NSTimer do not work in another thread

Question

I have a problem about using NSTimer and the code is as below, I have simplified the code:

- (void) mainThreadFun
{
    [NSTimer scheduledTimerWithTimeInterval:0.02 target:self selector:@selector(test) userInfo:nil repeats:YES];

    dispatch_async(dispatch_get_global_queue(0, 0), ^{
           [NSTimer scheduledTimerWithTimeInterval:0.02 target:self selector:@selector(test1) userInfo:nil repeats:YES];
    });

}

I found that the NSTimer in the mainThread work but the NSTimer in another thread did not work. Why does this happen and how can I fix this?

possible duplicate of [How do I create a NSTimer on a background thread?](http://stackoverflow.com/questions/8304702/), [Problem with thread and NSTimer](http://stackoverflow.com/questions/3335114/), [Running a timer within an NSThread](http://stackoverflow.com/questions/2083737/), [Threaded NSTimer](http://stackoverflow.com/questions/3156452/), [NSThread and NSTimer](http://stackoverflow.com/questions/1338891/) — jscs, May 30 '12 at 02:39

score 5 · Answer 1 · answered May 30 '12 at 03:07

5

You cannot use NSTimer on a GCD queue. NSTimer requires an NSRunLoop to operate, and GCD queues do not have an NSRunLoop.

If you want timer functionality with GCD queues, you should be using either dispatch_after() for a one-shot timer, or a dispatch_source for repeating timers.

answered May 30 '12 at 03:07

Lily Ballard

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@JacquesCousteau: That is incorrect. Queues do not get drained by runloops. The only exception is the main runloop does drain the main queue. You're confused because calling `+[NSRunLoop currentRunLoop]` will actually create one for the current thread if one doesn't exist. But that doesn't mean the runloop is actually running, because it's not. – Lily Ballard May 30 '12 at 04:16
@JacquesCousteau: Then either you're actually on the main queue or the documentation is wrong. Try pausing in the debugger and look at the backtrace. NSRunLoop is nowhere to be found. – Lily Ballard May 30 '12 at 04:18
@JacquesCousteau: BTW, you may be fooling yourself if you're using a `dispatch_sync()` onto a global queue to test this. `dispatch_sync()` will typically run the block on the current thread if it can. – Lily Ballard May 30 '12 at 04:20
@JacquesCousteau: Well yes, of *course* they're on threads. You can't run code outside of a thread. But they have no run loop. Here's a [sample file](https://gist.github.com/2833771) that demonstrates this. The first `dispatch_sync()` will run on thread 1 with a run loop, but the `dispatch_group_async()` runs on thread 2 with no run loop. – Lily Ballard May 30 '12 at 04:25

NSTimer do not work in another thread

1 Answers1