private val mUsers : SortedList<User> by lazy {
        SortedList(User::class.java, object : SortedList.Callback<User>() {
            override fun compare(u1: User, u2: User): Int {
                return if (u1.distanceTo(mGeoQuery!!.center) > u2.distanceTo(mGeoQuery!!.center))
                    1
                else if (u1.distanceTo(mGeoQuery!!.center) < u2.distanceTo(mGeoQuery!!.center))
                    -1
                else
                    0
            }

            override fun onChanged(position: Int, count: Int) {
                mListFragmentAdapter?.notifyItemChanged(position, count)
                mMapFragment?.reloadItems()
            }

            override fun areContentsTheSame(u1: User, u2: User): Boolean {
                return u1.toMap() == u2.toMap()
            }

            override fun areItemsTheSame(u1: User, u2: User): Boolean {
                return u1.uid == u2.uid
            }

            override fun onInserted(position: Int, count: Int) {
                mListFragmentAdapter?.notifyItemRangeInserted(position, count)

                for (i in position..position + count - 1)
                    mMapFragment?.clusterManager?.addItem(mUsers.get(i))

                mMapFragment?.clusterManager?.cluster()
            }

            override fun onRemoved(position: Int, count: Int) {
                mListFragmentAdapter?.notifyItemRangeRemoved(position, count)
                mMapFragment?.reloadItems()
            }

            override fun onMoved(fromPosition: Int, toPosition: Int) {
                mListFragmentAdapter?.notifyItemMoved(fromPosition, toPosition)
            }
        })
    }