How do you call executeFetchRequest asynchronously? - ios

Question

I am using Core Data in an IOS application and am experiencing a bottleneck while fetching data. I am calling executeFetchRequests multiple times in a loop to fetch 1 result each time. Each fetch takes a short time, but since I am calling it about 500 times the fetch takes at least a second. I am having trouble calling executeFetchRequest using GCD.
My code looks like this. (I removed code that saves the data, since it is not the problem).
Setup Code (I am unsure if this should go inside the threaded code, it doesn't work either way).
NSManagedObjectContext *context = [self managedObjectContext];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Entity"
inManagedObjectContext:context];
NSFetchRequest *fetchrequest = [[NSFetchRequest alloc]init];
[fetchrequest setEntity:entity];
Setup GCD stuff
dispatch_group_t x_group = dispatch_group_create();
dispatch_queue_t x_queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
Iterate through each predicate
for (NSPredicate *predicate in arrayOfPredicates) {
[fetchrequest setPredicate:predicate];
dispatch_group_async(x_group, x_queue, ^{
NSError *error;
NSArray *array = [context executeFetchRequest:fetchrequest error:&error];
for (Entity *managedObject in array) {
// save stuff to array inside of thread to pass to an array using locks.
}
});
}
dispatch_group_wait(x_group, DISPATCH_TIME_FOREVER);
... more code here...
However this code never gets past dispatch_group_wait and when fetching using this method the fetched managedObject is always an empty string. How can I do this asynchronously, so that there isn't a long delay period?

Answer 1

The short answer is a context is not thread safe. See: Concurrency with Core Data
The longer answer is you may have the wrong approach. I would setup the code so all required objects can be fetched using a single request: even if I needed to change the schema to make that happen.
Update Example of Single Predicate
NSPredicate *predicate = [NSCompoundPredicate orPredicateWithSubpredicates:arrayOfPredicates];
[fetchrequest setPredicate:predicate];