在后台上下文中保存核心數據時出現NSInternalInconsistencyException異常

Question

我已經閱讀了有關此錯誤的所有內容，但仍然無法確定為什么它會在我的應用程序中發生。

使用后台上下文保存多個核心數據對象時出現以下錯誤：

*** Terminating app due to uncaught exception "NSInternalInconsistencyException", reason: "Failed to process pending changes before save. The context is still dirty after 100 attempts. Typically this recursive dirtying is caused by a bad validation method, -willSave, or notification handler.

在下面的代碼， ArticleManager的addArticle被稱為在主線程上的環。 可能需要添加0-200篇以上的文章。 此錯誤通常發生在商品數100-150之間。

//ArticleManager.m

-(id)init
{
    ... //normal init stuff
    dispatch_queue_t request_queue = dispatch_queue_create("com.app.articleRequest", NULL);
}    

-(void) addArticle:(Article *)article withURLKey:(NSString *)url
{
    //check if exists
    if ([downloadedArticles objectForKey:url] == nil && article != nil)
    {
        //add locally
        [downloadedArticles setObject:article forKey:url];

        //save to core data
        SaveArticle *saveArticle = [[SaveArticle alloc] init];
        [saveArticle saveArticle:article withURL:url onQueue:request_queue];
    }
}

//SaveArticle.m

@implementation SaveArticle

@synthesize managedObjectContext;
@synthesize backgroundContext;

-(id)init
{
    if (![super init]) return nil;

    AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
    managedObjectContext = [appDelegate managedObjectContext];

    backgroundContext = [[NSManagedObjectContext alloc] init];
    [backgroundContext setPersistentStoreCoordinator:[managedObjectContext persistentStoreCoordinator]];

    return self;
}

-(void)saveArticle:(Article *)article withURL:(NSString *)url onQueue:(dispatch_queue_t)queue
{       
    //save persistently in the background
    dispatch_async(queue, ^{
        ArticleCache *articleCacheObjectModel = (ArticleCache *)[NSEntityDescription insertNewObjectForEntityForName:@"ArticleCache" inManagedObjectContext:backgroundContext];

        if (article != nil)
        {
            [articleCacheObjectModel setArticleHTML:article.articleHTML];
            [articleCacheObjectModel setUrl:url];

            NSError *error;

            //Save the background context and handle the save notification 
            [[NSNotificationCenter defaultCenter] addObserver:self
                                                     selector:@selector(backgroundContextDidSave:)
                                                         name:NSManagedObjectContextDidSaveNotification
                                                       object:backgroundContext];

            if(![backgroundContext save:&error]) //ERROR OCCURS HERE, after many saves
            {  
                //This is a serious error saying the record  
                //could not be saved. Advise the user to  
                //try again or restart the application.
            }

            [[NSNotificationCenter defaultCenter] removeObserver:self
                                                    name:NSManagedObjectContextDidSaveNotification
                                                  object:backgroundContext];

        }
    });
}

/* Save notification handler for the background context */
- (void)backgroundContextDidSave:(NSNotification *)notification {
    /* Make sure we're on the main thread when updating the main context */
    if (![NSThread isMainThread]) {
        [self performSelectorOnMainThread:@selector(backgroundContextDidSave:)
                               withObject:notification
                            waitUntilDone:NO];
        return;
    }

    /* merge in the changes to the main context */
    [self.managedObjectContext mergeChangesFromContextDidSaveNotification:notification];
}

@end

Answer 1

好的，因此閱讀官方文檔會有所幫助。

從蘋果公司（重點是我）：

並發

Core Data使用線程（或序列化隊列）限制來保護托管對象和托管對象上下文（請參閱“ Core Data的並發性”）。 這樣的結果是，上下文假定默認所有者是分配給它的線程或隊列，這是由調用其init方法的線程確定的。 因此，您不應在一個線程上初始化上下文，然后將其傳遞給另一個線程。 相反，您應該將引用傳遞給持久性存儲協調器，並讓接收線程/隊列創建一個從中派生的新上下文。 如果使用NSOperation，則必須在main（對於串行隊列）或start（對於並發隊列）中創建上下文。

所以我的問題是，我在主線程上初始化了后台上下文，但是隨后通過dispatch_async使用了Grand Central Dispatch，它在后台線程上執行保存（使用在主線程上創建的上下文）。

我通過將上下文初始化添加到后台塊來修復它：

-(void)saveArticle:(Article *)article withURL:(NSString *)url onQueue:(dispatch_queue_t)queue
{       
    //save persistently in the background
    dispatch_async(queue, ^{

        NSManagedObjectContext *backgroundContext = [[NSManagedObjectContext alloc] init];
        [backgroundContext setPersistentStoreCoordinator:[managedObjectContext persistentStoreCoordinator]];

        ArticleCache *articleCacheObjectModel = (ArticleCache *)[NSEntityDescription insertNewObjectForEntityForName:@"ArticleCache" inManagedObjectContext:backgroundContext];

        if (article != nil)
        {
            [articleCacheObjectModel setArticleHTML:article.articleHTML];
            [articleCacheObjectModel setUrl:url];

            NSError *error;

            //Save the background context and handle the save notification 
            [[NSNotificationCenter defaultCenter] addObserver:self
                                                     selector:@selector(backgroundContextDidSave:)
                                                         name:NSManagedObjectContextDidSaveNotification
                                                       object:backgroundContext];

            if(![backgroundContext save:&error])
            {  
                //This is a serious error saying the record  
                //could not be saved. Advise the user to  
                //try again or restart the application.
            }

            [[NSNotificationCenter defaultCenter] removeObserver:self
                                                            name:NSManagedObjectContextDidSaveNotification
                                                          object:backgroundContext];
        }
    });
}

Answer 2

是的，如果您使用限制並發模型（這是通過init獲得的），那么必須保證僅在創建MOC的線程中使用MOC。

您可以使用NSPrivateQueueConcurrencyType創建MOC，然后使用

[moc performBlock:^{
}];

執行操作。 它具有自己的內部隊列，並且將在后台運行所有請求，從而將訪問與其他調用同步。

您可以使用NSMainQueueConcurrencyType將MOC綁定為僅在主線程上運行。