMAIN - PLATFORM LAYERS CORE DATA / REALM OBJECT DOMAIN OBJECT 

MULTITHREADING, CORE DATA AND REALM DOMAIN - PLATFORM LAYERS protocol DomainConvertible { associatedtype DomainType var asDomain: DomainType { get } } protocol RealmConvertible { associatedtype RealmType: Object var uid: String { get } var asRealm: RealmType { get } } protocol CoreDataConvertible { associatedtype CoreDataType: NSManagedObject var uid: String { get } func sync(in context: NSManagedObjectContext) } 

MULTITHREADING, CORE DATA AND REALM PROTOCOL protocol DatabaseProtocol { func create(object: T) throws where T: PlatformConvertible func retrive(_ type: T.Type, predicate: NSPredicate?) -> [T] where T: PlatformConvertible, T.PlatformType: DomainConvertible, T.PlatformType.DomainType == T func update(objects: T) throws where T: PlatformConvertible func delete(object: T) throws where T: PlatformConvertible } 

MULTITHREADING, CORE DATA AND REALM CORE DATA - SAMPLE IMPLEMENTATION func create(object: T) where T: CoreDataConvertible { context.perform { [unowned self] in let entity = T.CoreDataType.entity() let object = NSManagedObject(entity: entity, insertInto: self.context) as! T object.sync(in: self.context) self.context.saveWithParents() } } func retrive(_ type: T.Type, predicate: NSPredicate?) -> [T] where T: CoreDataConvertible, T == T.CoreDataType.DomainType, T.CoreDataType: DomainConvertible { let request = T.CoreDataType.fetchRequest() request.predicate = predicate var result: [T.CoreDataType]! context.performAndWait { [unowned self] in result = try! self.context.fetch(request) as! [T.CoreDataType] } return result.compactMap { $0.asDomain } } 

MULTITHREADING, CORE DATA AND REALM REALM - SAMPLE IMPLEMENTATION func create(object: T) throws where T: RealmConvertible { try syncQueue.sync { do { try realm.write { realm.add(object.asRealm) } } } } func retrive(_ type: T.Type, predicate: NSPredicate) -> [T] where T: RealmConvertible, T == T.RealmType.DomainType, T.RealmType: DomainConvertible { return syncQueue.sync { do { return realm .objects(type.RealmType.self) .map { $0.asDomain } } } } 

MULTITHREADING, CORE DATA AND REALM REPOSITORY ▸ Abstraction of a data layer ▸ Way of handling domain objects ▸ One repository for each Domain Object 

MULTITHREADING, CORE DATA AND REALM REPOSITORY PLATFORM BUSINESS LOGIC REPOSITORY MAPPER CRUD Domain Object Domain Object Platform Object Platform Object 

MULTITHREADING, CORE DATA AND REALM REPOSITORY - PROTOCOL protocol AbstractRepository { associatedtype T func query(with predicate: NSPredicate?, sortDescriptors: [NSSortDescriptor]?) -> [T] func save(entity: T) throws func delete(entity: T) throws } 

MULTITHREADING, CORE DATA AND REALM REPOSITORY - IMPLEMENTATION func query(with predicate: NSPredicate? = nil, sortDescriptors: [NSSortDescriptor]? = nil) -> Observable<[T]> { let request = T.CoreDataType.fetchRequest() request.predicate = predicate request.sortDescriptors = sortDescriptors return context.rx.entities(fetchRequest: request) .mapToDomain() .subscribeOn(scheduler) } func save(entity: T) -> Observable { return entity.sync(in: context) .mapToVoid() .flatMapLatest(context.rx.save) .subscribeOn(scheduler) } func delete(entity: T) -> Observable { return entity.sync(in: context) .map({$0 as! NSManagedObject}) .flatMapLatest(context.rx.delete) } 

MULTITHREADING, CORE DATA AND REALM SOURCES ▸ https://developer.apple.com/documentation/coredata - Apple Documentation ▸ https://realm.io/docs/swift/latest/ - Realm Documentation ▸ https://github.com/sergdort/CleanArchitectureRxSwift - Rx Swift Clean Architecture open source project ▸ https://github.com/objcio/core-data - Core Data by objc.io 

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