Tracker VS Trackable for reseting Extended tracking

Hi,

I have a missunderstanting about how to reset the extended tracking.

Before Vuforia 3, I was looping on dataset.trackables and start/stop them (also reset extended tracking but it not the point).

Now I use the TrackerManager to get the Tracker instance (type Image in my case) and then start/stop it.

Do I still need any loop ? because TrackerManager returns me a single Tracker instance... the whole trackables of the dataset are concerned ?

Here is the point : 

 final TrackerManager tManager = TrackerManager.getInstance();
                final ImageTracker imageTracker = (ImageTracker) tManager.getTracker(ImageTracker.getClassType());

                if ((imageTracker != null)) {

                    imageTracker.stop();

                    if (!imageTracker.resetExtendedTracking()) {
                        Log.e(ARConfiguration.LOGTAG, "Unable to reset Extendedtracking");
                        res = false;
                    }

                    imageTracker.start();
                }

(Before that, I loop on trackable to stop extendedTracking, and after that I loop again on trackables to enable it)

Thanks !