Escaping closures (@escaping) in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a function with an escaping closure

What will be printed when this Swift code runs?

Swift

func performTask(completion: @escaping () -> Void) {
    DispatchQueue.global().async {
        print("Task started")
        completion()
    }
}

performTask {
    print("Task completed")
}

print("Function called")

A Function called\nTask started\nTask completed

B Task started\nTask completed\nFunction called

C Function called\nTask completed\nTask started

D Task completed\nTask started\nFunction called

💡 Hint

Think about how async code runs in the background and when the print statements execute.

The function performTask starts an asynchronous task. The print("Function called") runs immediately after calling performTask, before the async block executes. So the output order is: Function called, then Task started, then Task completed.

🧠 Conceptual

intermediate

Why use @escaping in Swift closures?

Which statement best explains why the @escaping attribute is required for a closure parameter in Swift?

A Because the closure is called after the function returns, so it escapes the function's scope.

B Because the closure modifies variables outside its own scope.

C Because the closure is guaranteed to run synchronously within the function.

D Because the closure is optional and might not be called.

💡 Hint

Think about when the closure is executed relative to the function call.

The @escaping attribute is needed when the closure is stored or called after the function returns, meaning it escapes the function's lifetime.

🔧 Debug

advanced

Identify the error with closure escaping

What error will this Swift code produce and why?

Swift

func fetchData(completion: () -> Void) {
    DispatchQueue.global().async {
        completion()
    }
}

fetchData {
    print("Data fetched")
}

A No error, prints 'Data fetched' asynchronously

B Error: Missing return statement in closure

C Runtime crash due to closure escaping

D Error: Closure use of non-escaping parameter 'completion' may allow it to escape

💡 Hint

Check the closure parameter declaration and how it's used inside an async block.

The closure parameter is non-escaping by default. Using it inside an async block means it escapes the function scope, causing a compile-time error.

📝 Syntax

advanced

Correct syntax for an escaping closure parameter

Which option correctly declares a function with an escaping closure parameter in Swift?

A func loadData(completion: escaping () -> Void) {}

B func loadData(@escaping completion: () -> Void) {}

C func loadData(completion: @escaping () -> Void) {}

D func loadData(completion: () -> Void @escaping) {}

💡 Hint

Remember the position of @escaping in the parameter declaration.

The @escaping attribute must appear before the closure type in the parameter declaration.

🚀 Application

expert

Determine the final value after escaping closure modifies a variable

What is the value of 'counter' after this Swift code runs?

Swift

var counter = 0

func incrementLater(completion: @escaping () -> Void) {
    DispatchQueue.global().async {
        counter += 1
        completion()
    }
}

incrementLater {
    print("Increment done")
}

// Wait briefly to let async code finish
Thread.sleep(forTimeInterval: 0.1)

print(counter)

A 0

B 1

C 2

D Compilation error

💡 Hint

Consider that the increment happens asynchronously and the main thread waits before printing.

The async block increments counter by 1. The main thread waits briefly to let the async code finish, so counter is 1 when printed.