method not called with dispatch_async and repeating NSTimer - ios

Question

I am developing an app where i want to call method in separate queue using dispatch_async. I want to call that method repeatedly after certain interval of time. But the method is not getting called.
I don't know whats wrong. Here is my code:
dispatch_async( NotificationQueue, ^{
NSLog(#"inside queue");
timer = [NSTimer scheduledTimerWithTimeInterval: 20.0
target: self
selector: #selector(gettingNotification)
userInfo: nil
repeats: YES];
dispatch_async( dispatch_get_main_queue(), ^{
// Add code here to update the UI/send notifications based on the
// results of the background processing
});
});
-(void)gettingNotification {
NSLog(#"calling method ");
}

Answer 1

If you want a repeating timer to be invoked on a dispatch_queue_t, use dispatch_source_create with a DISPATCH_SOURCE_TYPE_TIMER:
dispatch_queue_t queue = dispatch_queue_create("com.firm.app.timer", 0);
dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
dispatch_source_set_timer(timer, dispatch_walltime(NULL, 0), 20ull * NSEC_PER_SEC, 1ull * NSEC_PER_SEC);
dispatch_source_set_event_handler(timer, ^{
// stuff performed on background queue goes here
NSLog(#"done on custom background queue");
// if you need to also do any UI updates, synchronize model updates,
// or the like, dispatch that back to the main queue:
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(#"done on main queue");
});
});
dispatch_resume(timer);
That creates a timer that runs once every 20 seconds (3rd parameter to dispatch_source_set_timer), with a leeway of a one second (4th parameter to dispatch_source_set_timer).
To cancel this timer, use dispatch_source_cancel:
dispatch_source_cancel(timer);

Answer 2

Try this code
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
dispatch_async( dispatch_get_main_queue(), ^{
timer = [NSTimer scheduledTimerWithTimeInterval: 1.0 target: self
selector: #selector(gettingNotification) userInfo: nil repeats: YES];
// Add code here to update the UI/send notifications based on the
// results of the background processing
});
});
-(void)gettingNotification {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//background task here
dispatch_async( dispatch_get_main_queue(), ^{
// update UI here
);
});
}

Answer 3

int parameter1 = 12;
float parameter2 = 144.1;
// Delay execution of my block for 10 seconds.
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 10 * NSEC_PER_SEC), dispatch_get_current_queue(), ^{
NSLog(#"parameter1: %d parameter2: %f", parameter1, parameter2);
});
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