Dealing with Blocks, completion handlers, dispatch_async vs dispatch_sync

Question

I'm executing an online fetch of data in a thread and I want to do something immediately after the block is executed.

Here's my code:

- (IBAction)refresh:(UIBarButtonItem *)sender {
    NSLog(@"checking");
    [self editToolbar];
    dispatch_queue_t fetchQ = dispatch_queue_create("Refreshing", NULL);
    dispatch_async(fetchQ, ^{
        [self setupFetchedResultsController];
        [self fetchImonggoItemsDataIntoDocument: self.itemDatabase];
    });
    dispatch_release(fetchQ); 

    NSLog(@"done checking");
    //do something here
}

The thing is dispatch_async returns immediately and "done checking" prints immediately even before the block is done executing. How do I solve this?

Why don't you put whatever it is you want to do at the end of the Block? Am I misunderstanding your question? — jscs, Mar 01 '12 at 03:35
Is the problem that `performBlock:` returns right away (runs asynchronously)? There's always `performBlockAndWait:`, as long as it's on a background thread. — gregheo, Mar 01 '12 at 04:04
@acecapades i think you need to debug your code and after that get what is the exact point from where your control returns !! — aks.knit1108, Mar 04 '12 at 10:57
@acecapades try these links http://stackoverflow.com/questions/3379008/how-can-i-be-notified-when-a-dispatch-async-task-is-complete http://stackoverflow.com/questions/8668585/dispatch-sync-call-into-a-dispatch-async-call — aks.knit1108, Mar 04 '12 at 11:50
thanks @Amit I tried that, but it still didn't work. Thanks though — acecapades, Mar 05 '12 at 01:24

score 4 · Accepted Answer · answered Mar 04 '12 at 15:38

I think it's an architectural issue. The tasks are something like:

edit toolbar
fetchImonggoItemsDataIntoDocument
do something else

If these must be done exactly in order then I don't quite understand the use of blocks or queues; just run the statements after each other and you'll be set.

Otherwise, alternative #1 would be to use dispatch_sync rather than dispatch_async. Again, I'm not quite sure what the benefit of using a queue would be but there it is.

Alternative #2 would be to use a callback from the block. Something like:

- (IBAction)refresh:(UIBarButtonItem *)sender {
    NSLog(@"checking");
    [self editToolbar];
    dispatch_queue_t fetchQ = dispatch_queue_create("Refreshing", NULL);
    dispatch_async(fetchQ, ^{
        [self setupFetchedResultsController];
        [self fetchImonggoItemsDataIntoDocument: self.itemDatabase];
        [self doneChecking]; // <-- NOTE! call the callback
    });
    dispatch_release(fetchQ); 
}

// NOTE! refresh: has been split up into two methods
- (void)doneChecking:
    NSLog(@"done checking");
    //do something here
}

It's also possible to enqueue another block to do the callback business, again using `dispatch_async`, but there's really no reason not to do it the way you've written here. (I don't even understand why this was a question...) — jscs, Mar 05 '12 at 01:24
It's worth pointing out that if `doneChecking` is going to operate on any UI elements that it should be done in the main thread and not in fetchQ. Use another dispatch call but just request the main queue. — Joey J, Apr 04 '13 at 06:09

score 1 · Answer 2 · answered Mar 01 '12 at 04:07

As others have already suggested, this is probably what you need.

NSArray *items = [iMonggoFetcher fetchImonggoData:IMONGGO_GENERIC_URL_FOR_PRODUCTS withFormat:@"json" withDateRangeArgs:args];
[document.managedObjectContext performBlock:^{
    for (NSDictionary *itemInfo in items){
        [Product productWithImonggoInfo:itemInfo inManagedObjectContext:document.managedObjectContext];
    }
    // Put here what you need :)
}];

Dealing with Blocks, completion handlers, dispatch_async vs dispatch_sync

2 Answers2

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