flutter_redux_middleware.dart

class AssetsPrepareLevelsAction { }

class AssetMiddleware implements MiddlewareClass<AppState> {

  final String pathToAssets;

  AssetMiddleware(this.pathToAssets);
  
  @override
  Future call(Store<AppState> store, action, NextDispatcher next) async {
    if (action is AssetsPrepareLevelsAction) {
      String pathToLevels = await Asset("$pathToAssets/levels", "levels").prepare();
      String levelsString = await File(pathToLevels).readAsString();
      List<dynamic> levelsJson = jsonDecode(levelsString);
      store.dispatch(LoadLevelsAction(Level.levelHeirarchyFromJson(levelsJson)));
    }
    next(action); // don't forget, since other middlewares might look at this action
    }    
  }
}