how to assign future to widget in flutter?

Using a FutureBuilder should solve your problem. I modified you code so you can see how to use it. initialData is not required.

  @override
  Widget build(BuildContext context) {
    return new FutureBuilder(
      future: getTextFromFile(),
      initialData: "Loading text..",
      builder: (BuildContext context, AsyncSnapshot<String> text) {
        return new SingleChildScrollView(
          padding: new EdgeInsets.all(8.0),
          child: new Text(
            text.data,
            style: new TextStyle(
              fontWeight: FontWeight.bold,
              fontSize: 19.0,
            ),
          ));
      });
  }

  Future<String> getFileData(String path) async {
    return await new Future(() => "test text");
  }

  Future<String> getTextFromFile() async {
    return getFileData("test.txt");
  }
}