ITrackableEventHandler

// 1- add Reference 
using Vuforia;

// 2- public class Class_name : MonoBehaviour, ITrackableEventHandler {}

// 3-  protected TrackableBehaviour mTrackableBehaviour;

// 4 - on Start class 
    void Start()
    {
        mTrackableBehaviour = GetComponent<TrackableBehaviour>();
        if (mTrackableBehaviour)
        {
            mTrackableBehaviour.RegisterTrackableEventHandler(this);
        }
        
// 5-  
   public void OnTrackableStateChanged(
   TrackableBehaviour.Status previousStatus,
   TrackableBehaviour.Status newStatus)
    {
        if (newStatus == TrackableBehaviour.Status.DETECTED ||
            newStatus == TrackableBehaviour.Status.TRACKED ||
            newStatus == TrackableBehaviour.Status.EXTENDED_TRACKED)
        {
   
            Debug.Log("Trackable " + mTrackableBehaviour.TrackableName + " found");

            if (mTrackableBehaviour.TrackableName == "Marker_name")
            {         
            
            }
           OnTrackingFound();
         }
       else if (previousStatus == TrackableBehaviour.Status.TRACKED &&
                 newStatus == TrackableBehaviour.Status.NOT_FOUND)
        {
            Debug.Log("Trackable " + mTrackableBehaviour.TrackableName + " lost");
            OnTrackingLost();
        }
        else
        {
            // For combo of previousStatus=UNKNOWN + newStatus=UNKNOWN|NOT_FOUND
            // Vuforia is starting, but tracking has not been lost or found yet
            // Call OnTrackingLost() to hide the augmentations
            OnTrackingLost();
        }
    }

6- // Use this for initialization


    protected virtual void OnTrackingFound()
    {
        var rendererComponents = GetComponentsInChildren<Renderer>(true);
        var colliderComponents = GetComponentsInChildren<Collider>(true);
        var canvasComponents = GetComponentsInChildren<Canvas>(true);

        // Enable rendering:
        foreach (var component in rendererComponents)
            component.enabled = true;

        // Enable colliders:
        foreach (var component in colliderComponents)
            component.enabled = true;

        // Enable canvas':
        foreach (var component in canvasComponents)
            component.enabled = true;
    }


    protected virtual void OnTrackingLost()
    {
        var rendererComponents = GetComponentsInChildren<Renderer>(true);
        var colliderComponents = GetComponentsInChildren<Collider>(true);
        var canvasComponents = GetComponentsInChildren<Canvas>(true);

        // Disable rendering:
        foreach (var component in rendererComponents)
            component.enabled = false;

        // Disable colliders:
        foreach (var component in colliderComponents)
            component.enabled = false;

        // Disable canvas':
        foreach (var component in canvasComponents)
            component.enabled = false;
    }