NSTimer callback on background thread - ios

Question

I have a NSTimer defined as follows:
timer = [NSTimer scheduledTimerWithTimeInterval:30
target:self
selector:#selector(fooBar)
userInfo:nil
repeats:YES];
[[NSRunLoop mainRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
I want it to call the call back function fooBar in this case using a background thread. But when I check with if ([NSThread mainThread]) i'm always getting it on the main thread. Is there any other way aside from dispatching a thread from the callback function?

Answer 1

You are adding the timer to main thread. Your call back will also be in main thread. To schedule the timer in a background thread, I think you need to use NSOperation subclass and schedule the timer to [NSRunLoop currentRunLoop] from inside the operation's main method.
#import <Foundation/Foundation.h>
#interface BackgroundTimer : NSOperation
{
BOOL _done;
}
#end
#import "BackgroundTimer.h"
#implementation BackgroundTimer
-(void) main
{
if ([self isCancelled])
{
return;
}
NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:30
target:self
selector:#selector(fooBar)
userInfo:nil
repeats:YES];
[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
//keep the runloop going as long as needed
while (!_done && [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode
beforeDate:[NSDate distantFuture]]);
}
#end

Answer 2

If you want to run a timer on a background thread, the most efficient way to do that is with a dispatch timer:
#property (nonatomic, strong) dispatch_source_t timer;
and you can then configure this timer to fire every two seconds:
- (void)startTimer {
dispatch_queue_t queue = dispatch_queue_create("com.domain.app.timer", 0);
self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
dispatch_source_set_timer(self.timer, dispatch_walltime(NULL, 0), 2.0 * NSEC_PER_SEC, 0.1 * NSEC_PER_SEC);
dispatch_source_set_event_handler(self.timer, ^{
// call whatever you want here
});
dispatch_resume(self.timer);
}
- (void)stopTimer {
dispatch_cancel(self.timer);
self.timer = nil;
}