ios - Grand Central Dispatch, viewWillAppear, viewDidAppear order of execution confusion

Question

I'm using GCD for background downloading in my Tab Bar app.

First step is to do some background downloading in -viewWillAppear: (to setup some basic data before the view is loaded).

Second step is to the rest of the background downloading in -viewDidAppear:

For some reason, the dispatch block in -viewDidAppear: gets called before the dispatch block in -viewWillAppear:.

This only happens once after loading the application for the first time switching to the tab with the GCD background methods. Switching to another tab and then switching back to the tab with the GCD background methods. The third (and all the rest subsequent times) time I'm switching back it's works as expected (-viewWillAppear: firing first and then -viewDidAppear:).

Here are excerpts of my code (-viewWillAppear: and -viewDidAppear:):

-viewWillAppear:

- (void)viewWillAppear:(BOOL)animated {
    DLog(@"viewWillAppear method running");

    [super viewWillAppear:animated];

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES];

        [self setDiskCareerIds:[CareersParser idsFrom:@"disk"]];
        [self setDownloadedCareerIds:[CareersParser idsFrom:@"web"]];


        DLog(@"diskCareerIds after being set in viewWillAppear: %@", [self diskCareerIds])
        DLog(@"downloadedCareerIds after being set in viewWillAppear: %@", [self downloadedCareerIds])

        if ([[self downloadedCareerIds] isEqualToArray:[self diskCareerIds]]) {

            DLog(@"viewWillAppear: ids equal, loading careers from disk.");
            self.careers = [CareersParser loadCareersFromDisk];

            dispatch_async(dispatch_get_main_queue(), ^{

                [self.table reloadData];

                [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];


            });
        }

    });

    //[self downloadData];
}

-viewDidAppear:

- (void)viewDidAppear:(BOOL)animated {
    DLog(@"viewDidAppear method running");

    [super viewDidAppear:animated];

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES];

        if (![[self downloadedCareerIds] isEqualToArray:[self diskCareerIds]]) {

            DLog(@"ids not equal, saving careers to disk.");

            dispatch_async(dispatch_get_main_queue(), ^{

                [self showLoadingView];

            });

            [CareersParser saveCareersToDisk];
            self.careers = [CareersParser loadCareersFromDisk];
        }



        dispatch_async(dispatch_get_main_queue(), ^{

            [self.table reloadData];

            [[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];

            [self removeLoadingView];

        });
    });

    //[self download3];

    //[self downloadData];
}

Check the debug log at Pastie.