Loader

Sometimes you need to load some data before building your widget. Because initState doesn't support asynchronous loading you need to find another way to load your data. The most common way of loading data is using a FutureBuilder but FutureBuilders are tedious. Another way is using flags to rebuild the widget after all the loading is done.

Loader uses the flag method.

LoadingMixin

The LoadingMixin adds all the necessary flags to your stateful widget's state to turn it to a FutureBuilder like widget.

load is called before the first didChangeDependencies, so you can use context to access inherited widgets.

their are two flags:

1. loading : true if the load function is still running
2. hasError: true if the load function has thrown an exception.

the exception text is stored in the error variable.

class HomePage extends StatefulWidget {

  @override
  _HomePageState createState() => _HomePageState();
}


class _HomePageState extends State<HomePage> with LoadingMixin<HomePage> {

  Data _data;

  @override
  Future<void> load() async {
    var loader = FileLoader();
    _data = await loader.loadData();
  }

  @override
  Widget build(BuildContext context) {
    Widget body;
    if (loading) {
      body = Container();
    } else if (hasError) {
      body = Text(error);
    } else {
      body = DataViewer(_data);
    }
    
    return Scaffold(
      appbar: AppBar(),
      body: body,
    );
  }
}

StatelessLoadingMixin

For this mixin to work, you need to delete the build method and use the futureBuild method instead.

class FutureText extends StatelessWidget with StatelessLoadingMixin {
  final Future<String> futureText;
  final TextStyle style;

  FutureText(this.futureText, {this.style});

  String text;

  @override
  Future<void> load() async {
    text = await futureText;
  }

  @override
  Widget futureBuild(BuildContext context) {
    return Text(
      text,
      style: style,
    );
  }
}

Loader

Loader is a widget which uses the LoadingMixin mixin.

This widget will run it's builder method, only after the load function is done.

The builder will get the value returned in the load function as the value parameter.

class Banner extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      child: Center(
        child: Loader<String>(
          load: () async{
            return await retriveBannerText();
          },
          builder: (context, value){
            return Text(value);
          },
          errorBuilder: (error) => Text(error, style: TextStyle(color: Colors.red),),
        ),
      ),
    );
  }
}

Implementations details

StatelessLoadingMixin is implemented using the Loader widget.

The Loader widget is implemented using LoadingMixin.

LoadingMixin is implemented using flags on a stateful widget.