爲什麼在一個視圖中有不同的上下文？

Question

我有一個購物車控制器。用戶可以將項目添加到他的願望清單。如果我們沒有任何願望清單，我們應該創建默認的清單。這是工程2-3次，如果我添加，然後刪除，然後再添加。然後我得到的錯誤：

Illegal attempt to establish a relationship 'wishList' between objects in different contexts

class CartViewController: UIViewController, NSFetchedResultsControllerDelegate { 
     var fetchResultController:NSFetchedResultsController! 
     var shopItems:[ShopItem] = [] 
     let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate 

     override func viewDidLoad() { 
      self.fetchResultController = NSFetchedResultsController(fetchRequest: fetchRequest, 
        managedObjectContext: self.appDelegate.managedObjectContext, sectionNameKeyPath: nil, cacheName: nil) 
      self.fetchResultController.delegate = self 
      do { 
       try fetchResultController.performFetch() 
       self.shopItems = fetchResultController.fetchedObjects as! [ShopItem] 
      } catch { 
       fatalError("Failure to save context: \(error)") 
      } 

     } 
     func tableView(tableView: UITableView, editActionsForRowAtIndexPath indexPath: NSIndexPath) -> [UITableViewRowAction]? { 

      let addToWishListAction = UITableViewRowAction(style: UITableViewRowActionStyle.Default, title: "Add to wish list", handler: { (action:UITableViewRowAction!, indexPath:NSIndexPath!) -> Void in 
       var wishListResults:[WishList] = [] 
       let fetchRequest = NSFetchRequest(entityName: "WishList") 
       var wishList:WishList 
       fetchRequest.fetchLimit = 1 

       //...some other code 

       wishList = NSEntityDescription.insertNewObjectForEntityForName("WishList", inManagedObjectContext: self.appDelegate.managedObjectContext) as! WishList 
       wishList.setValue("Default wish list", forKey: "title") 
       wishList.setValue("My wish list", forKey: "desc") 

       let shopItem = self.fetchResultController.objectAtIndexPath(indexPath) as! ShopItem 

       shopItem.setValue(true, forKey: "inWishList") 
       shopItem.setValue(wishList, forKey: "WishList") 

       do { 
        try self.appDelegate.managedObjectContext.save() 
       } catch { 
        fatalError("Failure to save context: \(error)") 
       } 
      }) 
     } 
    } 

爲什麼語境變了嗎？

Answer 1

您正在嘗試在與創建託管對象上下文不同的線程上創建並保存CoreData對象。 UITableViewRowAction的處理程序提供了一個塊回調，它將在不同的線程上異步發生。

爲了在不同的線程上創建和保存對象，您需要創建另一個NSManagedObjectContext的concurrencyTypePrivateQueueConcurrencyType然後使用後臺線程中的對象。

因此，這裏是我會怎樣重新寫你的editActionsForRowAtIndexPath方法（因爲每個人都希望複製和粘貼，對吧？）：

func tableView(tableView: UITableView, editActionsForRowAtIndexPath indexPath: NSIndexPath) -> [UITableViewRowAction]? { 

    let privateMOC = NSManagedObjectContext(concurrencyType: .PrivateQueueConcurrencyType) 
    privateMOC.parentContext = self.appDelegate.managedContext 
    let addToWishListAction = UITableViewRowAction(style: UITableViewRowActionStyle.Default, title: "Add to wish list", handler: { (action:UITableViewRowAction!, indexPath:NSIndexPath!) -> Void in 
     var wishListResults:[WishList] = [] 
     let fetchRequest = NSFetchRequest(entityName: "WishList") 
     var wishList:WishList 
     fetchRequest.fetchLimit = 1 

     //...some other code 

     wishList = NSEntityDescription.insertNewObjectForEntityForName("WishList", inManagedObjectContext: privateMOC) as! WishList 
     wishList.setValue("Default wish list", forKey: "title") 
     wishList.setValue("My wish list", forKey: "desc") 

     let shopItem = self.fetchResultController.objectAtIndexPath(indexPath) as! ShopItem 

     shopItem.setValue(true, forKey: "inWishList") 
     shopItem.setValue(wishList, forKey: "WishList") 

     do { 
      try privateMOC.save() 
     } catch { 
      fatalError("Failure to save context: \(error)") 
     } 
    }) 
} 

此外，請確保您所創建的ManagedObjectContext使用MainQueueConcurrencyType爲：

var managedObjectContext = NSManagedObjectContext(concurrencyType: .MainQueueConcurrencyType)