在Flutter中不能异步的方法中，我如何从共享首选项中获取首选项？

import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  String _preferenceValue;

  @override
  void initState() {
    super.initState();
    Future.microtask(() async {
      final prefs = await SharedPreferences.getInstance();
      setState(() {
        _preferenceValue = prefs.getString('key') ?? 'default_value';
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('SharedPreferences Example'),
      ),
      body: Center(
        child: Text('Preference Value: $_preferenceValue'),
      ),
    );
  }
}