running operation in background thread and have a completion block? - iphone

Question

I have a task which is reading from a disk, potentially going to take quite some time, so don't want to do it in a main thread.. and what I want is to call a function X after reading from the disk. What is the best way to do this in iOS?
So far this is what I've tried:
NSInvocationOperation *processDataOperation = [[NSInvocationOperation alloc] initWithTarget:self selector:#selector(readDisk:) object:nil];
[processDataOperation setQueuePriority:NSOperationQueuePriorityVeryHigh];
[processDataOperation setCompletionBlock:^(void){
NSMutableArray *feedItemsArray = [self generateFeedItemsFromDictionary:streamDiskData];
[self postFetchCompletedNotificationForDict:queryStringDict withFeedItems:feedItemsArray isFresh:NO];
}];
basically I am using NSInvocationOperation and then set it's completion block, however the issue is that in my completion block I need the result that is generated in readDisk. How do I access that in the completion block? It's nearly imposible right?

Answer 1

Using NSInvocations it is possible, but far more complicated than necessary, to achieve a trivial amount of work beyond the main thread.
Both GCD and NSOperations can be used to implement a wide array of concurrency strategies. From an object-oriented perspective, NSOperations are more highly abstracted than CGD blocks, which makes them (imo) easier to "design" with, and potentially optimized beyond the scope of where I'm implementing them. GCD is lower-level: This makes interacting with it appear slightly more complicated (it really isn't), but people who are in to that sorta stuff will tell you that it is "more efficient" and carries "less overhead".
My personal approach is to use NSOperations in scenarios where I have a designed/orchestrated concurrency pattern in my application, and use GCD for trivial concurrent/background operations.
If all I need to do is fire some arbitrary task that is not relevant to the design but needs to be done in the background, I'd use CGD. That's what I'd probably use in this case:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
[self readDisk];
NSMutableArray *feedItemsArray = [weakSelf generateFeedItemsFromDictionary:streamDiskData];
dispatch_sync(dispatch_get_main_queue(), ^{
//Call back to the main thread before performing/posting anything touching UIKit
[self postFetchCompletedNotificationForDict:queryStringDict withFeedItems:feedItemsArray isFresh:NO];
})
})];

Answer 2

You could always use grand central dispatch to do your operation in the background instead.
Since it is a block you can just call the method normally and store the result. Then grab the main queue if you need to update any UI or do whatever you need to after completion.
dispatch_queue_t queue = dispatch_queue_create("read disc", NULL);
dispatch_async(queue, ^{
result = [self readDisc];
dispatch_async(dispatch_get_main_queue(), ^{
//update UI or do whatever you need to do with the result of readDisc
});
});
dispatch_release(queue);