iPhone - Grand Central Dispatch main thread

Dispatching a block to the main queue is usually done from a background queue to signal that some background processing has finished e.g.

- (void)doCalculation
{
    //you can use any string instead "com.mycompany.myqueue"
    dispatch_queue_t backgroundQueue = dispatch_queue_create("com.mycompany.myqueue", 0);

    dispatch_async(backgroundQueue, ^{
        int result = <some really long calculation that takes seconds to complete>;

        dispatch_async(dispatch_get_main_queue(), ^{
            [self updateMyUIWithResult:result];
        });    
    });
}

In this case, we are doing a lengthy calculation on a background queue and need to update our UI when the calculation is complete. Updating UI normally has to be done from the main queue so we 'signal' back to the main queue using a second nested dispatch_async.

There are probably other examples where you might want to dispatch back to the main queue but it is generally done in this way i.e. nested from within a block dispatched to a background queue.

• background processing finished -> update UI
• chunk of data processed on background queue -> signal main queue to start next chunk
• incoming network data on background queue -> signal main queue that message has arrived
• etc etc

As to why you might want to dispatch to the main queue from the main queue... Well, you generally wouldn't although conceivably you might do it to schedule some work to do the next time around the run loop.

Dispatching blocks to the main queue from the main thread can be useful. It gives the main queue a chance to handle other blocks that have been queued so that you're not simply blocking everything else from executing.

For example you could write an essentially single threaded server that nonetheless handles many concurrent connections. As long as no individual block in the queue takes too long the server stays responsive to new requests.

If your program does nothing but spend its whole life responding to events then this can be quite natural. You just set up your event handlers to run on the main queue and then call dispatch_main(), and you may not need to worry about thread safety at all.

Hopefully I'm understanding your question correctly in that you are wondering about the differences between dispatch_async and dispatch_sync?

dispatch_async

will dispatch the block to a queue asynchronously. Meaning it will send the block to the queue and not wait for it to return before continuing on the execution of the remaining code in your method.

dispatch_sync

will dispatch the block to a queue synchronously. This will prevent any more execution of remaining code in the method until the block has finished executing.

I've mostly used a dispatch_async to a background queue to get work off the main queue and take advantage of any extra cores that the device may have. Then dispatch_async to the main thread if I need to update the UI.

Good luck