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

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

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

Answer 2

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.