title: How to find if NSTimer is active or not?
body: <p>I have a timer something like this:</p>
  <pre><code> NSTimer* timer = [NSTimer scheduledTimerWithTimeInterval:1.0
                                                     target:self
                                                   selector:@selector(updateCountdown)
                                                   userInfo:nil
                                                    repeats:YES]; 
  </code></pre>
  <p>I am updating a label's text using this timer. At a certain condition, I want to check if the timer is active then invalidate the timer. My question is how do I find that timer is active or not?</p>