title: NSTimer automatically increases when I change view controller
body: <p>In my viewController I inserted a timer for a calculation of numbers taken directly from my database of Parse.com.</p>
  
  <p>The timer is working correctly, were inserted in viewDidAppear and in ViewDidDisappear in such a way as to lock the sequence when changing view controller.</p>
  
  <p>The problem I have is that when I change the view controller with the Push the timer does not stop you give an example:</p>
  
  <p>I open the application and the calculation is done correctly with the animation of NSTimer.</p>
  
  <p>Change View, and then I go back with the back button to view where the timer at this point I see that the numbers contiunano to increase every time I change view and go back ...</p>
  
  <p>Can you explain where I'm wrong?</p>
  
  <pre><code>-(void)viewDidAppear:(BOOL)animated {
      [[UIApplication sharedApplication] setStatusBarHidden:NO withAnimation:UIStatusBarAnimationSlide];
      [self.navigationController.navigationBar setBackgroundImage:[UIImage new]forBarMetrics:UIBarMetricsDefault];
      self.navigationController.navigationBar.shadowImage = [UIImage new];
      [self.navigationController.navigationBar setBackgroundColor:[UIColor clearColor]];
      self.title = NSLocalizedString(@"", nil);
      [self animatelayertopoint:0];
      [self AnimationMenuView:600];
      [self QueryForViewPrincipal];
      [self QueryForCollectionView];
      [self ShowBadgeNumber];
      [self Timer];
      [collectionView reloadData];
  
  }
  -(void)Timer{
  
    timer = [NSTimer scheduledTimerWithTimeInterval:0.012 target:self selector:@selector(CalcoloMediadiLaurea) userInfo:nil repeats:YES];
  
  }
  
  -(void) CalcoloMediadiLaurea{
      PFUser *CurrentUser = [PFUser currentUser];
      NSNumber *NumeroUndici = [NSNumber numberWithInt:11];
      NSNumber *NumeroTre = [NSNumber numberWithInt:3];
      NSNumber *ValoreMediaPonderata = [CurrentUser objectForKey:FF_USER_MEDIA_PONDERATA];
      int FFValoreTre = [NumeroTre intValue];
      int FFValoreUndici = [NumeroUndici intValue];
      double MediaPonderata = [ValoreMediaPonderata doubleValue];
      double RisultatoMoltiplicazioneValoriPonderataUndici;
      RisultatoMoltiplicazioneValoriPonderataUndici = MediaPonderata * FFValoreUndici;
      double RisultatoMediaLaurea;
      RisultatoMediaLaurea = RisultatoMoltiplicazioneValoriPonderataUndici / FFValoreTre;
      CalcoloMediaLaureaLabel.text = [NSString stringWithFormat:@"%.1f", FFTickValoreMediaLaurea ];
      FFTickValoreMediaLaurea++;
  
      if(FFTickValoreMediaLaurea &gt; RisultatoMediaLaurea){
          [timer invalidate];
          timer = nil;
      }
  
     }
  
  -(void)viewDidDisappear:(BOOL)animated {
      [self animatelayertopoint:0];
      [self AnimationMenuView:600];
      [self CalcoloMediadiLaurea];
      }
  </code></pre>