GCD timer never fires

Question

I have a bare single-view iOS app with the following in the -viewDidLoad of the view:

dispatch_queue_t q_default;
q_default = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, q_default); //run event handler on the default global queue
dispatch_time_t now = dispatch_walltime(DISPATCH_TIME_NOW, 0);
dispatch_source_set_timer(timer, now, 30ull*NSEC_PER_SEC, 5000ull);
dispatch_source_set_event_handler(timer, ^{
    printf("test\n");
});
dispatch_resume(timer);

This is taken directly from the docs (except for a simplified printf() argument). The block is never executed--can someone tell me why??

Additional Information

I was trying this in a larger app to no avail. I then backed out to the barebones app, tried with ARC both on and off, tried this code in -appDidFinishLaunching..., all with no luck. I can surround this code with NSLogs, both of which are printed. I've checked timer--it is not nil.

Answer 1

So, the problem was that I'd lost my reference to timer when the surrounding scope was destroyed. Changing timer to an ivar instead of an automatic variable fixed things...

Answer 2

Per the documentation of dispatch_source_create:

Dispatch sources are created in a suspended state. After creating the source and setting any desired attributes (for example, the handler or the context), your application must call dispatch_resume to begin event delivery.

So your timer never fires because it's suspended. To end its suspension you need to call dispatch_resume.