Creating queue of element with GCD - ios

Question

I have downloaded images and saved it to a GCD, updated the count of images and performed a post notification in queue. What is happening now is that it does not register that I have downloaded the images. Sometimes I am missing some images, and I think it is because I have missed something in my GCD logic.
Here is my code:
for (NSString *i in items)
{
[[RequestAPI sharedInstance]downloadImage:i completion:^(AFHTTPRequestOperation *operation, UIImage *image, NSError *error) {
//1. here main thread I receive images and go to BG
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
//2. here I save image on disk and get path
NSString *path = [ImageManager saveImageToDisk:image toEntity:entity withparams:#{#"save" : #"lala"}];
__block NSMutableDictionary *attachments = [NSMutableDictionary dictionary];
__block NSMutableArray *photoPaths = [NSMutableArray array];
dispatch_async(dispatch_get_main_queue(), ^{
//3. here I load entity and dictionary from it with NSKeyedUnarchiver from CD and set to it image path
if (entity.attachments)
{
attachments = [NSKeyedUnarchiver unarchiveObjectWithData:entity.attachments];
if (attachments[type])
{
photoPaths = attachments[type];
}
}
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
//4. here I check all images equality ti themselves in entity
BOOL haveDublicate = NO;
NSData *i = [ImageManager imageDataFromPath:path];
NSArray *photoImages = [ImageManager imageDatasFromPaths:photoPaths];
for (NSData *saved in photoImages)
{
if ([saved isEqualToData: i])
{
haveDublicate = YES;
}
}
if (!photoPaths)
{
photoPaths = [NSMutableArray array];
}
dispatch_async(dispatch_get_main_queue(), ^{
//5. and finally if all ok I save image path, change load counter and post notification
if (path.length
&& ![photoPaths containsObject:path]
&& !haveDublicate
)
{
[photoPaths addObject:path];
[savedLinks setObject:photoPaths forKey:type];
entity.attachments = [NSKeyedArchiver archivedDataWithRootObject:savedLinks];
[self saveContext];
}
[RequestAPI sharedInstance].downloadsCount -= 1;
[[NSNotificationCenter defaultCenter]postNotificationName:kReloadFeedData object:nil];
});
});
});
});
}];
I think here I need process 1-2-3-4-5 to get desired result. Am I right or how can I do this queuing?