Question

dispatch_queue_t callerQueue = dispatch_get_current_queue();
dispatch_retain(callerQueue);
dispatch_queue_t downloadQueue = dispatch_queue_create("Download Queue",NULL);

dispatch_async(downloadQueue, 
^{
    //some code that accesses a web service
    dispatch_async(callerQueue,
    ^{
      //some code that accesses UI
    });
});
dispatch_release(downloadQueue);
NSLog(@"great successing!");

The problem is that “Great successing!” never shows up, and nothing ever happens beyond the end of the code outermost dispatch_async block. I’m not sure what I’m doing wrong but I know there is something seriously wrong with this.

Answer 1

You are releasing your download queue too early. You need to wait until after it has executed the block. The dispatch_async man page suggests putting the release at the end of the block.

It is important to remember to retain the destination queue before the first call to dispatch_async(), and to release that queue at the end of the completion callback to ensure the destination queue is not deallocated while the completion callback is pending.