NSManagedObjectContext and NSArrayController reset / update problem

Question

I'm having trouble getting my user interface to reflect external changes (made by another process) in the sqlite data store.

I have a standard base view of NSArrayController / table based on master data. My application receives a notification that external changes have been made to the data, after which I do

[managedObjectContext reset]; // brute force, but data set is small 

The problem is that this clears all the data from the table. An array controller placed in objects is also empty. I thought the next

 [arrayController fetch:nil]; 

may help, but it is not. Executing a fetch request in the managedObjectContext file indicates that the data is present and updated, so managedObjectContext is aware of the changes.

Any tips on how to “restore” from reset? Or maybe the reset approach is completely wrong, in which case is there a better way to load external changes?

Answer 1

I do not think that two projects should work in one Core Data database. It is probably best to let one process act as a server that owns (and opens) the database and allows others to send commands to make changes. I don’t think that Core Data has always been intended to support several processes talking to the same db.

Answer 2

So, I played with this particular situation, and I will not contradict the first answer - you probably shouldn't do this, and if you need it, iCloud might work. Do not use this code. I provide it to help others get to the point that they can detect all the problems that I have not fixed, and move on.

However, you can do what the original question wanted to ask - to force the controller to reboot from disk:

 // Tear down bindings and context, create new context & rebind [self.watcherAC unbind:@"managedObjectContext"]; [self saveAction:self]; // Optional, dependent on NSMergePolicy, etc self.managedObjectContext = [[NSManagedObjectContext alloc] init]; self.managedObjectContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy; [self.managedObjectContext setPersistentStoreCoordinator:self.persistentStoreCoordinator]; [self.watcherAC bind:@"managedObjectContext" toObject:self withKeyPath:@"managedObjectContext" options:nil]; // Force controller to refetch and rearrange NSError* error; [self.watcherAC fetchWithRequest:nil merge:NO error:&error]; // Immediate fetch [self.watcherAC prepareContent]; [self.watcherAC rearrangeObjects]; 

This updates the contents of the tableView from storage on disk. (TableView is bound to watcherAC array controller)

I found that fetch: is not immediate - this was done the next time through the launch loop for the application. So that the fetch / swap is done in the correct order, you need to use fetchWithRequest:

I'm not sure if readyContent: is needed, although it may have helped fix errors in the contents of the controller.

I was not able to get it to restore the selection of tableView, although it may be because I am doing this in a delegate call to tableview, so the choice of the view does not synchronize with the choice of the controller no matter what attempts I try. Maybe this might work for someone, but I suggest trying to figure out how not to allocate a new MOC if your view has bindings to it.