         viewControllers.foreach(updateMulti)
     }
     
-}+}

        pageViewControllers = ["devices", "sensorDataGroup", "classification"].map { pagesStoryboard.instantiateViewControllerWithIdentifier($0) as UIViewController }
        setViewControllers([pageViewControllers.first!], direction: UIPageViewControllerNavigationDirection.Forward, animated: false, completion: nil)
        
        startTime = NSDate()
        timer = NSTimer.scheduledTimerWithTimeInterval(1, target: self, selector: "tick", userInfo: nil, repeats: true)
    }

    private var startTime: NSDate?
    private var exerciseSession: ExerciseSession?
    private var pageViewControllers: [UIViewController] = []
    private var pageControl: UIPageControl!
    @IBOutlet var stopSessionButton: UIBarButtonItem!
    
    // MARK: main
    override func viewWillDisappear(animated: Bool) {
        if let x = timer { x.invalidate() }
        navigationItem.prompt = nil
    }
    
    override func viewDidAppear(animated: Bool) {
        super.viewDidAppear(animated)
    }
    
    @IBAction
