Object released within block - objective-c

Question

I'm pretty sure I know what is happening, however I want to know if there is a nice way of stopping it from happening.
Basically, I have a class method which looks something up from the core data store, and if nothing exists attempts to fetch it from a web server. The core data lookup and request are performed in the managed object contexts performBlock method.
I have the following block of code:
[context performBlock:^{
__block NSError *error;
NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:NSStringFromClass([self class])];
[request setSortDescriptors:#[[[NSSortDescriptor alloc] initWithKey:key ascending:asc selector:#selector(caseInsensitiveCompare:)]]];
NSFetchedResultsController *controller = [[NSFetchedResultsController alloc] initWithFetchRequest:request
managedObjectContext:context
sectionNameKeyPath:keyPath
cacheName:nil];
[controller performFetch:&error];
if (!controller.fetchedObjects || controller.fetchedObjects.count == 0) {
// Nothing found or an error, query the server instead
NSString *url = [NSString stringWithFormat:#"%#%#", kMP_BASE_API_URL, [self baseURL]];
MPRequest *objRequest = [MPRequest requestWithURL:url];
[objRequest setRequestMethod:#"GET"];
[MPUser signRequest:objRequest];
[objRequest submit:^(MPResponse *resp, NSError *err) {
if (err) {
block(nil, err);
} else {
NSArray *objects = [self createListWithResponse:resp];
objects = [MPModel saveAllLocally:objects forEntityName:NSStringFromClass([self class])];
[controller performFetch:&error];
block(controller, nil);
}
}];
} else {
// Great, we found something :)
block (controller, nil);
}
}];
What is happening, is that the MPRequest object is created, and fired, however the submit method triggers an asynchronous request and thus returns almost instantly. I assume ARC is then releasing the MPRequest object. When the request is performed, the internal request object's delegate no longer exists, as ARC has released it (the MPRequest object is the delegate for the internal request it has). Because of this, the block that the submit method is provided with isn't called.
Is there any way I can prevent ARC for doing this without making the request synchronous?
Edit
The submit method of MPRequest looks like this
_completionBlock = block;
_responseData = [[NSMutableData alloc] init];
[self prepareRequest];
[self prepareRequestHeaders];
_connection = [[NSURLConnection alloc] initWithRequest:_urlRequest
delegate:self];
[self requestStarted];

Answer 1

Your MPRequest object needs to keep itself alive while the connection is running. The simplest way to do this is probably to retain itself when the connection is started, and then release itself after it calls the completion block. Under ARC, the simplest way to do this is
CFRetain((__bridge CFTypeRef)self);
and
CFRelease((__bridge CFTypeRef)self);
Conceptually, the run loop keeps the request alive. This is how NSURLConnection operates. But since MPRequest isn't actually attached to the run loop, it's just a wrapper around NSURLConnection, you need to do some work to keep it alive for the same period of time.
Also note that the NSURLConnection needs to be added to the appropriate runloop. The convenience method will add it to the current thread's runloop, but if you're in a background queue, that's not going to work. You can use something like
_connection = [[NSURLConnection alloc] initWithRequest:request delegate:delegate startImmediately:NO];
[_connection scheduleInRunLoop:[NSRunLoop mainRunLoop] forMode:NSDefaultRunLoopMode];

Answer 2

set this MPRequest outside of the context performBlock:^{...} like so:
__strong MPRequest = // allocate or set the MPRequest.
[context performBlock:^{...}
Now once the performBlock is finished, the MPRequest will not have been released, and still be set to whatever variable you designated it to in the __strong clause.