Question

To start with it all works without using the the GCD but I want this happening in a separate thread so trying GCD. I’ve got a login screen where on pressing the login button i’ve got the following action:

- (void)login
{
    dispatch_queue_t buckyballLoginFetcherQ = dispatch_queue_create("Login Queue", NULL);
    dispatch_async(buckyballLoginFetcherQ, ^
    {
        NSDictionary *resultDictionary = [MyService login:self.name.text password:self.password.text];
        self.userDetails = [resultDictionary valueForKey:USER_DETAILS_ATTRIBUTE];
        [self performSegueWithIdentifier:@"Login" sender:self];
    });
}

In MyService method being called above:

+ (NSDictionary *)executeRequest:(NSDictionary *)requestDictionary
{
    // Prepare the URL request and do the following
    NSData *results = [NSURLConnection sendSynchronousRequest:urlRequest returningResponse:&response error:&urlRequestError];

    // Process results
    ...
}

NOW the bit that crashes:

-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
if ([segue.identifier isEqualToString:@"Login"])
{
    MyDestinationTableViewController *myDestinationTableViewController = nil;
    UITabBarController *tbc = (UITabBarController *)[segue destinationViewController];
    for (UIViewController *vc in [tbc viewControllers])
    {
        if ([vc isKindOfClass:[UINavigationController class]])
        { // in our case all view controlers are navigation controllers :-)
            UINavigationController *nc = (UINavigationController *)vc;
            if ([[[nc viewControllers] lastObject] isKindOfClass:[BuckyballsTableViewController class]])
            {
                myDestinationTableViewController = [[nc viewControllers] lastObject];

                /**************CRASH LINE************/
                buckyballsTableViewController.userDetails = self.userDetails;
            }
        }
    }
}

Again without GCD it works, but it holds up screen so i’d want to do it asynchronously. Is it the instance member causing a problem? OR do i need to use it differently or do more with it? Thank you…

Answer 1

Only the main thread may manipulate the UI, so use this code fragment to call those bits on the main thread:

dispatch_async(dispatch_get_main_queue(), ^{
    [self performSegueWithIdentifier:@"Login" sender:self];
});