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.

Practice

1. What keyword must you add to a Swift test function to test asynchronous code?

easy

A. defer

B. await

C. async

D. try

Solution

1. Step 1: Understand async test function requirement

   To test asynchronous code, the test function itself must be marked as asynchronous.

2. Step 2: Identify the correct keyword

   The keyword to mark a function as asynchronous in Swift is async.

3. Final Answer:

   async -> Option C

4. Quick Check:

   async = async [OK]

Hint: Mark test functions with async to test async code [OK]

Common Mistakes:

• Using await instead of async on test function
• Forgetting to mark test function as async
• Using try instead of async
• Confusing defer with async

2. Which of the following is the correct syntax to wait for an async function fetchData() inside a test?

easy

A. let result = await fetchData()

B. let result = async fetchData()

C. await let result = fetchData()

D. fetchData().await()

Solution

1. Step 1: Recall await usage in Swift

   In Swift, to wait for an async function, you use the await keyword before the function call.

2. Step 2: Check correct syntax

   The correct syntax is let result = await fetchData(). The keyword await comes before the function call.

3. Final Answer:

   let result = await fetchData() -> Option A

4. Quick Check:

   await before function call = let result = await fetchData() [OK]

Hint: Put await before async function call to wait [OK]

Common Mistakes:

• Placing await after let or variable
• Using async instead of await before call
• Calling await as a method
• Omitting await keyword

3. Given this test code, what will be printed?

func testFetch() async {
  let data = try? await fetchData()
  print(data == nil ? "No Data" : "Got Data")
}

func fetchData() async throws -> String {
  return "Hello"
}

medium

A. No Data

B. Compilation error

C. Runtime error

D. Got Data

Solution

1. Step 1: Analyze fetchData() behavior

   The function fetchData() is async and throws but always returns "Hello" without throwing an error.

2. Step 2: Understand try? await usage

   The try? await fetchData() will return the string "Hello" wrapped as optional, not nil, because no error is thrown.

3. Step 3: Evaluate print statement

   Since data is not nil, the print outputs "Got Data".

4. Final Answer:

   Got Data -> Option D

5. Quick Check:

   try? await returns value = Got Data [OK]

Hint: try? returns nil only if error thrown, else returns value [OK]

Common Mistakes:

• Assuming try? always returns nil
• Confusing async throws with throwing error always
• Ignoring optional unwrapping
• Expecting compilation error due to async

4. Identify the error in this async test function:

func testDownload() async {
  let result = fetchData()
  XCTAssertEqual(result, "Success")
}

func fetchData() async -> String {
  return "Success"
}

medium

A. XCTAssertEqual cannot compare strings

B. Missing await before fetchData() call

C. Test function should not be async

D. fetchData() must throw an error

Solution

1. Step 1: Check async function call

   The function fetchData() is async, so calling it requires await to get the result.

2. Step 2: Identify missing await

   In the test, fetchData() is called without await, causing a compile error.

3. Final Answer:

   Missing await before fetchData() call -> Option B

4. Quick Check:

   Async calls need await = Missing await before fetchData() call [OK]

Hint: Always await async calls inside async functions [OK]

Common Mistakes:

• Calling async function without await
• Thinking test functions can't be async
• Misusing XCTAssertEqual with strings
• Assuming async functions must throw

5. You want to test an async function loadUser(id:) that throws an error if the user is not found. Which test code correctly checks that an error is thrown for an invalid id?

hard

A. func testLoadUserError() async throws {
do {
_ = try await loadUser(id: -1)
XCTFail("Expected error not thrown")
} catch {
XCTAssertTrue(true)
}
}

B. func testLoadUserError() async {
let user = try await loadUser(id: -1)
XCTAssertNil(user)
}

C. func testLoadUserError() async throws {
let user = await loadUser(id: -1)
XCTAssertNil(user)
}

D. func testLoadUserError() async throws {
_ = try await loadUser(id: -1)
XCTAssertTrue(false)
}

Solution

1. Step 1: Understand error throwing test pattern

   To test that an async function throws, use a do-catch block and call the function with try await.

2. Step 2: Verify correct error handling

   func testLoadUserError() async throws { do { _ = try await loadUser(id: -1) XCTFail("Expected error not thrown") } catch { XCTAssertTrue(true) } } calls loadUser(id: -1) with try await inside do, fails test if no error, and passes if error caught.

3. Step 3: Check other options

   Approaches expecting a nil user incorrectly assume throwing functions return nil instead of throwing. func testLoadUserError() async throws { _ = try await loadUser(id: -1) XCTAssertTrue(false) } fails test unconditionally.

4. Final Answer:

   do { _ = try await loadUser(id: -1) XCTFail("Expected error not thrown") } catch { XCTAssertTrue(true) } -> Option A

5. Quick Check:

   Use do-catch with try await to test errors [OK]

Hint: Use do-catch with try await to test thrown errors [OK]

Common Mistakes:

• Expecting nil instead of catching error
• Omitting try or await keywords
• Failing test unconditionally
• Not marking test async throws