The argument type 'Future<List<User>>' can't be assigned to the parameter type 'Future<void> Function()' in onRefresh when using RefreshIndicator

When you do this

onRefresh: _fetchUsersListUsingLoop(),

You are telling dart to call _fetchUsersListUsingLoop and expect it to return a function, then to call said function, what you want to do instead is tell it to just call _fetchUsersListUsingLoop:

onRefresh: _fetchUserListUsingLoop,

or ìf that doesn't work:

onRefresh: () async => _fetchUserListUsingLoop(),

But the problem is that this will not actually work, why? Because the future builder is already completed and nothing will change that, to fix your issue; you can reassign the future like this:

You need to declare a future:

late Future<List<User>> _user;

        child: FutureBuilder(
            future: _users,
            builder: (context, asyncSnapshot) {
              ...
                    onRefresh: _updateUsersFuture,
              ...

Then somewhere in your class you do:

Future<void> _updateUsersFuture() async {
  final newUsers = await _fetchUserListUsingLoop();
  setState(() => _users = newUsers);
}

and on initstate:

@override
void initState() {
  _users = _fetchUsersListUsingLoop();
  super.initState();
}