ios - 在后台上下文中保存核心数据时出现 NSInternalInconsistencyException

Question

我已经阅读了有关此错误的所有内容，但仍然无法确定为什么它会在我的应用程序中发生。

使用后台上下文保存多个 Core Data 对象时出现以下错误:

*** 由于未捕获的异常“NSInternalInconsistencyException”而终止应用程序，原因:“保存前无法处理挂起的更改。100 次尝试后上下文仍然是脏的。通常这种递归脏化是由错误的验证方法引起的、-willSave 或通知处理程序。

在下面的代码中，ArticleManager 的addArticle 在主线程中循环调用。可能有 0-200 多篇文章要添加。此错误通常发生在文章计数 100-150 之间。

//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，所以阅读官方文档还是有点用的。

来自 Apple(强调我的):

Concurrency

Core Data uses thread (or serialized queue) confinement to protect managed objects and managed object contexts (see “Concurrency with Core Data”). A consequence of this is that a context assumes the default owner is the thread or queue that allocated it—this is determined by the thread that calls its init method. You should not, therefore, initialize a context on one thread then pass it to a different thread. Instead, you should pass a reference to a persistent store coordinator and have the receiving thread/queue create a new context derived from that. If you use NSOperation, you must create the context in main (for a serial queue) or start (for a concurrent queue).

所以我的问题是我在主线程上初始化了后台上下文，但随后通过 dispatch_async 使用了 Grand Central Dispatch，它在后台线程上执行保存(使用创建的上下文主线程)。

我通过将上下文初始化添加到背景 block 来修复它:

-(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];
        }
    });
}