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.

Practice

1. What does the @escaping keyword indicate in Swift closures?

easy

A. The closure can be called after the function returns

B. The closure is executed immediately within the function

C. The closure cannot capture variables from the surrounding scope

D. The closure is only used for synchronous tasks

Solution

1. Step 1: Understand closure execution timing

   Closures without @escaping run inside the function before it returns.

2. Step 2: Role of @escaping

   Marking a closure @escaping means it can be stored and called later, after the function ends.

3. Final Answer:

   The closure can be called after the function returns -> Option A

4. Quick Check:

   @escaping means delayed closure call [OK]

Hint: Escaping closures run later, not immediately [OK]

Common Mistakes:

• Thinking @escaping closures run immediately
• Confusing @escaping with synchronous execution
• Believing @escaping closures cannot capture variables

2. Which of the following is the correct syntax to declare a function parameter as an escaping closure in Swift?

easy

A. func fetchData(completion: () -> Void) {}

B. func fetchData(completion: escaping () -> Void) {}

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

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

Solution

1. Step 1: Identify correct placement of @escaping

   The @escaping keyword must appear before the closure type in the parameter list.

2. Step 2: Check each option's syntax

   func fetchData(completion: @escaping () -> Void) {} correctly places @escaping before the closure type. Options A, C, and D have incorrect syntax.

3. Final Answer:

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

4. Quick Check:

   Correct syntax = func fetchData(completion: @escaping () -> Void) {} [OK]

Hint: Put @escaping before the closure type in parameters [OK]

Common Mistakes:

• Placing @escaping before parameter name
• Omitting @escaping for delayed closures
• Writing 'escaping' without @ symbol

3. What will be the output of the following Swift code?

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

performTask {
    print("Task completed")
}

medium

A. Task started

B. Task completed\nTask started

C. Task started\nTask completed

D. Task completed

Solution

1. Step 1: Understand async dispatch behavior

   The closure passed to DispatchQueue.main.async runs later, after the current function finishes.

2. Step 2: Trace print statements order

   "Task started" prints immediately. Then the closure runs and prints "Task completed".

3. Final Answer:

   Task started\nTask completed -> Option C

4. Quick Check:

   Async closure runs after print [OK]

Hint: Async closures run after current code finishes [OK]

Common Mistakes:

• Assuming closure runs before print
• Ignoring async dispatch delay
• Confusing order of print statements

4. Identify the error in this Swift function using an escaping closure:

func saveData(completion: () -> Void) {
    DispatchQueue.global().async {
        // Save operation
        completion()
    }
}

medium

A. Missing @escaping before closure parameter

B. Closure parameter should be optional

C. DispatchQueue.global() cannot be used here

D. Completion closure must return a value

Solution

1. Step 1: Analyze closure usage inside async block

   The closure is called asynchronously after the function returns, so it must be marked @escaping.

2. Step 2: Check function parameter declaration

   The parameter lacks @escaping, causing a compile error.

3. Final Answer:

   Missing @escaping before closure parameter -> Option A

4. Quick Check:

   Async closure needs @escaping [OK]

Hint: Async closure parameters must be @escaping [OK]

Common Mistakes:

• Forgetting @escaping for async closures
• Assuming closure is synchronous by default
• Adding unnecessary optional to closure

5. You want to store a closure to call later after a network request finishes. Which is the correct way to declare and use this escaping closure in Swift?

class NetworkManager {
    var completionHandler: (() -> Void)?

    func requestData(completion: ???) {
        self.completionHandler = completion
        // Simulate network delay
        DispatchQueue.global().asyncAfter(deadline: .now() + 1) {
            self.completionHandler?()
        }
    }
}

hard

A. completion: () -> Void

B. completion: @escaping () -> Void

C. completion: @autoclosure () -> Void

D. completion: @escaping @autoclosure () -> Void

Solution

1. Step 1: Understand closure storage and delayed call

   The closure is saved in a property and called later asynchronously, so it must be @escaping.

2. Step 2: Evaluate options for closure parameter

   Only completion: @escaping () -> Void correctly marks the closure as @escaping. Options A, B, and D are incorrect or unnecessary here.

3. Final Answer:

   completion: @escaping () -> Void -> Option B

4. Quick Check:

   Stored closures must be @escaping [OK]

Hint: Store closures? Use @escaping keyword [OK]

Common Mistakes:

• Omitting @escaping when storing closures
• Using @autoclosure unnecessarily
• Marking closure without @escaping for delayed call