How to avoid reloading data every time navigating to page

Question

I'm trying to avoid rebuilding FutureBuilder in flutter. I have tried solution suggested in below Q's.

How to parse JSON only once in Flutter

Flutter Switching to Tab Reloads Widgets and runs FutureBuilder

still my app fires API every time I navigate to that page. Please point me where I'm going wrong.

Util.dart

//function which call API endpoint and returns Future in list
class EmpNetworkUtils {
    ...
    Future<List<Employees>> getEmployees(data) async {
        List<Employees> emps = [];
        final response = await http.get(host + '/emp', headers: { ... });
        final responseJson = json.decode(response.body);
        for (var empdata in responseJson) {
            Employees emp = Employees( ... );
            emps.add(emp);
        }
        return emps;
    }
}

EmpDetails.dart

class _EmpPageState extends State<EmpPage>{
    ...
    Future<List<Employees>>_getAllEmp;
    @override
    initState() {
        _getAllEmp = _getAll();
        super.initState();
    }
    Future <List<Employees>>_getAll() async {
        _sharedPreferences = await _prefs;
        String authToken = AuthSessionUtils.getToken(_sharedPreferences);
        return await EmpNetworkUtils().getEmployees(authToken);
    }

    @override
    Widget build(BuildContext context) {
        return Scaffold(
            appBar: new AppBar( ... ),
            body: Container(
                child: FutureBuilder(
                    future: _getAllEmp,
                    builder: (BuildContext context, AsyncSnapshot snapshot) { ... }
        )))
    }
}

Update: I'm using bottomNavigationBar in my app, from which this page is loaded.

Answer 1

You are calling your getEmployees function in initState, which is meant to be called every time your widget is inserted into the tree. If you want to save the data after calling your function the first time, you will have to have a widget that persists.

An easy implementation would be using an InheritedWidget and a data class:

class InheritedEmployees extends InheritedWidget {
  final EmployeeData employeeData;

  InheritedEmployees({
    Key key,
    @required Widget child,
  })  : assert(child != null),
        employeeData = EmployeeData(),
        super(key: key, child: child);

  static EmployeeData of(BuildContext context) => (context.inheritFromWidgetOfExactType(InheritedEmployees) as InheritedEmployees).employeeData;

  @override
  bool updateShouldNotify(InheritedEmployees old) => false;
}

class EmployeeData {
  List<Employees> _employees;

  Future<List<Employees>> get employees async {
    if (_employees != null) return _employees;
    _sharedPreferences = await _prefs;
    String authToken = AuthSessionUtils.getToken(_sharedPreferences);
    return _employees = await EmpNetworkUtils().getEmployees(authToken);
  }
}

Now, you would only have to place your InheritedEmployees somewhere that will not be disposed, e.g. about your home page, or if you want, even about your MaterialApp (runApp(InheritedEmployees(child: MaterialApp(..));). This way the data is only fetched once and cached after that. You could also look into AsyncMemoizer if that suits you better, but the example I provided should work fine.

Now, you will want to call this employees getter in didChangeDependencies because your _EmpPageState is dependent on InheritedEmployees and you need to look that up, which cannot happen in initState:

class _EmpPageState extends State<EmpPage>{
    Future<List<Employees>>_getAllEmp;

    @override
    void didChangeDependencies() {
      _getAllEmp = InheritedEmployees.of(context).employees;
      super.didChangeDependencies();
    }

    @override
    Widget build(BuildContext context) {
        return Scaffold(
            appBar: new AppBar( ... ),
            body: Container(
                child: FutureBuilder(
                    future: _getAllEmp,
                    builder: (BuildContext context, AsyncSnapshot snapshot) { ... }
        )))
    }
}

I mentioned that your State is now dependent on your InheritedWidget, but that does not really matter as updateShouldNotify always returns false (there are not going to be any additional builds).