Passing parameters between screens + initialising the view model

view_model_example.dart

import 'package:flutter/material.dart';
import 'package:meta/meta.dart';
import 'package:provider/provider.dart';

abstract class ViewModel implements ChangeNotifier {
  Future<void> initState();
}

abstract class BaseViewModel with ChangeNotifier implements ViewModel {
  bool _isDisposed = false;

  bool _isInitialized = false;

  void setState(VoidCallback fn) {
    assert(fn != null);
    assert(_isInitialized);
    fn();
    if (!_isDisposed) {
      notifyListeners();
    }
  }

  @override
  void notifyListeners() {
    assert(_isInitialized);
    super.notifyListeners();
  }

  @override
  void dispose() {
    _isDisposed = true;
    super.dispose();
  }

  @override
  @mustCallSuper
  Future<void> initState() async {
    assert(!_isInitialized);
    _isInitialized = true;
  }
}

abstract class ExampleViewModel extends ViewModel {
  @override
  Future<void> initState({@required String param1, @required int param2});

  void onButtonClick();

  factory ExampleViewModel() => ExampleViewModelImpl();
}

class ExampleViewModelImpl extends BaseViewModel implements ExampleViewModel {
  String param1;
  int param2;

  @override
  Future<void> initState(
      {@required String param1, @required int param2}) async {
    assert(param1 != null);
    assert(param2 != null);
    super.initState();
    this.param1 = param1;
    this.param2 = param2;
    await fetchData();
  }

  Future<void> fetchData() {
    return Future.delayed(const Duration(seconds: 1));
  }

  @override
  void onButtonClick() {
    print("button clicked");
  }
}

class ExampleWidget extends StatelessWidget {
  final String param1;
  final int param2;

  const ExampleWidget({Key key, this.param1, this.param2}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
      create: (_) =>
          ExampleViewModel()..initState(param1: param1, param2: param2),
      child: Builder(builder: (context) {
        final vm = Provider.of<ExampleViewModel>(context);
        return RaisedButton(
          onPressed: vm.onButtonClick,
        );
      }),
    );
  }
}