在获取/创建用户核心数据对象时，我遇到类似奇怪的竞争条件问题。我有两种方法， handleGames（）和 handelUsers（），它们在发出各自的网络请求后被调用。
用户包含普通信息，如用户名和ID。游戏可以有很多用户参与者。我们在核心数据模型中创建了这种一对多的关系。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网络请求后调用的handleGames（）和handleUsers（）方法。这两种方法都由我们的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。但是，当我们的NetworkManager也同时调用handleUsers（）执行其他任务时，我们会抛出多个对象被提取的错误，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（）但仍然无效。
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)
}
}
答案 0 :(得分：0)
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()块移到每个handleMethod()内来解决了这个问题。我不认为这是绝对正确的做法，因为仍然会涉及一些竞争条件，但在小项目的背景下，它对我来说是一个合适的解决方案。如果我稍后找到一个，我会发一个更好的答案。