StackOverflow2013

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POCore Data and multithreading
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copied!<p>I have read Marcus Zarra's chapter on multithreading in his Core Data book and have looked fairly closely at his sample code. But his code and others that I have found elsewhere seem to be focused in background processes that do <em>not</em> need to be aware of each other. These examples are good for importing a tree structure - but they do not address the importing of a more general (complex) structure, like a directed acyclic graph.</p> <p>In my case, I am trying to parse a C++ class hierarchy and would like to use as many NSOperations as possible. I would like to create an NSManagedObject instance for each encountered class and I would like to merge different NSManagedObjectContexts whenever one is saved.</p> <p>As an aside: I am able to get things working with a single NSOperation that iterates of files and parse one at a time. In this implementation, the -mergeChanges: approach that calls -mergeChangesFromContextDidSaveNotification: on main thread's MOC works well.</p> <p>But ideally, I would have one NSOperation iterate over source files and spawn NSOperations to parse each file. I have tried several approaches - but can't seem to get it right. The most promising was to have each NSOperation observing NSManagedObjectContextDidSaveNotification. With -mergeChanges: looking like this:</p> <pre><code>- (void) mergeChanges:(NSNotification *)notification { // If locally originated, then trigger main thread to merge. if ([notification object] == [self managedObjectContext]) { AppDelegate *appDelegate = (AppDelegate*)[[NSApplication sharedApplication] delegate]; NSManagedObjectContext *mainContext = [appDelegate managedObjectContext]; // Merge changes into the main context on the main thread [mainContext performSelectorOnMainThread:@selector(mergeChangesFromContextDidSaveNotification:) withObject:notification waitUntilDone:YES]; return; } // If not locally originated, then flag need to merge with in this NSOperation's thread. [self setNeedsToMerge:YES]; [self setMergeNotification:notification]; } </code></pre> <p>Essentially, the parsing NSOperation's main() checked ivar 'needsToMerge' periodically. If it was true, then -mergeChangesFromContextDidSaveNotification: was called on local MOC with cached NSNotifications. And then needsToMerge was reset. If notification had originated locally, then main thread was told to perform -mergeChangesFromContextDidSaveNotification: on its MOC.</p> <p>I am sure that there is a good reason why this didn't work and why I get this:</p> <blockquote> <p>warning: Cancelling call - objc code on the current thread's stack makes this unsafe.</p> </blockquote> <p>I have also tried to use NSPeristentStoreCoordinator's lock to control access - but this is problematic if it is held during a call to NSManagedObjectContext's -save: method because -save: will notify interested observers of save event and -mergeChangesFromContextDidSaveNotification: appears to block trying to acquire PSC's lock.</p> <p>It just seems that this should be a lot easier.</p>

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