invalidate NSTimer inside dispatch_async - ios

Question

I am creating a game, and inside the game I need to get a time counter.
I works great but when I scroll the map it stops and then when I finish scrolling it start again. I fix this with dispatch_async. Here is the code
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:#selector(timeNext) userInfo:nil repeats:NO];
[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];
[[NSRunLoop currentRunLoop] run];
});
Timer:
__block NSTimer *timer;
But if the app goes to the background I need to stop this times (and when it get active it starts again).
With this way I can scroll the map and the timer works well I try:
- (void)timeNext {
[timer invalidate];
}
and didin't work. Anyone knows how to stop it? or any other idea to accomplish this task?

Answer 1

Don't create your timer on a global queue's thread and then start a run loop on that thread.
Just create your timer on the main thread (main run loop), and add it to the tracking run loop mode. A timer can be added to multiple run loop modes.
timer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:#selector(timeNext) userInfo:nil repeats:NO];
[[NSRunLoop currentRunLoop] addTimer:timer forMode:UITrackingRunLoopMode];

Answer 2

If you're using UIScrollView to scroll via your game world, timer will be paused while scroll is tracking touches - you need to setup timer to use it with different mode - UITrackingRunLoopMode, e.g.
[[NSRunLoop currentRunLoop] addTimer:timer forMode:UITrackingRunLoopMode];
instead of
[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];
This will allow your timer to fire events while controls such as scroll is tracking user's touches.
[EDIT]
You create timer e.g. via
+ (NSTimer *)timerWithTimeInterval:(NSTimeInterval)ti target:(id)aTarget selector:(SEL)aSelector userInfo:(id)userInfo repeats:(BOOL)yesOrNo;
then add this timer to runloop (basically, you should add it to your app main runloop):
NSTimer* timer = [NSTimer timerWithTimeInterval:1./60. target:self selector:#selector(tick) userInfo:nil repeats:YES];
[[NSRunLoop mainRunLoop] addTimer:timer forMode:UITrackingRunLoopMode];

Answer 3

I use NSRunLoopCommonModes instead of UITrackingRunLoopMode, everythings looks fine. The main thread ui operation will not block tick function anymore.
NSTimer* timer = [NSTimer timerWithTimeInterval:1./60. target:self selector:#selector(tick) userInfo:nil repeats:YES];
[[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];