Escaping closures (@escaping) in Swift

Introduction

Escaping closures let you use a piece of code later, even after the function that got it finishes. This helps when you want to do something after waiting or in the background.

When you want to run code after a delay or asynchronously, like fetching data from the internet.

When you store a closure to use it later, for example in a completion handler.

When you pass a closure to a function but it might be called after the function returns.

When you want to keep a closure around for event handling or callbacks.

Syntax

Swift

func someFunction(completion: @escaping () -> Void) {
    // function body
}

The @escaping keyword tells Swift the closure might be called later, not immediately.

Without @escaping, the closure must be used before the function ends.

Examples

This example shows a closure that runs after 1 second, so it must be marked @escaping.

Swift

func performTask(completion: @escaping () -> Void) {
    DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
        completion()
    }
}

This closure runs immediately, so @escaping is not needed.

Swift

func saveData(completion: () -> Void) {
    completion()
}

Sample Program

This program simulates fetching data asynchronously. The closure is marked @escaping because it runs after the function returns. The program waits 3 seconds to show the result.

Swift

import Foundation

func fetchData(completion: @escaping (String) -> Void) {
    DispatchQueue.global().async {
        // Simulate network delay
        sleep(2)
        let data = "Hello from network"
        DispatchQueue.main.async {
            completion(data)
        }
    }
}

print("Start fetching...")
fetchData { result in
    print("Received: \(result)")
}

// Keep the program running to see async result
RunLoop.main.run(until: Date(timeIntervalSinceNow: 3))

Important Notes

Use @escaping only when the closure is called after the function returns.

Escaping closures can capture variables and keep them alive longer.

Be careful with retain cycles when using escaping closures inside classes.

Summary

Escaping closures run after the function ends.

Mark closures with @escaping if they run later or asynchronously.

They help with callbacks, delays, and asynchronous tasks.

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