Escaping closures (@escaping) in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function parameter as an escaping closure.

Swift

func performTask(completion: [1] () -> Void) {
    // function body
}

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

A@discardableResult

B@escaping

C@autoclosure

D@inline

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to add @escaping causes a compile error when the closure is used asynchronously.

Using @autoclosure instead of @escaping.

The @escaping keyword is used to mark a closure parameter that can escape the function's scope.

2fill in blank

medium

Complete the code to store an escaping closure in a property.

Swift

class Handler {
    var completionHandler: (() -> Void)?
    func setHandler(handler: [1] () -> Void) {
        completionHandler = handler
    }
}

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

A@autoclosure

B@inline

C@discardableResult

D@escaping

Attempts:

3 left

💡 Hint

Common Mistakes

Not marking the closure as @escaping causes a compile error.

Confusing @escaping with other closure attributes.

The closure parameter must be marked @escaping because it is stored beyond the function's lifetime.

3fill in blank

hard

Fix the error in the function declaration by marking the closure parameter correctly.

Swift

func fetchData(completion: [1] () -> Void) {
    DispatchQueue.global().async {
        // some async work
        completion()
    }
}

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

A@escaping

B@autoclosure

C@discardableResult

D@inline

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting @escaping causes a compile-time error.

Using @autoclosure instead of @escaping.

The closure is called asynchronously, so it must be marked @escaping.

4fill in blank

hard

Fill both blanks to declare a function with an escaping closure and call it asynchronously.

Swift

func executeTask(task: [1] () -> Void) {
    DispatchQueue.main.[2] {
        task()
    }
}

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

A@escaping

Basync

Cawait

D@autoclosure

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting @escaping on the closure parameter.

Using 'await' instead of 'async' in DispatchQueue.

The closure parameter must be marked @escaping because it is called asynchronously. The DispatchQueue method uses async to schedule the closure.

5fill in blank

hard

Fill all three blanks to declare a class with an escaping closure property, a method to set it, and a method to call it.

Swift

class AsyncProcessor {
    var completion: (() -> Void)?
    func setCompletion(handler: [1] () -> Void) {
        completion = handler
    }
    func run() {
        DispatchQueue.global().[2] {
            // do work
            self.completion[3]()
        }
    }
}

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

A@escaping

Basync

C.

D?

Attempts:

3 left

💡 Hint

Common Mistakes

Not marking the closure as @escaping.

Forgetting to use optional chaining when calling the closure.

Using '.' instead of '?' for optional closure call.

The closure parameter must be @escaping because it is stored. The async keyword is used for asynchronous dispatch. The optional chaining operator '?' safely calls the closure if it exists.

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