Tip Calculator: The Tip

Part of the Making It Interactive section of Coddy's Flutter journey. Lesson 31 of 34.

Next, the tip. A Slider from 0 to 30 percent in steps of 5 picks it, and the app shows how much money that is.

Keep the percent in the State as a double starting at 10. The tip amount is not something you store: it follows from the bill and the percent, so work it out at the top of build, as a final:

final double tip = _bill * _tipPercent / 100;

Every setState, from the field or from the slider, builds the screen again, and tip is worked out again with the new values.

Challenge

Easy

Under the bill, show Tip: 10% and a Slider from 0 to 30 with 6 divisions, starting at 10.
Under the slider, show Tip amount: 4.00 for a bill of 40, with two decimals.

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> {
  double _bill = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Tip Calculator')),
      body: Padding(
        padding: const EdgeInsets.all(24),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: [
            TextField(
              keyboardType: const TextInputType.numberWithOptions(decimal: true),
              decoration: const InputDecoration(
                labelText: 'Bill',
                border: OutlineInputBorder(),
              ),
              onChanged: (String value) {
                setState(() {
                  _bill = double.tryParse(value) ?? 0;
                });
              },
            ),
            const SizedBox(height: 8),
            Text('Bill: ${_bill.toStringAsFixed(2)}'),
          ],
        ),
      ),
    );
  }
}