How to dispatch on main queue synchronously without a deadlock?

Question

I need to dispatch a block on the main queue, synchronously. I don’t know if I’m currently running on the main thread or no. The naive solution looks like this:

Answer 1

I need to use something like this fairly regularly within my Mac and iOS applications, so I use the following helper function (originally described in this answer):

void runOnMainQueueWithoutDeadlocking(void (^block)(void))
{
    if ([NSThread isMainThread])
    {
        block();
    }
    else
    {
        dispatch_sync(dispatch_get_main_queue(), block);
    }
}

which you call via

runOnMainQueueWithoutDeadlocking(^{
    //Do stuff
});

This is pretty much the process you describe above, and I've talked to several other developers who have independently crafted something like this for themselves.

I used [NSThread isMainThread] instead of checking dispatch_get_current_queue(), because the caveats section for that function once warned against using this for identity testing and the call was deprecated in iOS 6.