使用Flutter和Dart，如何使Future正常工作？

Question

我知道還有其他方法可以實現此目的，但是我想通過initState（）使用Future來使用SharedPreferences獲取列表。 以下內容說明了我想要實現的目標，但是它不能按要求工作，因為Future會在完成任務之前立即返回。

應該如何組織？

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

class _MyHomePageState extends State<MyHomePage> {
  SharedPreferences _prefs;
  String _sMessage = "No response from getting params";
  List<String> _lsCategories;

  @override
  void initState() {
    super.initState();
    _initPreferences().then((_) {
      try {
        _lsCategories = _prefs.getStringList("categories") ?? [];
        debugPrint("Categories = $_lsCategories");
        _sMessage = "Categories loaded OK";
      } catch (vError) {
        _sMessage = ("Error loading categories = $vError");
      }
      setState(() {});
    });
  }

  Future<void> _initPreferences() async {
    SharedPreferences.getInstance().then((prefs) {
      _prefs = prefs;
      debugPrint("Preferences initialized OK");
    }).catchError((vError) {
      debugPrint("Error initializing preferences = ${vError.toString()}");
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(_sMessage,
                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

將您的_initPreferences方法更改為以下內容：

Future<void> _initPreferences() async {
  try {
    final prefs = await SharedPreferences.getInstance();
    _prefs = prefs;
    debugPrint("Preferences initialized OK");
  } catch (e) {
    debugPrint("Error initializing preferences = ${e.toString()}");
  }
}

問題在於，在Future上使用.then()時.then()您不會等待該將來完成，而使用await會等待。

Answer 2

試試吧。

Future<void> _initPreferences() async {
    _prefs = await SharedPreferences.getInstance().catchError((vError) { 
          debugPrint("Error initializing preferences = ${vError.toString()}");
    };
    debugPrint("Preferences initialized OK");
}