NSTimer requiring me to add it to a runloop

Question

I am wondering if someone can explain why dispatching back to the main queue and creating a repeating NSTimer I am having to add it to RUN LOOP for it too fire? Even when using performselectorOnMainThread I still have to add it to a RUN LOOP to get it to fire.

Below is an example of my question:

#define queue dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
#define mainqueue dispatch_get_main_queue()

- (void)someMethodBeginCalled
{
    dispatch_async(queue, ^{
        int x = 0;
        dispatch_async(mainqueue, ^(void){
            if([_delegate respondsToSelector:@selector(complete:)])
                [_delegate complete:nil];
        });
    });
}

- (void)compelete:(id)object
{
    [self startTimer];

    //[self performSelectorOnMainThread:@selector(startTimer) withObject:nil waitUntilDone:NO];
}

- (void)startTimer
{
    NSTimer timer = [NSTimer timerWithTimeInterval:3 target:self selector:@selector(callsomethingelse) userInfo:nil repeats:YES];

    //NSDefaultRunLoopMode
    [[NSRunLoop currentRunLoop] addTimer:_busTimer forMode:NSRunLoopCommonModes];
}

EDIT: I believe I worded this question very poorly. I would like to know why [[NSRunLoop currentRunLoop] addTimer:_busTimer forMode:NSRunLoopCommonModes]; is necessary in startTimer if I call someMethodBeginCalled. If I don't include that line, the timer doesn't fire.

If I call startTimer from viewDidLoad for example, I can remove the NSRunLoop line and the timer will fire every 60 seconds.

Answer 1

You could always use this method instead:

NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:3 target:self selector:@selector(getBusLocation) userInfo:nil repeats:YES];

This will save you a line, as it will add it to the run loop automatically.

Answer 2

And here's how to add an NSTimer to a runloop:

NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
[runLoop addTimer:timer forMode:NSDefaultRunLoopMode];

Answer 3

Because, as the docs say:

Timers work in conjunction with run loops. To use a timer effectively, you should be aware of how run loops operate—see NSRunLoop and Threading Programming Guide. Note in particular that run loops retain their timers, so you can release a timer after you have added it to a run loop.

It is a design decision that Apple made when they wrote the code for NSTimer (and I'm sure they had good reason to do so) and there is nothing we can do to get around it. Is it really that burdensome?

Answer 4

Like @sosborn said, NSTimers depend on NSRunLoops, and since GCD queues create threads that don't have run loops, NSTimer doesn't play well with GCD.

Check out this other StackOverflow question on the matter: Is it safe to schedule and invalidate NSTimers on a GCD serial queue?

To solve that problem, I implemented MSWeakTimer: https://github.com/mindsnacks/MSWeakTimer (and had the implementation checked by a libdispatch engineer at the last WWDC!)

Answer 5

Adding the timer to the runloop didn't work in my case. I had to create the timer on the main thread. I was doing this thread creation in a MultipeerConnectivity delegate.

    dispatch_async(dispatch_get_main_queue(), ^{
        self.timer = [NSTimer scheduledTimerWithTimeInterval:self.interval  invocation: self.invocation repeats:YES];
    });

Answer 6

Timer method won't be called since GCD queues create threads that don't have run loops

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
   [NSTimer scheduledTimerWithTimeInterval:1 repeats:YES block:^(NSTimer * _Nonnull timer) {
       NSLog(@"Timer method from GCD main queue");
   }];

});

However when dispatched on main queue the timer method will be called as it will get added to main threads run loop.

dispatch_async(dispatch_get_main_queue(), ^{
   [NSTimer scheduledTimerWithTimeInterval:1 repeats:YES block:^(NSTimer * _Nonnull timer) {
       NSLog(@"Timer method from GCD main queue");
   }];

});