Updating NSManagedObjects in the background

Question

I'm trying to update NSManagedObject's in the background based on properties from a network fetch. Having trouble wrapping my head around concurrency. What I have tried is

fetch NSManagedObjects(Asset) on a localcontext (NSPrivateQueueType)
enumerate over assets array perform a network GET request for each asset and add it to a dispatch_group
Within the completion block from the network call, map updated values to the asset (This is my problem)

My code looks something like this

__block dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    __block dispatch_group_t taskGroup = dispatch_group_create();

    [MagicalRecord saveWithBlock:^(NSManagedObjectContext *localContext) {
        NSArray *assets = [Asset MR_findAllInContext:localContext];
        for (Asset *asset in assets) {
            dispatch_group_enter(taskGroup);
            dispatch_async(queue, ^{
                [stockClient GET:@"/v1/public/yql" parameters:asset.assetSymbol success:^(NSURLSessionDataTask *task, id responseObject) {
                   //TODO:Check response
                    NSDictionary *stockData = [[[responseObject objectForKey:@"query"] objectForKey:@"results"] objectForKey:@"quote"];
                    [asset mapPropertiesFrom:stockData];//----------> How do I access the localcontext queue?
                    dispatch_group_leave(taskGroup);

                } failure:^(NSURLSessionDataTask *task, NSError *error) {
                    //TODO:Error handling here
                    dispatch_group_leave(taskGroup);
                }];
            });
        }

        dispatch_group_notify(taskGroup, dispatch_get_main_queue(), ^{
            NSLog(@"All Tasks are completed");
        });
    }];
}

When I try to update the asset(NSManagedObject) from the network completion block

[asset mapPropertiesFrom:stockData];

I receive

CoreData could not fulfill a fault for

I suspect this is because the completion block is on the main queue and my asset was fetched on a private queue and you can't access NSManagedObjects from different queues...but I'm not sure is this is the problem and if it is how to resolve it.

So,how could I complete such a task? Am I going about this completely wrong?

EDIT

Made some changes based on comments below, but I'm receiving error

//Setup Dispatch Group
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_group_t taskGroup = dispatch_group_create();

    NSArray *assets = [Asset MR_findAll];
    for (Asset *asset in assets) {
        dispatch_group_enter(taskGroup);
        dispatch_async(queue, ^{
            NSManagedObjectContext *localContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSConfinementConcurrencyType];
            localContext.parentContext = [NSManagedObjectContext MR_defaultContext];
            Asset *localAsset = [asset MR_inContext:localContext];
            [stockClient updateStockDataFor:localAsset completionHandler:^(NSDictionary *stockData, NSError *error) {
                NSManagedObjectContext *responseContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSConfinementConcurrencyType];
                responseContext.parentContext = [NSManagedObjectContext MR_defaultContext];
                Asset *responseAsset = [localAsset MR_inContext:responseContext];
                [responseAsset mapPropertiesFrom:stockData];
                [responseContext MR_saveToPersistentStoreAndWait];
                 dispatch_group_leave(taskGroup);

            }];
        });
    }
    dispatch_group_notify(taskGroup, dispatch_get_main_queue(), ^{
        NSLog(@"Tasks are Completed");
    });
}

Error:

Can only use -performBlock: on an NSManagedObjectContext that was created with a queue.

casademora casademora · Accepted Answer · 2014-07-25T16:43:51

As far as MagicalRecord is concerned here, you are dispatching a background task in the background. Basically, your queues are out of sync. I suggest you use a single background queue to save your data.

Try something like this:

__block dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
__block dispatch_group_t taskGroup = dispatch_group_create();


    NSArray *assets = [Asset MR_findAll];
    for (Asset *asset in assets) {
        dispatch_group_enter(taskGroup);
        dispatch_async(queue, ^{
            NSManagedObjectContext = //create a confinement context
            Asset *localAsset = [asset MR_inContext:localContext];
            [stockClient GET:@"/v1/public/yql" parameters:asset.assetSymbol success:^(NSURLSessionDataTask *task, id responseObject) {
               //TODO:Check response
                NSManagedObjectContext *responseContext = //create a confinement context
                Asset *responseAsset = [localAsset MR_inContext:responseContext];
                NSDictionary *stockData = [[[responseObject objectForKey:@"query"] objectForKey:@"results"] objectForKey:@"quote"];
                [asset mapPropertiesFrom:stockData];//----------> How do I access the localcontext queue?
                [responseContext MR_saveToPersistentStoreAndWait];
                dispatch_group_leave(taskGroup);

            } failure:^(NSURLSessionDataTask *task, NSError *error) {
                //TODO:Error handling here
                dispatch_group_leave(taskGroup);
            }];
        });
    }

    dispatch_group_notify(taskGroup, dispatch_get_main_queue(), ^{
        NSLog(@"All Tasks are completed");
    });

}

Updating NSManagedObjects in the background

2 Answers