issue with NSOperationQueue, weakSelf and blocks - iphone

Question

I have the following code:
[[AHPinterestAPIClient sharedClient] getPath:requestURLPath parameters:nil
success:^(AFHTTPRequestOperation *operation, id response) {
[weakSelf.backgroundQueue_ addOperationWithBlock:^{
[self doSomeHeavyComputation];
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
[weakSelf.collectionView_ setContentOffset:CGPointMake(0, 0)];
[weakSelf.collectionView_ reloadData];
[weakSelf.progressHUD_ hide:YES];
[[NSNotificationCenter defaultCenter] performSelector:#selector(postNotificationName:object:) withObject:#"UIScrollViewDidStopScrolling" afterDelay:0.3];
[weakSelf.progressHUD_ hide:YES];
}];
}];
}
failure:^(AFHTTPRequestOperation *operation, NSError *error) {
[weakSelf.progressHUD_ hide:YES];
[weakSelf.collectionView_.pullToRefreshView stopAnimating];
NSLog(#"Error fetching user data!");
NSLog(#"%#", error);
}];
For some reason this worked just fine in iOS 5, but not iOS 6 (it crashes). Now I am not going to ask about iOS 6 because it's still under NDA. What I want to know is, whether the code above is wrong? If yes how do I fix it.
If I put the code inside the block outside of the mainQueue then it's fine.
What I am trying to do here is to do the NSOperationQueue mainQueue only after the [self doSomeHeavyComputation] is done. So this is a dependency, how should I add this dependency?
Update:
Here's the crash log if it helps:

Answer 1

It is recommended to “unwind” weak references in the block, so please try this:
__weak id weakSelf = self;
[client getPath:path parameters:nil success:^(id op, id response) {
id strongSelf = weakSelf;
if (strongSelf == nil) return;
__weak id internalWeakSelf = strongSelf;
[strongSelf.backgroundQueue_ addOperationWithBlock:^{
id internalStrongSelf = internalWeakSelf;
if (internalStrongSelf == nil) return;
[internalStrongSelf doSomeHeavyComputation];
__weak id internalInternalWeakSelf = internalStrongSelf;
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
id internalInternalStrongSelf = internalInternalWeakSelf;
if (internalInternalStrongSelf == nil) return;
[internalInternalStrongSelf reloadCollectionView];
}];
}];
}
failure:^(id op, NSError *error) {
id strongSelf = weakSelf;
if (strongSelf == nil) return;
[strongSelf stopProgress];
NSLog(#"Error fetching user data: %#", error);
}];