final Variables

Part of the Making It Interactive section of Coddy's Flutter journey. Lesson 16 of 34.

final is the other word for a value that does not change. A final variable gets its value once and can never be given another:

final int total = _tickets * 12;
total = 50;   // error: total is final

The difference from const is when the value is known. A const value is known when you write the code, like 12. A final value can be worked out while the app runs, like _tickets * 12, which depends on how many tickets there are right now.

const int price = 12;                // always 12
final int total = _tickets * price;  // known when build runs

A final variable at the top of build is worked out again on every build, so after setState it holds the new total. Use final for a value you set once and only read, and a plain int or String for one you change, like _tickets.

Challenge

Easy

A ticket costs 12. At the top of build, add const int price = 12; and a final variable total that is _tickets times price. Show it as Total: 12 in place of Total: ?, so it reads Total: 24 after one more ticket.

Try it yourself

import 'package:flutter/material.dart';

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

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

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

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

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

class _HomePageState extends State<HomePage> {
  int _tickets = 1;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Tickets')),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            Text('Tickets: $_tickets', style: const TextStyle(fontSize: 24)),
            Text('Total: ?', style: const TextStyle(fontSize: 24)),
            const SizedBox(height: 16),
            ElevatedButton(
              onPressed: () {
                setState(() {
                  _tickets++;
                });
              },
              child: const Text('Add a ticket'),
            ),
          ],
        ),
      ),
    );
  }
}