* 겪고 있는 문제 상황을 최대한 자세하게 작성해주세요.

* 문제 해결을 위해 어떤 시도를 해보았는지 구체적으로 함께 알려주세요.

좋아요 누른 책 정보 저장하는 숙제에서 답안 그대로 했을 시, 시뮬레이터에서 type 'String' is not a subtype of type 'List<dynamic>' 에러가 납니다. 그래서 book.dart 부분 수정하니까 해결 됐는데 왜 답안대로 하면 에러가 나는지 궁금합니다.

작성한 코드 및 에러 메세지

오류 발생 시, 작성한 코드 전체와 에러 메시지를 첨부해 주세요.

Tip 1) </> 아이콘을 눌러 코드박스를 만들어 보세요.

Tip 2) Ctrl+A(맥의 경우 Command+A) 단축키로 코드를 한 번에 선택할 수 있어요

[main.dart]

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


import 'book.dart';
import 'book_service.dart';


import 'package:webview_flutter/webview_flutter.dart';


late SharedPreferences prefs;


void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  prefs = await SharedPreferences.getInstance();
  runApp(
    MultiProvider(
      providers: [
        ChangeNotifierProvider(create: (context) => BookService()),
      ],
      child: const MyApp(),
    ),
  );
}


class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);


  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: HomePage(),
    );
  }
}


class HomePage extends StatefulWidget {
  HomePage({Key? key}) : super(key: key);


  @override
  State<HomePage> createState() => _HomePageState();
}


class _HomePageState extends State<HomePage> {
  var bottomNavIndex = 0;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: [
        SearchPage(),
        LikedBookPage(),
      ].elementAt(bottomNavIndex),
      bottomNavigationBar: BottomNavigationBar(
        selectedItemColor: Colors.black,
        unselectedItemColor: Colors.grey,
        showUnselectedLabels: true,
        selectedFontSize: 12,
        unselectedFontSize: 12,
        iconSize: 28,
        type: BottomNavigationBarType.fixed,
        onTap: (value) {
          setState(() {
            bottomNavIndex = value;
          });
        },
        items: [
          BottomNavigationBarItem(
            icon: Icon(Icons.search),
            label: '검색',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.star),
            label: '좋아요',
          ),
        ],
        currentIndex: bottomNavIndex,
      ),
    );
  }
}


class SearchPage extends StatelessWidget {
  SearchPage({super.key});


  @override
  Widget build(BuildContext context) {
    return Consumer<BookService>(
      builder: (context, bookService, child) {
        return Scaffold(
          appBar: AppBar(
            backgroundColor: Colors.white,
            toolbarHeight: 80,
            title: TextField(
              onSubmitted: (value) {
                bookService.search(value);
              },
              cursorColor: Colors.grey,
              decoration: InputDecoration(
                prefixIcon: Icon(Icons.search, color: Colors.grey),
                hintText: "작품, 감독, 배우, 컬렉션, 유저 등",
                border: OutlineInputBorder(
                  borderSide: BorderSide(color: Colors.white),
                  borderRadius: BorderRadius.all(Radius.circular(10)),
                ),
                focusedBorder: OutlineInputBorder(
                  borderSide: BorderSide(color: Colors.grey),
                  borderRadius: BorderRadius.all(Radius.circular(10)),
                ),
              ),
            ),
          ),
          body: Padding(
            padding: const EdgeInsets.symmetric(horizontal: 12),
            child: ListView.separated(
              itemCount: bookService.bookList.length,
              separatorBuilder: (context, index) {
                return Divider();
              },
              itemBuilder: (context, index) {
                if (bookService.bookList.isEmpty) return SizedBox();
                Book book = bookService.bookList.elementAt(index);
                return BookTile(book: book);
              },
            ),
          ),
        );
      },
    );
  }
}


class BookTile extends StatelessWidget {
  const BookTile({
    super.key,
    required this.book,
  });


  final Book book;


  @override
  Widget build(BuildContext context) {
    BookService bookService = context.read<BookService>();


    return ListTile(
      // isThreeLine: true,
      onTap: () {
        Navigator.push(
          context,
          MaterialPageRoute(
            builder: (context) => WebViewPage(
              url: book.previewLink.replaceFirst("http", "https"),
            ),
          ),
        );
      },
      leading: Image.network(
        book.thumbnail,
        fit: BoxFit.fitHeight,
      ),
      title: Text(
        book.title,
        style: TextStyle(fontSize: 16),
      ),
      subtitle: Text(
        "${book.authors.join(", ")}\n${book.publishedDate}",
        style: TextStyle(color: Colors.grey),
      ),
      trailing: IconButton(
        onPressed: () {
          bookService.toggleLikeBook(book: book);
        },
        icon: bookService.likedBookList.map((book) => book.id).contains(book.id)
            ? Icon(
                Icons.star,
                color: Colors.amber,
              )
            : Icon(Icons.star_border),
      ),
    );
  }
}


class LikedBookPage extends StatelessWidget {
  const LikedBookPage({super.key});


  @override
  Widget build(BuildContext context) {
    return Consumer<BookService>(
      builder: (context, bookService, child) {
        return Scaffold(
          body: Padding(
            padding: const EdgeInsets.symmetric(horizontal: 12),
            child: ListView.separated(
              itemCount: bookService.likedBookList.length,
              separatorBuilder: (context, index) {
                return Divider();
              },
              itemBuilder: (context, index) {
                if (bookService.likedBookList.isEmpty) return SizedBox();
                Book book = bookService.likedBookList.elementAt(index);
                return BookTile(book: book);
              },
            ),
          ),
        );
      },
    );
  }
}


class WebViewPage extends StatelessWidget {
  WebViewPage({super.key, required this.url});


  String url;


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.grey,
        title: Text(url),
      ),
      body: WebView(initialUrl: url),
    );
  }
}

[book.dart] @@@@수정 부분@@@@@@

class Book {
  String id;
  String title;
  String subtitle;
  String thumbnail; // 썸네일 이미지 링크
  String previewLink; // ListTile 을 눌렀을 때 이동하는 링크
  List authors;
  String publishedDate;


  Book({
    required this.id,
    required this.title,
    required this.subtitle,
    required this.thumbnail,
    required this.previewLink,
    required this.authors,
    required this.publishedDate,
  });


  Map toJson() {
    return {
      'id': id,
      'title': title,
      'subtitle': subtitle,
      'thumbnail': thumbnail,
      'previewLink': previewLink,
      'authors': authors.join(", "),
      'publishedDate': publishedDate,
    };
  }


  factory Book.fromJson(json) {
    return Book(
      id: json['id'],
      title: json['title'],
      subtitle: json['subtitle'],
      thumbnail: json['thumbnail'],
      previewLink: json['previewLink'],
      authors: [json['authors']],
      publishedDate: json['publishedDate'],
    );
  }
}

[book_service.dart]

import 'dart:convert';


import 'package:dio/dio.dart';
import 'package:flutter/material.dart';
import 'package:watcha_pedia/main.dart';


import 'book.dart';


class BookService extends ChangeNotifier {
  BookService() {
    loadLikedBookList();
  }


  List<Book> bookList = []; // 책 목록
  List<Book> likedBookList = [];


  void toggleLikeBook({required Book book}) {
    String bookId = book.id;
    if (likedBookList.map((book) => book.id).contains(bookId)) {
      likedBookList.removeWhere((book) => book.id == bookId);
    } else {
      likedBookList.add(book);
    }
    notifyListeners();
    saveLikedBookList();
  }


  void search(String q) async {
    bookList.clear(); // 검색 버튼 누를때 이전 데이터들을 지워주기


    if (q.isNotEmpty) {
      Response res = await Dio().get(
        "https://www.googleapis.com/books/v1/volumes?q=$q&startIndex=0&maxResults=40",
      );
      List items = res.data["items"];


      for (Map<String, dynamic> item in items) {
        // List authorlist = item['volumeInfo']['authors'] ?? List.empty();


        Book book = Book(
          id: item['id'],
          title: item['volumeInfo']['title'] ?? "",
          subtitle: item['volumeInfo']['subtitle'] ?? "",
          thumbnail: item['volumeInfo']['imageLinks']?['thumbnail'] ??
              "https://thumbs.dreamstime.com/b/no-image-available-icon-flat-vector-no-image-available-icon-flat-vector-illustration-132482953.jpg",
          previewLink: item['volumeInfo']['previewLink'] ?? "",
          // authors: authorlist,
          authors: item['volumeInfo']['authors'] ?? [],
          publishedDate: item['volumeInfo']['publishedDate'] ?? "",
        );
        bookList.add(book);
      }
    }
    notifyListeners();
  }


  saveLikedBookList() {
    List bookJsonList = likedBookList.map((book) => book.toJson()).toList();
    // [{"content": "1"}, {"content": "2"}]


    String jsonString = jsonEncode(bookJsonList);
    // '[{"content": "1"}, {"content": "2"}]'


    prefs.setString('likedBookList', jsonString);
  }


  loadLikedBookList() {
    String? jsonString = prefs.getString('likedBookList');
    // '[{"content": "1"}, {"content": "2"}]'


    if (jsonString == null) return; // null 이면 로드하지 않음


    List bookJsonList = jsonDecode(jsonString);
    // [{"content": "1"}, {"content": "2"}]


    likedBookList = bookJsonList.map((json) => Book.fromJson(json)).toList();
  }
}