ScopedModel.dart

// Create our App, which will provide the `UserModel` to 
// all children that require it! 
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // First, create a `ScopedModel` widget. This will provide 
    // the `model` to the children that request it. 
    return new ScopedModel<UserModel>(
      model: new UserModel(),
      child: new Column(children: [
        // Create a ScopedModelDescendant. This widget will get the
        // UserModel from the nearest ScopedModel<UserModel>. 
        // It will hand that model to our builder method, and rebuild 
        // any time the UserModel changes (i.e. after we 
        // `notifyListeners` in the Model). 
        new ScopedModelDescendant<UserModel>(
	// This text widget will display the value of userName and update when the value changes
          builder: (context, child, model) => new Text('${model.userName}'),
        ),
      ])
    );
  }
}