axiom-swift-testing

Swift Testing

Overview

Swift Testing is Apple's modern testing framework introduced at WWDC 2024. It uses Swift macros (@Test, #expect) instead of naming conventions, runs tests in parallel by default, and integrates seamlessly with Swift concurrency.

Core principle: Tests should be fast, reliable, and expressive. The fastest tests run without launching your app or simulator.

The Speed Hierarchy

Tests run at dramatically different speeds depending on how they're configured:

Configuration	Typical Time	Use Case
swift test (Package)	~0.1s	Pure logic, models, algorithms
Host Application: None	~3s	Framework code, no UI dependencies
Bypass app launch	~6s	App target but skip initialization
Full app launch	20-60s	UI tests, integration tests

Key insight: Move testable logic into Swift Packages or frameworks, then test with swift test or "None" host application.

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Building Blocks

@Test Functions

import Testing

@Test func videoHasCorrectMetadata() {
    let video = Video(named: "example.mp4")
    #expect(video.duration == 120)
}

Key differences from XCTest:

• No test prefix required — @Test attribute is explicit
• Can be global functions, not just methods in a class
• Supports async, throws, and actor isolation
• Each test runs on a fresh instance of its containing suite

#expect and #require

// Basic expectation — test continues on failure
#expect(result == expected)
#expect(array.isEmpty)
#expect(numbers.contains(42))

// Required expectation — test stops on failure
let user = try #require(await fetchUser(id: 123))
#expect(user.name == "Alice")

// Unwrap optionals safely
let first = try #require(items.first)
#expect(first.isValid)

Why #expect is better than XCTAssert:

• Captures source code and sub-values automatically
• Single macro handles all operators (==, >, contains, etc.)
• No need for specialized assertions (XCTAssertEqual, XCTAssertNil, etc.)