A List From JSON

Part of the Data and the Network section of Coddy's Flutter journey. Lesson 18 of 27.

Many answers are lists: every book, every message, every day of a forecast. In JSON a list is written in square brackets, and here each item is an object:

[
  {"title": "Frankenstein", "author": "Mary Shelley"},
  {"title": "Pride and Prejudice", "author": "Jane Austen"}
]

jsonDecode turns square brackets into a List. Its items are dynamic too, so say what you expect with as: List<dynamic> for the list, Map<String, dynamic> for each item.

Then turn every item into a Book with map, and collect the results with toList():

final items = jsonDecode(body) as List<dynamic>;
return items
    .map((item) => Book.fromJson(item as Map<String, dynamic>))
    .toList();

map alone gives a lazy Iterable, not a List, which is why toList() is there. The screen then has a List<Book> for ListView.builder.

Challenge

Medium

The shelf shows only the first book. Make loadBooks turn every item of the list into a Book, with items.map(...) and .toList().

Try it yourself

import 'dart:convert';

import 'package:flutter/material.dart';

import 'api.dart';

void main() {
  runApp(const MyApp());
}

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

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

class BooksScreen extends StatefulWidget {
  const BooksScreen({super.key});

  @override
  State<BooksScreen> createState() => _BooksScreenState();
}

class _BooksScreenState extends State<BooksScreen> {
  late Future<List<Book>> _books;

  @override
  void initState() {
    super.initState();
    _books = loadBooks();
  }

  Future<List<Book>> loadBooks() async {
    final body = await fetch('/books');
    final items = jsonDecode(body) as List<dynamic>;
    return [Book.fromJson(items[0] as Map<String, dynamic>)];
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Books')),
      body: FutureBuilder<List<Book>>(
        future: _books,
        builder: (context, snapshot) {
          if (snapshot.hasError) {
            return Center(child: Text('Could not load: ${snapshot.error}'));
          }
          if (snapshot.hasData) {
            final books = snapshot.data!;
            return ListView.builder(
              itemCount: books.length,
              itemBuilder: (context, index) {
                final book = books[index];
                return ListTile(
                  title: Text(book.title),
                  subtitle: Text(book.author),
                );
              },
            );
          }
          return const Center(child: CircularProgressIndicator());
        },
      ),
    );
  }
}

class Book {
  Book({required this.title, required this.author});

  final String title;
  final String author;

  factory Book.fromJson(Map<String, dynamic> json) {
    return Book(
      title: json['title'] as String,
      author: json['author'] as String,
    );
  }
}

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

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