Timer inside global queue is not calling in iOS

Question

-(void)viewDidLoad{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        [NSTimer scheduledTimerWithTimeInterval:0.10 
                                         target:self 
                                       selector:@selector(action_Timer) 
                                       userInfo:nil 
                                        repeats:YES];
        }
    );
}

-(void)action_Timer{
    LOG("Timer called");
}

action_Timer is not being called. I dont know why. Do you have any idea?

Answer 1

You're calling +[NSTimer scheduledTimerWithTimeInterval:...] from a GCD worker thread. GCD worker threads don't run a run loop. That's why your first try didn't work.

When you tried [[NSRunLoop mainRunLoop] addTimer:myTimer forMode:NSDefaultRunLoopMode], you were sending a message to the main run loop from a GCD worker thread. The problem there is NSRunLoop is not thread-safe. (This is documented in the NSRunLoop Class Reference.)

Instead, you need to dispatch back to the main queue so that when you send the addTimer:... message to the main run loop, it's done on the main thread.

-(void)viewDidLoad {
    [super viewDidLoad];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSTimer *timer = [NSTimer timerWithTimeInterval:0.10 
                                         target:self 
                                       selector:@selector(action_Timer) 
                                       userInfo:nil 
                                        repeats:YES];
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSRunLoop mainRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];
        });        
    });
}

Realistically, there's no reason to create the timer on the background queue if you're going to schedule it in the main run loop. You can just dispatch back to the main queue to create and schedule it:

-(void)viewDidLoad {
    [super viewDidLoad];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"on background queue");
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"on main queue");
            [NSTimer scheduledTimerWithTimeInterval:0.10 
                                         target:self 
                                       selector:@selector(action_Timer) 
                                       userInfo:nil 
                                        repeats:YES];
        });        
    });
}

Note that both of my solutions add the timer to the main run loop, so the timer's action will run on the main thread. If you want the timer's action to run on a background queue, you should dispatch to it from the action:

-(void)action_Timer {
    // This is called on the main queue, so dispatch to a background queue.
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        LOG("Timer called");
    });
}

Answer 2

You have to add the timer to the main run loop for the timer to fire, but first, you should hold reference to to the timer in a private ivar or a property:

-(void)viewDidLoad{
        // on the main queue
        self.mytimer = [NSTimer scheduledTimerWithTimeInterval:0.10 
                                                        target:self 
                                                      selector:@selector(action_Timer) 
                                                      userInfo:nil 
                                                        repeats:YES];
        [[NSRunLoop mainRunLoop] addTimer:myTimer forMode:NSDefaultRunLoopMode];
}

-(void)action_Timer{
    dispatch_async( dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
        LOG("Timer called");
    });
}

I find easier to get off the main queue in the called method. At somepoint, maybe in viewDidUnlod or in dealloc, you will have call [self.myTimer invalidate]; self.myTimer = nil;.

• NSTimer
• NSRunLoop
• NSObject