class FbManager private constructor() {

    class CallbackHolder(val dr: DatabaseReference, val vel: ValueEventListener)

    private object Holder {
        val INSTANCE = FbManager()
    }

    private val mFb: DatabaseReference
    private val callbacks: MutableList<CallbackHolder>

    init {
        mFb = FirebaseDatabase.getInstance().reference
        callbacks = ArrayList()
    }

    companion object {
        val instance: FbManager by lazy { Holder.INSTANCE }
    }
 }