I'm currently having an issue where creating a new object on a background child thread (whose parent is the main UI thread context) and saving causes my NSFetchedResultsController to show two new objects: one with a temporary objectID, and one with a permanent objectID. This seems to be a bug of some sort, unless I'm missing something.
So I thought I would manually obtain permanent IDs for any new objects I create. This fixes the duplicate row issue, but introduces new random errors (such as "could not fulfill fault for object", refering to the new object I created). If anyone has any ideas as to why any of the previously mentioned is happening, please share.
I'm guessing obtainPermanentIDs is a step in the right direction. But when do I call this method? Before saving to the child context? After saving the child and before the parent? After the parent?
Currently my setup is this:
masterMOC - private queue tied to the persistent store, so physical saves happen here
----mainMOC - main queue tied to the UI, child of masterMOC
-------backgroundMOC - private queue, child of mainMOC
So if I create a new object on backgroundMOC, and I intend to immediatly save to disk (which means I'll have to call save: on all three contexts), where should I be calling obtainPermanentIDs?
(or if anyone has a different solution other than calling obtain permanent ids? What problem was this method introduced to solve anyway? Why would I want to call this method?)
Update:
I think I figured out what's going on (it's only a theory though), though not how to solve it. Core Data apparently generates permanent IDs for objects when they are saved physically to disk. So in my case, this won't happen until I call save on the masterMOC. Currently what I do when creating a new object on the backgroundMOC is:
save on backgroundMOC (so that changes are pushed up one level to the mainMOC and the my table view can insert the new rows)
save on mainMOC (so that I can prepare for saving to disk)
save on masterMOC (which finally saves to disk)
What's happening here is that calling save on the backgroundMOC triggers a UI update, and causes the fetched results controller to insert a new object that still has only a temporary ID. But then calling save on masterMOC causes all objects to get assigned permanent IDs, which causes another UI update, inserting another row for this "new" object! By commenting out the last masterMOC save, I no longer see duplicate entries. Am I doing something wrong here, or is this some kind of bug?
Another update: I think I've pretty much confirmed the bug. I call save on the backgroundMOC and then set up a timer to call save on the mainMOC and masterMOC 5 seconds later. Immediatley upon saving to the backgroundMOC, a new row is inserted into my table. 5 seconds later (upon saving main and master), another new row is inserted. (the row inserted first has a temp id, and the newest insert has permanent id).
I had the exact same issue, of course after a particularly difficult and dispiriting day of debugging everything to find out the issue was temporary IDs. :)
I have the exact same structure as you, and I also have subclasses of NSManagedObjectContext to codify the behavior I expect of saves in the background and main contexts – namely, a save in the background context should save the main context (and the main context should sync any objects that changed with the external service, which is irrelevant but worth mentioning as an explanation for why I have two subclasses), and a save in the main context should save the master context.
In my RFSImportContext subclass (equivalent to your backgroundMOC), I implement - save: to call [super save:], then call [self.parentContext performBlock:] (self.parentContext here is equivalent to your mainM)C, where the block calls obtainPermanentIDsForObjects: with the contents of the main context's - updatedObjects and - insertedObjects arrays, then I save the main context.
I no longer have the leaking of temporary objects into my NSFetchedResultsController as you describe. A way to improve the situation a bit would be to use the RFSMainContext subclass (again, equivalent to your mainMOC) to implement - save: to obtain permanent object IDs, save itself, then save the master context. This codifies the behavior that we always want the main context to have permanent IDs for objects in it when it is saved.
Related
I'm reading the doc of refresh(_:mergeChanges:) of NSManagedObjectContext.
If flag is NO, then object is turned into a fault and any pending
changes are lost. The object remains a fault until it is accessed
again, at which time its property values will be reloaded from the
store or last cached state.
If flag is YES, then object is turned into a fault and object’s property values are reloaded from the values from the store or the
last cached state then any changes that were made (in the local
context) are re-applied over those (now newly updated) values. (If
flag is YES the merge of the values into object will always succeed—in
this case there is therefore no such thing as a “merge conflict” or a
merge that is not possible.)
I have a mainQueueConcurrencyType typed NSManagedObjectContext as a Child of an privateQueueConcurrencyType one.
When I create objects in the private/parent one, even they are not saved, I can access them using NSManagedObjectID in the child context.
So in this case, is the Parent Context the provider of "last cached state"?
In the "one liner" description of refresh(_:mergeChanges:), it only says
Updates the persistent properties of a managed object to use the
latest values from the persistent store.
That's really confusing to me.
I've read the Parent Store section of the doc. So the child context doesn't use a coordinator. That conflicts with the refresh(_:mergeChanges:)'s statement, does it?
I'm trying to build a caching system for a feed reading application. The idea is each time a new feed is successfully pulled, remove all stored entities in Core Data, and store the first twenty items of the feed (this is used as an offline cache).
The issue I'm running into is my managed object context may have hundreds of items in it when a pull to refresh is performed. I'd like to keep those items in the context while removing any stored items from Core Data and then store the twenty items returned from the refresh call.
For what it's worth, I'm using Magical Record. I've tried looking around for this solution, but either I'm using the wrong keywords or the information is hard to find.
I'm not sure what code to show exactly, but here's the handling of the feed call:
for (id dict in feedArray){
WFeedItem *item = [WFeedItem feedItemWithAttributes:[dict dictionaryByReplacingNullsWithBlanks] inManagedObjectContext:[NSManagedObjectContext defaultContext]];
[parsedArray addObject:item];
}
This gets passed back from the subclassed HTTPClient it's defined in to a view controller that has called it. Bear in mind, this all works fine, it's all a matter of deleting stored items while retaining everything I've gathered during this session in the context.
Just use a different context for importing and storing the new records. Your original object context can remain as it is.
I seem to be having an issue with my managed objects not releasing memory. I have an NSOperation that downloads new information, saves it to a temp context, then merges it to the main context. All this works well but in the allocations instrument all the newly created properties stick around in memory after the merge is complete and the entire operation is deallocated. Is there any way to fix this? I've tried to reset both the temp and main contexts and refreshObject:, both don't fix this.
Thanks!
Make sure that you create an NSAutoreleasePool first thing in the operations main. Before you return out of main make sure you drain the pool. This includes returns before the end of the method such as inside an if statement.
I am having a few nagging issues with NSFetchedResultsController and CoreData, any of which I would be very grateful to get help on.
Issue 1 - Updates: I update my store on a background thread which results in certain rows being delete, inserted or updated. The changes are merged into the context on the main thread using the "mergeChangesFromContextDidSaveNotification:" method. Inserts and deletes are updated properly, but updates are not (e.g. the cell label is not updated with the change) although I have confirmed the updates to come through the contextDidSaveNotifcation, exactly like the inserts and deleted. My current workaround is to temporarily change the staleness interval of the context to 0, but this does not seem like the ideal solution.
Issue 2 - Deleting objects: My fetch batch size is 20. If an object is deleted by the background thread which is in the first 20 rows, everything works fine. But if the object is after the first 20 rows and the table is scrolled down, a "CoreData could not fulfill a fault" error is raised. I have tried resaving the context and reperforming the frc fetch - all to no avail. Note: In this scenario, the frc delegate method "didChangeObject...." is not called for the delete - I assume this is because the object in question had not been faulted at that time (as it is was outside the initial fetch range). But for some reason, the context still thinks the object is around, although is has been deleted from the store.
Issue 3 - Deleting sections : When the deletion of a row leads to the deletion of a section, I have gotten the "invalid number of rows in section???" error. I have worked around this by removing the "reloadSection" line from the NSFetchedResultsChangeMove: section and replacing it with "[tableView insertRowsAtIndexPaths...." This seems to work, but once again, I am not sure if this is the best solution.
Any help would be greatly appreciated. Thank you!
I think all your problems relate to the fetched results controller's cache.
Issue 1 is caused by the FRC using the cached objects (whose IDs have not changed.) When you add or remove an object that changes the IDs and forces an update of the cache but changing the attributes of an object doesn't do so reliably.
Issue 2 is caused by the FRC checking for the object in cache. Most likely, the object has an unfaulted relationship that persist in the cache. When you delete it in the background the FRC tries to fault in the object at the other end of the relationship and cannot.
Issue 3: Same problem. The cache does not reflect the changes.
You really shouldn't use a FRC's cache when some object other than the FRC is modifying the data model. You have two options:
(Preferred) Don't use the cache. When creating the FRC set the cache property to nil.
Clear the cache anytime the background process alters the data model.
Of course, two defeats the purpose of using the cache in the first place.
The cache is only useful if the data is largely static and/or the FRC manages the changes. In any other circumstance, you shouldn't use it because FRC need to check the actual data model repeatedly to ensure that it has a current understanding of the data. It can't rely on the object copies it squirreled away because another input may have changed the real objects.
My advice:
Detect the changes needed on the background thread
Post the changes to the main thread as a payload
Make the actual changes and save on the main thread (Managed Object Context on the main thread)
DO use the cache for the FRC; you'll get better performance
Quote from "Pro Core Data for iOS" by Michael Privat, Robert Warner:
"Core Data manages its caches intelligently so that if the results are updated by another call, the cache is removed if affected."
Setup:
I have several UITableViewControllers (tvc) that all instantiate a separate NSFetchedResultsController (frc). All these frc-s, though, use the same ManagedObjectContext (moc).
Scenario:
Sometimes, from one of the tvc-s, I launch some background process to update data. The background process updates the data in a separate thread and finally calls [moc mergeChangesFromContextDidSaveNotification:] on the main thread. These changes should then propagate to frc, and [frcDelegate controllerDidChangeContent:] should be called. This works fine... but only for the frcDelegate whose associated tvc initiated the update. (all the tvc-s are acting as frcDelegate for their frc.)
Expected behavior:
since all the frc-s are bound to the same moc, all the frc-s should pick up the changes, and [frcDelegate controllerDidChangeContent:] should be called for all of them.
Actual behavior:
only one [frcDelegate controllerDidChangeContent:] gets called.
Why am I seeing actual behavior instead of expected? How should I debug this? Or is this the correct behavior?
This is the expected behavior.
Each fetched results controller will only observe changes in the entity that is specified in its fetch request. Presumably you are only modifying objects of the entity that corresponds to the controller that initiated the update.