Question

I have NSOperationQueue with 3 NSInvocationOperation in a prepareForSegue callback and I need after finishing all async tasks move to another view controller.
How can I be notified about finishing all async tasks and only after this move to another screen?

I tried this without success:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

});

EDIT:

It seems that I not understand something :

-(void)initPurchase{

    [operationQueue cancelAllOperations];

    NSInvocationOperation *downloadImageOperation = [[NSInvocationOperation alloc] initWithTarget:[BSImageDownloader getInstance] selector:@selector(downloadImageSync:) object:@"http://.........jpg"];


    NSInvocationOperation *createImageOperation = [[NSInvocationOperation alloc] initWithTarget:[BSImageCreator getInstance] selector:@selector(createImage:) object:@"dsadsadsa"];


    NSInvocationOperation *saveImageOperation = [[NSInvocationOperation alloc] initWithTarget:[BSImageSaver getInstance] selector:@selector(saveImageAsPng:) object:[BSSharedObject getInstance].createdImage];

    [createImageOperation addDependency:downloadImageOperation];
    [saveImageOperation addDependency:createImageOperation];


    [operationQueue addOperation:downloadImageOperation];
    [operationQueue addOperation:createImageOperation];
    [operationQueue addOperation:saveImageOperation];


}


-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender{

    [[BSPopupManager getInstance]showWaitingPopup];


    dispatch_group_t group = dispatch_group_create();

    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), 
                         ^{
                             Purchase *purchase = [[Purchase alloc] init];
                             [purchase initPurchase];
                         });

    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        [[BSPopupManager getInstance] closeWaitingPopup];
        BSPurchaseViewController *purchaseViewController = [segue destinationViewController];
        purchaseViewController.pngImage = [BSSharedObject getInstance].createdImage;


 NSLog(@"2");
    });

    dispatch_release(group);

}
}

I still get NSLog(@”2″); earlier than I save image in 3rd NSInvocationOperation

Answer 1

You can use the dispatch_group_async() function if you intend to use GCD to perform your asynchronous tasks: Grand Central Dispatch (GCD) Reference

dispatch_group_t group = dispatch_group_create();

dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), 
^{
     //Code here is executed asynchronously
});

dispatch_group_notify(group, dispatch_get_main_queue(), 
^{
   //Do something when async has completed
   //Note: You are not required to use the main 
   //queue if you aren't performing any UI work.
});

dispatch_release(group);