How do I get call back when the image target was detected

I was doing the project using vuforia native on android studio where the native api was called by JNI-C++ and then called by java-android, I want to get notification on android layer when the vuforia detects/loses the image target. 

I found out that on Unity API, we get the event of "OnTargetStatusUpdated" under "ObserverBehaviour" class which we can subscribe our own function to thus we can get the status when the observer status changed.

The problem for me is that I want to know where to bind the function which can be called when the observer changed in the native API just like in Unity API