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PORace Condition of coreData executeFetchRequest method cause the issue of nil data
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  1. PORace Condition of coreData executeFetchRequest method cause the issue of nil data
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    copied!<p>I run into a race condition that I have no clue how to solve it. getPurchasedBookTitles and getBookEntryIDs are in different threads using NSOperationQueue, they're competing with each other calling fetchBooks (which is a method in a different class). I put managedObjecContext (parent, child, root) with lock/unlock to avoid the problem of racing, but it doesn't seems to solve the root problem. </p> <p>The issue is that book.bookTitle in the for-loop (Book *book in allBooks) will be turned to nil sometime and leads to app crashing or hanging. </p> <p>Thanks in advance!</p> <p>// in a singleton class, dispatch_once</p> <pre><code>- (NSArray *)getPurchasedBookTitles { NSMutableArray *bookTitles = [[NSMutableArray alloc] init]; NSArray *allBooks = [[CoreDataManager sharedInstance] fetchBooks]; for (Book *book in allBooks) { if (book.bookTitle != nil &amp;&amp; book.is_owned != nil ) if (![bookTitles containsObject:book.bookTitle] &amp;&amp; [[book is_owned] boolValue] == YES) [bookTitles addObject:book.bookTitle]; } return bookTitles; } - (NSArray *)getBookEntryIDs { NSMutableArray *bookTitles = [[NSMutableArray alloc] init]; NSArray *allBooks = [[CoreDataManager sharedInstance] fetchBooks]; for (Book *book in allBooks) { if (book.bookTitle != nil) if (![bookTitles containsObject:book.bookTitle]) [bookTitles addObject:book.bookTitle]; } return bookTitles; } </code></pre> <p>// in the class coreDataManager // managedObjectContextChild, managedObjectContext, writerManagedObjectContext are declared and instantiated in the appDelegate and coreDataManager classes in details. </p> <pre><code>- (NSArray *)fetchBooks { NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"bookTitle" ascending:YES]; NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil]; NSFetchRequest *request = [[NSFetchRequest alloc] init]; [request setEntity:bookEntity]; [request setPredicate:nil]; [request setSortDescriptors:sortDescriptors]; [managedObjectContextChild lock]; [managedObjectContext lock]; [writerManagedObjectContext lock]; NSError *error = NULL; NSArray *results = [managedObjectContextChild executeFetchRequest:request error:&amp;error]; if (error != NULL) NSLog(@"Error fetching - %@", error); [writerManagedObjectContext unlock]; [managedObjectContext unlock]; [managedObjectContextChild unlock]; return results; } </code></pre>
 

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