Question

I’ve got a bunch of server requests which I can run asynchronously, but I need to wait for them all to finish before I continue.

dispatch_group_async and it seems quite reasonable, but I can’t get it to work. It either blocks forever or doesn’t block at all. My latest attempt looks something like….

dispatch_group_t group;

- (void)cleanRoom {
    NSAssert(![NSThread isMainThread], @"not on main thread.");
    group = dispatch_group_create();

    for (Junk *thing in myRoom) {
    // take it off the current thread
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            // register code against group
            dispatch_group_enter(attachmentGroup);
            NSURLConnectionWrapper *wrapper = [[NSURLConnectionWrapper alloc] init];
            wrapper.delegate = self;
            [wrapper sendRequestFor:thing];
        }];
    }

    // wait till the stuff is the group is done
    dispatch_group_wait(attachmentGroup, DISPATCH_TIME_FOREVER);
    NSLog(@"waiting complete!!!!");

    // process the results now that I have them all
}

- (void)wrapperConnectionDone {
    // do a bit more junk
    dispatch_group_leave(group);
}

This causes it to block forever because NSURLConnectionDelegate and NSURLConnectionDataDelegate methods are never getting called. I’m assume I’ve somehow block their thread, but using NSLog I can confirm the NSURLConnection is on a different thread than my cleanRoom method.

I read a bit about other threads not having run loops to make the callbacks, so I’ve tried things like connection setDelegateQueue:[NSOperationQueue mainQueue]] and [[NSRunLoop currentRunLoop] runUntilDate:[NSDate distantFuture]] but to no noticable effect.

+ sendAsynchronousRequest:queue:completionHandler: doesn’t work for me, I’ve got some ugly authentication. I’ve seen some good example with that, but I’ve failed at adapting.

I’m obviously missing some fundamental bit, but I can’t find it.

Answer 1

NSURLConnection needs to run on a thread with a processed run-loop. The simplest such thread is the main thread. So just dispatch_async these connection creations to dispatch_get_main_queue() and the rest of your dispatch_group logic should be fine. Keep in mind that the delegate methods will be called on the main thread (from the NSURLConnection Overview):

These delegate methods are called on the thread that started the asynchronous load operation for the associated NSURLConnection object.