Question

I’m trying to add multiple timers to a thread, not the main thread.Here is code:

- (IBAction)addTimer:(id)sender 
{
  if (!_timerQueue) {
    _timerQueue = dispatch_queue_create("timer_queue", NULL);
  }

  dispatch_async(_timerQueue, ^{
    NSTimer *tempTimer = [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(timerAction) userInfo:nil repeats:YES];
    [[NSRunLoop currentRunLoop] addTimer:tempTimer forMode:NSRunLoopCommonModes];
    [[NSRunLoop currentRunLoop] run];
  });
}

The method above is triggered by a button action. But the code in the dispatch block runs only once not matter how many times i click the button. So only one Timer in that thread. I wonder why?

Answer 1

The reason why you only see one timer at a time is in the last line of your dispatch block:

-[NSRunLoop run] is a blocking call that returns when the last input source of the run loop finishes and no timers are scheduled anymore.

In addition, GCD-queues are strictly FIFO and you are creating a serial queue.
Thus, the result of you tapping that button several times is a queue that gets fuller and fuller without the first block ever finishing:
Since the timer is repeating, there always is something scheduled on the run loop and thus run never returns, barring all subsequent blocks from ever being invoked.