Reacting to updated and removed tracked images in AR Foundation (part 2)

ARPlaceTrackedImages.cs

// Disable instantiated prefabs that are no longer being actively tracked
foreach (var trackedImage in eventArgs.updated) { 
    _instantiatedPrefabs[trackedImage.referenceImage.name] 
        .SetActive(trackedImage.trackingState == TrackingState.Tracking); 
} 

// Remove is called if the subsystem has given up looking for the trackable again.
// (If it's invisible, its tracking state would just go to limited initially).
// Note: ARCore doesn't seem to remove these at all; if it does, it would delete our child GameObject
// as well.
foreach (var trackedImage in eventArgs.removed) { 
    // Destroy the instance in the scene.
    // Note: this code does not delete the ARTrackedImage parent, which was created
    // by AR Foundation, is managed by it and should therefore also be deleted by AR Foundation.
    Destroy(_instantiatedPrefabs[trackedImage.referenceImage.name]);
    // Also remove the instance from our array
    _instantiatedPrefabs.Remove(trackedImage.referenceImage.name);

    // Alternative: do not destroy the instance, just set it inactive
    //_instantiatedPrefabs[trackedImage.referenceImage.name].SetActive(false);
}