Unit Testing with XCTest

Tests prove your code works and keep it working as you change it. You'll write XCTestCase tests, assert with XCTAssertEqual , use setUp / tearDown , test async code, mock with protocols, and meet the new Swift Testing framework.

Learn Unit Testing with XCTest in our free Swift course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick…

Part of the free Swift course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn in This Lesson

• • Write tests in an XCTestCase subclass
• • Verify results with XCTAssertEqual and friends
• • Prepare and clean state with setUp / tearDown
• • Test async functions with await
• • Mock dependencies by coding to a protocol
• • Meet the new Swift Testing framework ( @Test , #expect )

📊 Common Assertions

Assertion

Passes when

XCTAssertEqual(a, b)

a equals b

XCTAssertTrue(x)

x is true

XCTAssertFalse(x)

x is false

XCTAssertNil(x)

x is nil

XCTUnwrap(x)

x is non-nil (returns it)

1️⃣ Your First Test

Subclass XCTestCase , write methods that start with test , and assert the result. Xcode finds and runs them automatically.

2️⃣ setUp and tearDown

setUp() runs before each test to build fresh state; tearDown() runs after to clean it up. This keeps every test isolated and repeatable.

Your turn. Fill in the base class, the method prefix, and the assertion.

3️⃣ Async Tests and Mocking

Mark a test method async to await async code directly. Coding to a protocol lets you swap in a fast, offline mock during tests.

Now you try. Fill in the keywords that make a test async-aware.

4️⃣ The New Swift Testing Framework

Apple's newer Swift Testing framework uses the @Test macro and #expect instead of XCTest's class and assertions. It reads more naturally and coexists with XCTest.

Pro Tips

• 💡 One behaviour per test. A focused test points straight at the broken thing when it fails.
• 💡 Inject dependencies via protocols so you can mock the network, clock, or database.
• 💡 Keep tests fast and offline. Real network calls make tests flaky and slow.
• 💡 Name tests for behaviour, e.g. testDivideByZeroReturnsNil .

Common Errors (and the fix)

• Test never runs — its method name doesn't start with test .
• Tests interfere with each other — reset shared state in setUp / tearDown .
• "async" call not awaited — mark the test method async and use await .
• Flaky tests — you're hitting the real network; inject a mock instead.

📋 Quick Reference

Task

Code

Notes

Test class

class T: XCTestCase

subclass

Test method

func testThing()

test* prefix

Equality

a == b

Setup

override func setUp()

before each

Async

func testX() async

use await

Swift Testing

@Test func x() { #expect(…) }

newer API

Frequently Asked Questions

Mini-Challenge: Test a Calculator

Write a small test class with setUp, an equality test, and a nil test for a divide function.

🎉 Lesson Complete!

• ✅ Tests live in an XCTestCase subclass; methods start with test
• ✅ Verify with XCTAssertEqual , XCTAssertTrue , XCTAssertNil
• ✅ setUp / tearDown keep tests isolated
• ✅ Mark a test async to test async code; mock via protocols
• ✅ Swift Testing's @Test and #expect are the modern alternative
• ✅ Next lesson: Swift Package Manager

Practice quiz

Continue this course

• Previous: SwiftData Basics
• Next: Swift Package Manager