Chapter 56 — Calculator

Level: intermediate Estimated time: 12 h Prerequisites: chapters 08, 09, 13, 16 (Dart) and 45, 46, 48, 49, 51 (Flutter), plus chapter 55 (project 1) What you'll be able to do by the end: write a Flutter app whose business core is a pure Dart class, unit-tested, and whose UI is nothing more than a display layer plugged into it.

56.0 — Chapter objectives

By the end of this chapter, you will be able to:

• turn a specification into a state machine before writing a single line of code;
• isolate the business logic in a Dart class with no import 'package:flutter/...' at all;
• model digit-by-digit number entry, including the decimal separator;
• chain operations like a real pocket calculator, and handle repeated =, C, CE, the sign, and the percentage;
• throw and catch a domain exception (DivisionParZeroException);
• format a double for human display: thousands separator, decimal comma, scientific notation;
• keep text that is too long from overflowing the screen with FittedBox;
• build a button grid with Row and Expanded, without GridView;
• write a reusable button widget, parameterized by an enum and colored by the ColorScheme;
• produce a proper press effect with Material + InkWell;
• wire the logic to the UI with a single setState, and display a history in a ListView;
• accept the physical keyboard with KeyboardListener and LogicalKeyboardKey;
• write unit tests for the logic that run without an emulator.