ios - 核心数据同步创建和获取

Question

在获取/创建我的用户核心数据对象时，我遇到了奇怪的类似竞争条件的问题。我有两个方法，handleGames() 和 handelUsers()，在发出各自的网络请求后调用。

用户包含普通信息，例如用户名和 ID。一个游戏可以有许多用户参与者。我们在核心数据模型中创建了这种一对多关系。对于每个核心数据模型，我们创建了扩展来处理获取和创建相应的核心数据对象。

import UIKit
import CoreData
import SwiftyJSON

extension User {
    func set(withJson json: JSON) {
        self.id = json["id"].int32Value
        self.username = json["username"].stringValue
        self.email = json["email"].stringValue
        self.money = json["money"].int32 ?? self.money
        self.rankId = json["rank_id"].int32 ?? self.rankId
        self.fullname = json["fullName"].stringValue

        if let pictureUrl = json["picture"].string {
            self.picture = json["picture"].stringValue
        }
        else {
            let pictureJson = json["picture"]
            self.pictureWidth = pictureJson["width"].int16Value
            self.pictureHeight = pictureJson["height"].int16Value
            self.picture = pictureJson["url"].stringValue
        }
    }

    // Peform a fetch request on the main context for entities matching the predicate
    class func fetchOnMainContext(withPredicate predicate: NSPredicate?) -> [User]? {
        do {
            let appDelegate = UIApplication.shared.delegate as! AppDelegate
            let mainContext = appDelegate.persistentContainer.viewContext
            let fetchRequest: NSFetchRequest<User> = User.fetchRequest()
            fetchRequest.predicate = predicate
            let objects = try mainContext.fetch(fetchRequest)
            return objects
        } catch {
            print(error)
        }

        return nil
    }

    // Fetch a single entity that matches the predicate
    class func fetch(id: Int32, context: NSManagedObjectContext) -> User? {
        let predicate = NSPredicate(format: "id = %d", id)
        return fetchAndPerformBlockIfNotFound({return nil}, withPredicate: predicate, inContext: context)
    }

    // Fetch a single entity that matches the predicate or create a new entity if it doesn't exist
    class func fetchOrCreate(withPredicate predicate: NSPredicate?, inContext context: NSManagedObjectContext) -> User? {
        return fetchAndPerformBlockIfNotFound({return User(context: context)}, withPredicate: predicate, inContext: context)
    }

    // Helper method for fetching an entity with a predicate
    private class func fetchAndPerformBlockIfNotFound(_ block: () -> User?, withPredicate predicate: NSPredicate?, inContext context: NSManagedObjectContext) -> User? {
        do {
            let fetchRequest: NSFetchRequest<User> = User.fetchRequest()
            fetchRequest.predicate = predicate
            let objects = try context.fetch(fetchRequest)

            if objects.count == 0 {
                return block()
            } else if objects.count == 1 {
                return objects.first
            } else {
                print("ERROR: fetch request found more than 1 object")
            }
        } catch {
            print(error)
        }

        return nil
    }
}


import UIKit
import CoreData
import SwiftyJSON

extension Game {
    func set(withJson json: JSON) {
        self.id = json["id"].int32Value
        let pickerJson = json["picker"]
        if let pickerId = pickerJson["id"].int32 {
            self.pickerId = pickerId
        }
        self.pickerChoiceId = json["pickerChoice_id"].int32 ?? self.pickerChoiceId
        self.creationDate = NSDate(timeIntervalSince1970: json["creationDate"].doubleValue)
        if let completedTimeInterval = json["completedDate"].double {
            self.completedDate = NSDate(timeIntervalSince1970: completedTimeInterval)
        }
        self.isPublic = json["isPublic"].boolValue
        self.maturityRating = json["rating"].int32Value
        self.beenUpvoted = json["beenUpvoted"].boolValue
        self.beenFlagged = json["beenFlagged"].boolValue
        let topCaptionJson = json["topCaption"]
        if let before = topCaptionJson["fitbBefore"].string,
            let after = topCaptionJson["fitbAfter"].string,
            let userEntry = topCaptionJson["userEntry"].string {
            self.topCaptionBefore = before
            self.topCaptionAfter = after
            self.topCaptionEntry = userEntry
        }
        if let picUrl = topCaptionJson["userPic"].string {
            self.topCaptionUserPicUrl = picUrl;
        }

        let pictureJson =  json["picture"]
        self.pictureUrl = pictureJson["url"].stringValue
        self.pictureWidth = pictureJson["width"].int16Value
        self.pictureHeight = pictureJson["height"].int16Value

        self.numUpvotes = json["numUpvotes"].int32Value
        self.topCaptionUserId = topCaptionJson["userId"].int32 ?? topCaptionUserId
        self.participants = NSSet()
    }
}

下面是我们在发出各自的网络请求后调用的handleGames()和handleUsers()方法。这两个方法都由我们的 NetworkManager 异步调用。在handleGames()中，我们还调用handleUsers()来设置每个游戏的参与者。然而，当我们的 NetworkManager 还同时为其他任务调用 handleUsers() 时，我们会抛出已获取多个对象的错误，这意味着在获取之前已经创建了多个对象。我们尝试使用performAndWait()，但仍然不起作用。

import CoreData
import SwiftyJSON

protocol CoreDataContextManager {
    var persistentContainer: NSPersistentContainer { get }
    func saveContext()
}

class CoreDataManager {
    static let shared = CoreDataManager()

    var contextManager: CoreDataContextManager!

    private init() {}

    // Perform changes to objects and then save to CoreData.
    fileprivate func perform(block: (NSManagedObjectContext)->()) {
        if self.contextManager == nil {
            self.contextManager = UIApplication.shared.delegate as! AppDelegate
        }
        let mainContext = self.contextManager.persistentContainer.viewContext

        block(mainContext)

        self.contextManager.saveContext()
    }
}

extension CoreDataManager: NetworkHandler {
    func handleUsers(_ usersJson: JSON, completion: (([User])->())? = nil) {
        var userCollection: [User] = []
        var idSet: Set<Int32> = Set()
        self.perform { context in

            for userJson in usersJson.arrayValue {

                guard userJson["id"].int != nil else {
                    continue
                }
                let predicate = NSPredicate(format: "id = %@", userJson["id"].stringValue)

                if  !idSet.contains(userJson["id"].int32Value), let user = User.fetchOrCreate(withPredicate: predicate, inContext: context) {
                    user.set(withJson: userJson)
                    userCollection.append(user)
                    idSet.insert(userJson["id"].int32Value)
                    //Establish Relations
                    if let rankId = userJson["rank_id"].int32, let rank = Rank.fetch(id: rankId, context: context) {
                        user.rank = rank
                    }
                }
            }
        }
        completion?(userCollection)
    }

func handleGames(_ gamesJson: JSON, completion: (([Game])->())? = nil) {
        var games: [Game] = []
        var idSet: Set<Int32> = Set()
        self.perform { context in

            for gameJson in gamesJson.arrayValue {

                guard gameJson["id"].int != nil else {
                    continue
                }

                let predicate = NSPredicate(format: "id = %@", gameJson["id"].stringValue)
                let gameId = gameJson["id"].int32Value

                // Make sure there are no duplicates
                if !idSet.contains(gameId), let game = Game.fetchOrCreate(withPredicate: predicate, inContext: context) {
                    game.set(withJson: gameJson)
                    games.append(game)

                    // Establish relationships
                    let userPredicate = NSPredicate(format: "id = %@", game.pickerId.description)
                    if let picker = User.fetch(id: game.pickerId, context: context) {
                        game.picker = picker
                    }
                    else if let picker = User.fetchOrCreate(withPredicate: userPredicate, inContext: context) {
                        picker.set(withJson: gameJson["picker"])
                        game.picker = picker
                    }
                    if let pickerChoiceId = gameJson["pickerChoice_id"].int32, let pickerChoice = Caption.fetch(id: pickerChoiceId, context: context) {
                        game.pickerChoice = pickerChoice
                    }
                    idSet.insert(gameId)

                    // add participants to game
                    handleUsers(gameJson["users"]) { users in
                        print("handleUsers for game.id: \(game.id)")
                        for user in users {
                            print("user.id: \(user.id)")
                        }
                        game.participants = NSSet(array: users)

                    }
                }
            }
        }
        completion?(games)
    }
}

Answer 1

当其他调用尝试获取用户核心数据对象时，我保存核心数据的方式发生得不够快。我通过将 self.contextManager.saveContext() 从 perform() block 移动到每个 handleMethod() 内解决了这个问题。我认为这不是绝对正确的方法，因为仍然可能涉及一些竞争条件，但在小项目的背景下，这对我来说是一个合适的解决方案。如果以后找到更好的答案，我会发布一个更好的答案。