What is the equivalent of Android LiveData in Flutter?

Question

Android's LiveData allows to update the UI when the activity is in an active state. So if a background operation has finished while the activity is paused, the activity won't be notified and thus the app won't crush. Can Flutter perform the same behavior?

Answer 1

For people interested in an equivalent of LiveData for other scenarios, I present you StreamController:

class ExampleViewModel {

  StreamController<bool> loggedInStream = StreamController<bool>();

  logIn() { loggedInStream.add(true); }
}


class ExampleScreen extends StatefulWidget {

  @override
  State<StatefulWidget> createState() => ExampleScreenState();
}

class ExampleScreenState extends State<ExampleScreen> {

  ExampleViewModel _viewModel;
  BuildContext _ctx;

  @override
  void initState() {
    super.initState();

    _viewModel = ExampleViewModel()
    _viewModel.loggedInStream.stream.listen( (loggedIn) {
      if ( loggedIn != null && loggedIn ) {
        Navigator.of(_ctx).pushReplacementNamed("/home");
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    _ctx = context;

    var loginBtn =
    RaisedButton(
      onPressed: _viewModel.logIn(true),
        child: Text(
          "LOGIN",
          style: new TextStyle(
          fontSize: 24.0,
        )
      ),
      color: Colors.green,
      textColor: Colors.white,
    );

    return loginBtn;
  }

  @override
  void dispose() {
    super.dispose();
    _viewModel.loggedInStream.close();
  }

}

You can subscribe to it just like a LiveData, using:

loggedInStream.stream.listen( (data) { code } )

And you should clear the listeners in dispose to avoid memory leaks:

loggedInStream.close()

This code basically do the following things:

1. Creates a screen with a button.
2. Listen to a Stream (observe a LiveData).
3. When you click the button, it changes the value.
4. The listener (observer) is triggered.
5. Launches new screen.