iOS核心數據dispatch_async後臺排隊崩潰

Question

我得到一個不可重現的崩潰，我不知道爲什麼。我在後臺隊列上緩存圖像。圖像名稱是Core Data NSManagedObject子類CCCard上的屬性。與此同時，我有一個訪問這些CCCards的集合視圖。

下面是相關的代碼和注意事項。

//CCDeckViewController.m -------------------------------------- 

- (void)viewDidLoad { 
    // This will fetch the CCCard objects from Core Data. 
    self.cards = [[CCDataManager shared] cards]; 

    [self cacheImages]; 
} 

- (void)cacheCardImages { 
    // Because these are NSManagedObjects, I access them in the background 
    // via their object ID. So pull the IDs out here. 
    NSArray *cardIds = [self.cards valueForKey:@"objectID"]; 

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{ 
     for (NSManagedObjectID *cardId in cardIds) { 

      // Fetch the actual CCCard object. 
      CCCard *card = (CCCard *)[[CCDataManager shared] objectWithId:cardId]; 

      // Cache the card's image if it's not already there. 
      NSString *key = [card thumbnailCacheKey]; 
      if ([self.cardImageCache objectForKey:key]) { 
       continue; 
      } 
      CCDiscardableImage *discardable = [CCHelper decompressedImageForPath:key size:[card thumbnailSize] tintable:[card tintable]]; 
      [self.cardImageCache setObject:discardable forKey:key]; 
     } 
    }); 
} 

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath { 
    CCCard *card = self.cards[indexPath.item]; 

    // This line calls the code that crashes. 
    UIColor *color = [card color]; 

    // More code that returns the cell. 
} 


// CCCard.m -------------------------------------- 

- (UIColor *)color { 
    // Crash happens on this line. 
    if (self.colorId.integerValue == 0) { 
     // some code 
    } 
} 

下面是從Crashlytics報告，我認爲堆棧跟蹤的部分最相關：

Thread #0: Crashed: com.apple.main-thread 
EXC_BREAKPOINT 0x000000000000defe 
0 CoreData      0x24c1b070 _sharedIMPL_pvfk_core + 247 
1 CoreData      0x24c1b071 _sharedIMPL_pvfk_core + 248 
2 myapp       0x4286f -[CCCard color] (CCCard.m:37) 
3 myapp       0x40bbb -[CCDeckViewController collectionView:cellForItemAtIndexPath:] (CCDeckViewController.m:127) 

Thread #4: com.apple.root.utility-qos 
0 libsystem_pthread.dylib  0x2321920c RWLOCK_GETSEQ_ADDR 
1 libsystem_pthread.dylib  0x23219295 pthread_rwlock_unlock + 60 
2 libobjc.A.dylib    0x22cb8e01 rwlock_tt<false>::unlockRead() + 8 
3 libobjc.A.dylib    0x22cb5af5 lookUpImpOrForward + 488 
4 libobjc.A.dylib    0x22cb5903 _class_lookupMethodAndLoadCache3 + 34 
5 libobjc.A.dylib    0x22cbbd7b _objc_msgSend_uncached + 26 
6 CoreData      0x24c1d3ab _PFObjectIDFastEquals64 + 38 
7 CoreFoundation     0x233eeed4 CFBasicHashFindBucket + 1820 
8 CoreFoundation     0x233ee775 CFDictionaryGetValue + 116 
9 CoreData      0x24c17037 _PFCMT_GetValue + 122 
10 CoreData      0x24c16ec7 -[NSManagedObjectContext(_NSInternalAdditions) _retainedObjectWithID:optionalHandler:withInlineStorage:] + 58 
11 CoreData      0x24c1b45d _PF_FulfillDeferredFault + 940 
12 CoreData      0x24c1afcf _sharedIMPL_pvfk_core + 86 
13 myapp       0x42991 -[CCCard imagePath] (CCCard.m:53) 
14 myapp       0x41d5b __39-[CCDeckViewController cacheCardImages]_block_invoke (CCDeckViewController.m:295) 

這是令人沮喪，因爲它的發展過程中從未發生過，所以無法測試任何理論。我正在嘗試瞭解代碼和崩潰報告以現在找出問題。我的猜測是它與錯誤有關，因爲它訪問CCCard對象的實例方法，但是當試圖從它讀取屬性時會崩潰。但爲什麼？

Answer 1

您違反了線程限制。

根據核心數據文檔，與其關聯的NSManagedObjectContext和任何NSManagedObject只能在分配給它的隊列上訪問。

您的dispatch_async違反該規則，需要更正。不再有像這樣在dispatch_async中使用核心數據的設計。

如果你想處理是異步的，那麼你應該產生一個私人的NSManagedObjectContext這是你的主要NSManagedObjectContext的孩子，然後執行一個-performBlock:反對它。這會給你正確配置的後臺處理。

此外，我強烈建議打開-com.apple.CoreData.ConcurrencyDebug 1，因爲這會在開發過程中發現類似問題。