Testing async code in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an async function.

Swift

func fetchData() async [1] String {
    return "Data" 
}

Drag options to blanks, or click blank then click option'

A->

Bthrows

Cawait

Dasync

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'async' instead of '->' to specify return type

Using 'throws' when not throwing errors

The arrow -> is used to specify the return type of the function.

2fill in blank

medium

Complete the code to call an async function with await.

Swift

Task {
    let result = try? await [1]()
    print(result ?? "No data")
}

Drag options to blanks, or click blank then click option'

AfetchData

BfetchData()

Cawait fetchData

DTask

Attempts:

3 left

💡 Hint

Common Mistakes

Adding parentheses inside the await expression

Calling Task instead of the function

You call the async function by its name without parentheses inside await.

3fill in blank

hard

Fix the error in the async test function declaration.

Swift

func testFetch() [1] async throws {
    let data = try await fetchData()
    XCTAssertEqual(data, "Data")
}

Drag options to blanks, or click blank then click option'

Aasync

Bthrows

Casync throws

Doverride

Attempts:

3 left

💡 Hint

Common Mistakes

Marking only async or only throws

Forgetting to mark the test function as async

Test functions that call async throwing functions must be marked async throws.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension filtering async results.

Swift

let filtered = try await Dictionary(uniqueKeysWithValues: results.compactMap { item in
    guard let value = item.value, value [1] 10 else { return nil }
    return (item.key, value[2]2)
})

Drag options to blanks, or click blank then click option'

A>

B*

C**

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > in the guard

Using ** instead of * for multiplication

The guard checks if value is greater than 10, and the value is multiplied by 2 using *.

5fill in blank

hard

Fill all three blanks to create an async test that awaits and asserts a value.

Swift

func testValue() async throws {
    let value = try await [1]()
    XCTAssertEqual(value, [2])
    XCTAssertTrue(value [3] 0)
}

Drag options to blanks, or click blank then click option'

AfetchData

B"Data"

C>

DfetchNumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using fetchData instead of fetchNumber

Using wrong expected value in XCTAssertEqual

The function fetchNumber() is awaited, the expected value is "Data", and the test checks if value is greater than 0.

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