StackOverflow2013

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POCore Data and NSOperation
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copied!<p>I'm currently working with an <code>NSPersistentDocument</code> subclass that uses <code>NSOperation</code> to import data in the background. As per the documentation, I'm observing the <code>NSManagedObjectContextDidSaveNotification</code> after saving in the background task and propagating the notification to the <code>NSManagedObjectContext</code> in the main thread using <code>-mergeChangesFromContextDidSaveNotification:</code>.</p> <p>Everything works fine, but it presents a weird workflow for a user who's importing data into a new document. They need to save an empty document before doing the import (otherwise the <code>-save:</code> fails because the document hasn't configured a URL for the <code>NSPersistentStoreCoordinator</code>.) I don't see a way around this other than some kind of "new document setup" wizard that ensures <code>-writeToURL:ofType:forSaveOperation:originalContentsURL:error:</code> gets called before the import.</p> <p>Also, it appears that an import task in the background precludes the use of an <code>NSUndoManager</code> on the main thread. (I'm assuming that it's unsafe to share the managed object context's undo manager across the threads.) From a user's point-of-view, there's no way to undo all the new objects created during the import.</p> <p>I've read both the <em>Core Data Programming Guide</em> and Marcus Zarra's book, but I'm still new to this aspect of the framework. Hopefully, I've overlooked something: if not, I'll adapt my app to these restrictions (the benefits of Core Data far outweigh these user interface limitations.)</p> <p>Thanks for your time!</p> <p>--</p> <p>Based on Peter Hosey's suggestion below, I added the following code to create a temporary persistent store prior to the import:</p> <pre><code>NSPersistentStoreCoordinator *persistentStoreCoordinator = [self.managedObjectContext persistentStoreCoordinator]; if ([[persistentStoreCoordinator persistentStores] count] == 0) { // create an in-memory store to use temporarily NSError *error; NSPersistentStore *persistentStore = [persistentStoreCoordinator addPersistentStoreWithType:NSInMemoryStoreType configuration:nil URL:nil options:nil error:&error]; if (! persistentStore) { NSLog(@"error = %@", error); // TODO: better error handling } } </code></pre> <p>Then, after a file is selected in the save panel, the temporary persistent store is migrated to a SQLite store at the selected URL:</p> <pre><code>- (BOOL)writeToURL:(NSURL *)absoluteURL ofType:(NSString *)typeName forSaveOperation:(NSSaveOperationType)saveOperation originalContentsURL:(NSURL *)absoluteOriginalContentsURL error:(NSError **)error { NSPersistentStoreCoordinator *persistentStoreCoordinator = [self.managedObjectContext persistentStoreCoordinator]; for (NSPersistentStore *persistentStore in [persistentStoreCoordinator persistentStores]) { if (persistentStore.type == NSInMemoryStoreType) { // migrate the in-memory store to a SQLite store NSError *error; NSPersistentStore *newPersistentStore = [persistentStoreCoordinator migratePersistentStore:persistentStore toURL:absoluteURL options:nil withType:NSSQLiteStoreType error:&error]; if (! newPersistentStore) { NSLog(@"error = %@", error); // TODO: better error handling } } } return [super writeToURL:absoluteURL ofType:typeName forSaveOperation:saveOperation originalContentsURL:absoluteOriginalContentsURL error:error]; } </code></pre>

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