Flutter：'Future <dynamic>' 不是 bool 类型的子类型

Question

我试图实现一个简单的登录/注销功能。我的场景是这样的：

我有 2 个页面（登录页面和主页），在 中main.dart，我使用 SharedPreferences 检查用户是否已登录，如果用户已登录，我在单击按钮时将布尔值设置为 true。

我遇到的问题是，我创建了一个 routeLogin 函数，用于在主页和登陆页面之间进行选择。我得到这个错误：

I/flutter ( 9026): ??? EXCEPTION CAUGHT BY WIDGETS LIBRARY ????????????????????????????????????????????????????????????
I/flutter ( 9026): The following assertion was thrown building MyApp(dirty):
I/flutter ( 9026): type 'Future<dynamic>' is not a subtype of type 'bool'
I/flutter ( 9026):
I/flutter ( 9026): Either the assertion indicates an error in the framework itself, or we should provide substantially
I/flutter ( 9026): more information in this error message to help you determine and fix the underlying cause.
I/flutter ( 9026): In either case, please report this assertion by filing a bug on GitHub:
I/flutter ( 9026):   https://github.com/flutter/flutter/issues/new?template=BUG.md

这是我的代码：

import 'package:credit/src/pages/landing.dart';
import 'package:flutter/material.dart';
import 'package:credit/src/pages/credit/home.dart';
import 'package:shared_preferences/shared_preferences.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.

 bool checkValue;
 checkLoginValue () async{
   SharedPreferences loginCheck = await SharedPreferences.getInstance();
   checkValue = loginCheck.getBool("login");
   }
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Test App',
       debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: routeLogin());
      //home: LandingPage());      
  }

  routeLogin()
  {
    print("Check value");
    if (checkValue == null){
      return LandingPage();
    }
    else{
      return HomePage();
    }
    
  }
}

请让我知道我哪里出错了，我是 Flutter 的新手。

Answer 1

您可以使用 future builder 轻松获得此行为。

Future<bool> checkLoginValue() async {
  SharedPreferences loginCheck = await SharedPreferences.getInstance();
  return loginCheck.getBool("login");
}

@override
Widget build(BuildContext context) {
  return MaterialApp(
    title: 'Test App',
    debugShowCheckedModeBanner: false,
    theme: ThemeData(
      primarySwatch: Colors.blue,
    ),
    home: FutureBuilder<bool>(
      future: checkLoginValue,
      builder: (BuildContext context, AsyncSnapshot<bool> snapshot) {
        if (snapshot.data == false) {
          return LandingPage();
        } else {
          return HomePage();
        }
      },
    ),
  );
}

Answer 2

假设 loginCheck 中的 getBool 函数返回 Future，

您正试图将 Future 放入 bool 中。

将该行更改为：

checkValue = await loginCheck.getBool("login");