ios - 为什么在一个 View 中有不同的不同上下文？

Question

我有一个购物车 Controller 。用户可以将项目添加到他的愿望 list 中。如果我们没有任何愿望 list ，我们应该创建默认的一个。如果我添加，然后删除，然后再次添加，这可以工作 2-3 次。然后我得到错误:

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)")
                }
            })
        }
    }

为什么上下文改变了？

Answer 1

您正试图在与创建托管对象上下文不同的线程上创建和保存 CoreData 对象。 UITableViewRowAction 的处理程序提供了一个 block 回调，它将在不同的线程上异步发生。

为了在不同的线程上创建和保存对象，您需要创建另一个 concurrencyType PrivateQueueConcurrencyType 的 NSManagedObjectContext，然后从后台线程。

下面是我将如何重写您的 editActionsForRowAtIndexPath 方法(因为每个人都想复制和粘贴，对吧？):

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，确保您正在创建 ManagedObjectContext 以使用 MainQueueConcurrencyType 作为:

var managedObjectContext = NSManagedObjectContext(concurrencyType: .MainQueueConcurrencyType)