Transformations.switchMap Function not called in ViewModel

Question

I am observer LiveData from Repository and transform to view,here Transformations.switchMap Function not called in ViewModel. If I use observeForEver it works good.

Transformations.switchMap(signInResponseMutableLiveData){signInResponse ->
            val mutableLiveData = MutableLiveData<Any>()

            if (signInResponse is FirebaseUser) {
                val firebaseUserEmail = signInResponse.email

                homeFragmentUIDataLiveData.value = HomeFragmentUIData(firebaseUserEmail ?: "")
            } else if (signInResponse is String) {
                loginErrorEventLiveData.value = signInResponse
            }

            mutableLiveData
        }