Testing async code in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this async function call?

Consider the following Swift async function and its call. What will be printed?

Swift

func fetchNumber() async -> Int {
    return 42
}

Task {
    let number = await fetchNumber()
    print(number)
}

A 0

B nil

C 42

D Compilation error

💡 Hint

Remember that await waits for the async function to complete and returns the value.

The function fetchNumber() returns 42 asynchronously. Using await gets the value 42, which is then printed.

❓ Predict Output

intermediate

What error does this async function cause?

What error will this Swift code produce when run?

Swift

func fetchData() async throws -> String {
    throw NSError(domain: "Test", code: 1)
}

Task {
    let data = try await fetchData()
    print(data)
}

A Runtime error: unhandled error

B Compilation error: missing try

C Prints empty string

D No error, prints data

💡 Hint

The function throws an error but the call site does not handle it.

The fetchData() function throws an error. Since the Task does not catch it, the program crashes with a runtime error.

🧠 Conceptual

advanced

Which statement about async testing is true?

In Swift async testing, which statement is correct?

A You must use <code>await</code> inside test functions marked with <code>async</code>.

B Async functions cannot be tested with XCTest.

C Async tests run synchronously by default.

D You cannot use <code>Task</code> inside test functions.

💡 Hint

Think about how async test functions handle asynchronous calls.

In Swift, test functions can be marked async to allow use of await inside them. This enables proper async testing.

❓ Predict Output

advanced

What is the output of this async sequence processing?

What will this Swift code print?

Swift

func numbers() async -> [Int] {
    return [1, 2, 3]
}

Task {
    let nums = await numbers()
    let doubled = nums.map { $0 * 2 }
    print(doubled)
}

A [1, 2, 3]

B [2, 4, 6]

C Compilation error: cannot use map here

D Runtime error: await on non-async context

💡 Hint

Remember await gets the array, then map doubles each element.

The async function returns [1, 2, 3]. After awaiting, map doubles each number, resulting in [2, 4, 6].

🔧 Debug

expert

Why does this async test fail to wait for completion?

Given this Swift test code, why does the test finish before the async call completes?

Swift

func testAsyncCall() {
    Task {
        let result = await fetchValue()
        XCTAssertEqual(result, 10)
    }
}

A fetchValue() is not an async function.

B The <code>await</code> keyword is missing inside the Task.

C XCTAssertEqual cannot be used inside Task closures.

D The test function is not marked async, so it does not wait for Task to finish.

💡 Hint

Think about how XCTest waits for async code to finish.

The test function is synchronous and returns immediately after starting the Task. Marking the test function async and using await on the async call ensures the test waits.