Prices With NumberFormat

Part of the Data and the Network section of Coddy's Flutter journey. Lesson 22 of 27.

Prices come from servers as plain numbers: 7.5, 12.25. Put 7.5 in a string and you get $7.5, which no shop would print. A price needs its currency sign and exactly two decimals.

NumberFormat.currency, from the same intl package, does both:

final price = NumberFormat.currency(symbol: '\$');
price.format(7.5)      // $7.50
price.format(1250)     // $1,250.00

It adds the sign, always writes two decimals, and puts commas between thousands. symbol is the sign to print. Without it you get the currency's code instead: USD7.50.

The $ sign has a backslash in front, '\$', because in a Dart string $ starts a variable. The backslash says "a plain dollar sign".

Challenge

Easy

The book prices read $7.5. Format every price with NumberFormat.currency(symbol: '\$'), so they read $8.99, $7.50 and $12.25.

Try it yourself

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:intl/intl.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 items.map((item) => Book.fromJson(item as Map<String, dynamic>)).toList();
  }

  @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: ${snapshot.error}'));
          }
          if (snapshot.hasData) {
            final books = snapshot.data!;
            return ListView.builder(
              itemCount: books.length,
              itemBuilder: (context, index) {
                final book = books[index];
                return ListTile(
                  title: Text(book.title),
                  subtitle: Text(book.author),
                  trailing: Text(
                    '\$${book.price}',
                    style: const TextStyle(fontSize: 18),
                  ),
                );
              },
            );
          }
          return const Center(child: CircularProgressIndicator());
        },
      ),
    );
  }
}

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

  final String title;
  final String author;
  final double price;

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

Test yourself

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