Testing async code in Swift

Introduction

We test async code to make sure it works correctly even when tasks happen at different times. This helps catch mistakes before users see them.

When you want to check if a network call returns the right data.

When you need to verify that a background task finishes successfully.

When you want to test code that waits for a delay or timer.

When you want to confirm that async functions handle errors properly.

Syntax

Swift

func testExample() async throws {
    let result = try await someAsyncFunction()
    XCTAssertEqual(result, expectedValue)
}

Use the 'async' keyword in your test function to test async code.

Use 'await' to wait for the async function to finish before checking results.

Examples

This test waits for fetchDataFromServer() to finish and checks that data is not nil.

Swift

func testFetchData() async throws {
    let data = try await fetchDataFromServer()
    XCTAssertNotNil(data)
}

This test checks that an async function throws an error as expected.

Swift

func testAsyncError() async {
    do {
        _ = try await asyncFunctionThatThrows()
        XCTFail("Expected error but got success")
    } catch {
        XCTAssertTrue(true)
    }
}

Sample Program

This program defines an async function that waits half a second and returns 42. The test checks if the returned number is 42 and prints a success message.

Swift

import XCTest

class AsyncTests: XCTestCase {
    func fetchNumber() async -> Int {
        // Simulate async delay
        try? await Task.sleep(nanoseconds: 500_000_000) // 0.5 seconds
        return 42
    }

    func testFetchNumber() async {
        let number = await fetchNumber()
        XCTAssertEqual(number, 42)
        print("Test passed: number is \(number)")
    }
}

// Run the test manually (for playground or script)
@main
struct Main {
    static func main() async {
        let tests = AsyncTests()
        await tests.testFetchNumber()
    }
}

Important Notes

Remember to mark test functions with async when testing async code.

Use await to pause until the async task finishes before checking results.

In XCTest, async tests can throw errors, so use async throws if needed.

Summary

Async tests let you check code that runs at different times.

Use async and await keywords in test functions.

Test both success and error cases for async functions.