Run repeating NSTimer with GCD?

Question

I was wondering why when you create a repeating timer in a GCD block it doesen't work?

This works fine:

-(void)viewDidLoad{
    [super viewDidLoad];
    [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(runTimer) userInfo:nil repeats:YES];
}
-(void)runTimer{
    NSLog(@"hi");
}

But this doesent work:

dispatch_queue_t myQueue;

-(void)viewDidLoad{
    [super viewDidLoad];

    myQueue = dispatch_queue_create("someDescription", NULL);
    dispatch_async(myQueue, ^{
        [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(runTimer) userInfo:nil repeats:YES];
    });
}
-(void)runTimer{
    NSLog(@"hi");
}

Answer 1

NSTimers are scheduled on the current thread's run loop. However, GCD dispatch threads don't have run loops, so scheduling timers in a GCD block isn't going to do anything.

There's three reasonable alternatives:

1. Figure out what run loop you want to schedule the timer on, and explicitly do so. Use +[NSTimer timerWithTimeInterval:target:selector:userInfo:repeats:] to create the timer and then -[NSRunLoop addTimer:forMode:] to actually schedule it on the run loop you want to use. This requires having a handle on the run loop in question, but you may just use +[NSRunLoop mainRunLoop] if you want to do it on the main thread.
2. Switch over to using a timer-based dispatch source. This implements a timer in a GCD-aware mechanism, that will run a block at the interval you want on the queue of your choice.
3. Explicitly dispatch_async() back to the main queue before creating the timer. This is equivalent to option #1 using the main run loop (since it will also create the timer on the main thread).

Of course, the real question here is, why are you creating a timer from a GCD queue to begin with?

Answer 2

NSTimer is scheduled to thread’s runloop. In code of question, runloop of thread dispatched by GCD is not running. You must start it manually and there must be a way to exit run loop, so you should keep a reference to the NSTimer, and invalidate it in appropriate time.

NSTimer has strong reference to the target, so target can't has strong reference to timer, and runloop has strong reference to the timer.

weak var weakTimer: Timer?
func configurateTimerInBackgroundThread(){
    DispatchQueue.global().async {
        // Pause program execution in Xcode, you will find thread with this name
        Thread.current.name = "BackgroundThreadWithTimer"
        // This timer is scheduled to current run loop
        self.weakTimer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(runTimer), userInfo: nil, repeats: true)
        // Start current runloop manually, otherwise NSTimer won't fire.
        RunLoop.current.run(mode: .defaultRunLoopMode, before: Date.distantFuture)
    }
}

@objc func runTimer(){
    NSLog("Timer is running in mainThread: \(Thread.isMainThread)")
}

If timer is invalidated in future, pause program execution again in Xcode, you will find that thread is gone.

Of course, threads dispatched by GCD have runloop. GCD generate and reuse threads internally, there threads are anonymous to caller. If you don't feel safe to it, you could use Thread. Don't afraid, code is very easy.

Actually, I try same thing last week and get same fail with asker, then I found this page. I try NSThread before I give up. It works. So why NSTimer in GCD can't work? It should be. Read runloop's document to know how NSTimer works.

Use NSThread to work with NSTimer:

func configurateTimerInBackgroundThread(){
    let thread = Thread.init(target: self, selector: #selector(addTimerInBackground), object: nil)
    thread.name = "BackgroundThreadWithTimer"
    thread.start()
}

@objc func addTimerInBackground() {
    self.weakTimer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(runTimer), userInfo: nil, repeats: true)
    RunLoop.current.run(mode: .defaultRunLoopMode, before: Date.distantFuture)
}

Answer 3

This is a bad Idea. I was about to delete this answer, but I left it here to avoid others from doing the same mistake I did. Thank you #Kevin_Ballard for pointing at this.

You'd only add one line to your example and it'd work just as you wrote it:

[[NSRunLoop currentRunLoop] run]

so you'd get:

    -(void)viewDidLoad{
        [super viewDidLoad];

        myQueue = dispatch_queue_create("someDescription", NULL);
        dispatch_async(myQueue, ^{
            [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(runTimer) userInfo:nil repeats:YES];
            [[NSRunLoop currentRunLoop] run]
        });
    }

Since your queue myQueue contains a NSThread and it contains a NSRunLoop and since the code in the dispatch_async is run the the context of that NSThread, currentRunLoop would return a stopped run loop associated with the thread of your queue.