Welcome to the first lesson of our course on Test-Driven Development (TDD) using Swift and XCTest. TDD is an iterative software development process where tests are written before the actual code. This approach enables programmers to focus on the requirements before diving into implementation, ultimately leading to code that is more reliable and maintainable.

In this lesson, we will introduce you to the essential elements of TDD, including the Red-Green-Refactor cycle, which forms the backbone of this methodology. We'll also introduce the tools we'll be using: Swift and XCTest. Swift is a powerful and intuitive programming language for iOS, macOS, watchOS, and tvOS, while XCTest is a robust testing framework for Swift. XCTest provides a simple yet comprehensive solution for defining and running tests. Let's begin by exploring TDD's core components with an example.

In TDD, the journey begins with writing a test that fails. This "Red" stage allows you to clarify your objective before implementation. Let's start by writing a test for a sum function, which will eventually add two numbers.

Create a new Swift file named MathFunctionsTests.swift in your test target:

This test script:

• Contains a test function to ensure sum(2, 3) returns 5 using an assertion.