CHAPTER 1 ■ INTRODUCTION TO TEST-DRIVEN DEVELOPMENT
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Central to the TDD approach is the concept that a developer not only writes actual
code to carry out the app’s functionality but also tests code that ensures his application’s
code does what it is supposed to do. The test code is not shipped with the product.
While writing test code in addition to code that carries out the app’s intended
functionality is indeed extra work, it should be seen as an up-front investment toward
improving the quality of the product that ships to customers. A team that practices TDD
techniques will, over time, observe a reduction in the number of regression defects.
■ Note TDD is often used interchangeably with the term Unit Test; however, these two
terms are not the same thing. TDD is an approach to software development where the test
code is written first: in essence, the tests drive development.
A unit test is just a piece of test code viewed in isolation. A unit test is one of the by-
products of adopting a TDD approach.
However, the mere existence of one or more unit tests does not necessarily imply
that the developer followed a TDD approach. The unit tests could, for instance, have been
fitted retrospectively to existing code.
If you find working on large problems overwhelming, you may find TDD to be a
useful technique to break down a problem into smaller ones, use tests to solve the smaller
problems, and in the process end up solving the larger problem. You will soon realize that
large problems are not as overwhelming once you approach them with a TDD mindset.
TDD Terminology
This section examines some of the common terminology associated with Test-Driven
Development.
Subject under Test
This is usually a piece of code, or unit of functionality you wish to test. In most situations
the subject under test is usually a single method of a Swift class. However, you may
encounter scenarios where a small group of methods or classes are being tested together.
In such cases, the subject under test usually represents a complete functionality or user
journey. The subject under test is sometimes also known as the system under test.
Unit Test
This is the piece of code that tests the subject under test. A unit test is also known as a
“test case.” Unit tests work by calling the subject under test under controlled conditions,
and verifying some kind of expected behavior. It is common for an application to have
hundreds of unit tests with each test testing a very small piece of the functionality.
Individual unit tests are implemented as independent methods of a Swift class that
derives from an XCTestCase. This Swift class is also commonly referred to as a test class.
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