firestore write using ArrayUnion

Future<void> updateUserData(String bus_id, String speed, GeoPoint geoPoint) async {
    Map<String, dynamic>data=new Map();
    data['coordinate']=geoPoint;
    data['velocity']=speed;
    var list=new List<Map<String, dynamic>>();
    list.add(data);
    print("write func invoked");

    firestoreref.document(bus_id).setData({
      'locations': FieldValue.arrayUnion(list)
    });


  }