Chapter 57 — Unit Converter

Level: intermediate Estimated time: 12 h Prerequisites: chapter 56 — Project 2: the calculator What you will be able to do by the end: build a complete form-driven application, cleanly separate testable business logic from its interface, and write the unit tests that prove your calculations are correct.

57.0 — Chapter Objectives

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

• model a business domain with an enhanced enum and a value class;
• explain what a pivot unit is and why it cuts the work by a factor of a hundred;
• demonstrate why temperature cannot be converted with a simple factor;
• generalize a "factor" model into a "pair of functions" model;
• write a pure conversion function with no dependency on Flutter;
• wire a TextField to a TextEditingController and react to every keystroke;
• restrict input to numeric characters with inputFormatters;
• distinguish what an inputFormatter filters from what double.tryParse validates;
• handle the French decimal comma and grouping spaces;
• build two synchronized DropdownButton<Unite> widgets;
• filter the list of units by the selected category with where;
• swap the source and target units in a single setState call;
• convert in real time via onChanged with no "Calculate" button;
• format a readable result with NumberFormat from the intl package;
• choose a number of decimal places suited to the order of magnitude of the result;
• switch to scientific notation for extreme numbers;
• organize six categories into tabs with TabController and TabBarView;
• preserve the state of each tab with AutomaticKeepAliveClientMixin;
• keep a history of confirmed conversions;
• persist the user's last choice with shared_preferences;
• write a suite of unit tests that verifies each family of conversions.