Add an (optional) way to make a ViewModel life cycle aware (foreground / background)

[gcx-service-bot]

• the ViewModel should have an empty method which can receive those notifications
• we need a mixin that can be used by State which automatically wires a WidgetBindingObserver to the viewModel

maybe s.th. like this (not tested)

mixin LifecycleAwareState<T extends StatefulWidget> on State<T>  implements WidgetsBindingObserver {
  ViewModel get _viewModel;
  @override
  @mustCallSuper
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }
  @override
  @mustCallSuper
  void dispose() {
    super.dispose();
    WidgetsBinding.instance.removeObserver(this);
  }
  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    // TODO: implement
  }
}

Currently unclear if this is possible (this way) with Dart

Original by @bquentin