使用Flutter和Dart，这是实现这一未来的最好方法吗？

Question

I know that there are other ways of achieving this, but I want to use a Future via initState() to obtain a List using SharedPreferences without using await. The following illustrates what I want to achieve, and it does work. Because I have never used this pattern previously, I'm just unsure if it's the best way (without using await). Is there a better way without using await?

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

class _MyHomePageState extends State<MyHomePage> {
  SharedPreferences _prefs;
  String _sMessage1;
  String _sMessage2;
  List<String> _lsCategories;

  @override
  void initState() {
    super.initState();
    Future<void> future = _initPreferences();
    future.then((_) {
      if (_prefs != null) {
        try {
          _lsCategories = _prefs.getStringList("categories") ?? [];
          debugPrint("Categories = $_lsCategories");
          _sMessage2 = "Categories loaded OK";
        } catch (vError) {
          _sMessage2 = ("Error loading categories = $vError");
        }
      }
      setState(() {});
    });
  }

  Future<void> _initPreferences() {
    return SharedPreferences.getInstance().then((prefs) {
      _prefs = prefs;
      _sMessage1 = "Preferences initialized OK";
    }).catchError((vError) {
      _sMessage1 = "Error initializing preferences = ${vError.toString()}";
      _sMessage2 = "Unable to load categories";
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            _createText(_sMessage1),
            SizedBox(height: 20),
            _createText(_sMessage2),
          ],
        ),
      ),
    );
  }
}

Text _createText(String sText) {
  return Text(sText == null ? "" : sText,
      style: TextStyle(
          color: Colors.red[500], fontWeight: FontWeight.bold, fontSize: 20));
}

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Future Test',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: "Flutter Future Test"),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

Answer 1

What you did is fine in terms of asynchrony. However, it's too verbose. For example, you do not need to transfer the prefs instance through state.

Another improvement could be to check whether the widget is still mounted when the Future is done. (If you use a FutureBuilder , you don't need to worry about this.)

A problem with your code is that you're setting state variables outside setState() . It is not guaranteed to work well that way, I swear I had old state used sometimes when I did that. You should set them within setState()

Here are a couple of different ways I would prefer to code it:

  @override
  void initState() {
    super.initState();
    SharedPreferences.getInstance().then((prefs) {
      if(!mounted) return;
      setState(() {
        _prefs = prefs;
        _lsCategories = _prefs.getStringList("categories") ?? [];
        debugPrint("Categories = $_lsCategories");
        _sMessage2 = "Categories loaded OK";
      });
    }).catchError((vError) {
      setState(() {
        _sMessage2 = ("Error loading categories = $vError");
      });
    });
  }

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

  Future<void> _initPreferences() async {
    final prefs = await SharedPreferences.getInstance();
    if (!mounted) return;
    setState(() {
      try {
        _prefs = prefs;
        _lsCategories = _prefs.getStringList("categories") ?? [];
        debugPrint("Categories = $_lsCategories");
        _sMessage2 = "Categories loaded OK";
      } catch (vError) {
        _sMessage2 = ("Error loading categories = $vError");
      }
    });
  }