Swift Closures

Swift Tutorial

🧠 Swift Closures – Complete Beginner to Interview Guide (With Clear Examples)

Closures in Swift are self-contained blocks of code that you can store, pass, and execute later.
They are heavily used in iOS development, functional programming, callbacks, and async code.

1️⃣ What is a Closure in Swift? ⭐

A closure is like an anonymous function (function without a name).


 

Output

Hello Swift

2️⃣ Why Closures Are Important ⭐

Closures are used for:

  • Callbacks (networking, animations)

  • Sorting & filtering

  • Async operations

  • Functional programming (map, filter, reduce)

📌 Almost every real Swift app uses closures.

3️⃣ Basic Closure Syntax ⭐


 

4️⃣ Closure with Parameters ⭐


 

Output

8

5️⃣ Type Inference (Cleaner Syntax) ⭐⭐

Swift can infer types:


 

6️⃣ Shorthand Argument Names ($0, $1) ⭐⭐


 

7️⃣ Trailing Closures ⭐⭐⭐ (Very Important)

If a function’s last parameter is a closure, write it outside parentheses.


 

📌 Makes code clean and readable.

8️⃣ Closures as Function Parameters ⭐⭐⭐


 

Output

50

9️⃣ Capturing Values ⭐⭐⭐

Closures can capture variables from their surrounding scope.


 

Output

1
2

📌 Captured values persist between calls.

🔟 Escaping Closures ⭐⭐⭐ (Interview Favorite)

A closure escapes when it is executed after the function returns.


 

📌 Required for:

  • Async tasks

  • Stored closures

1️⃣1️⃣ Avoiding Retain Cycles ([weak self]) ⭐⭐⭐

❌ Problem


 

✅ Solution


 

📌 Prevents memory leaks.

1️⃣2️⃣ Closures with Collections ⭐⭐⭐


 

1️⃣3️⃣ Closure vs Function ⭐⭐

Feature Closure Function
Name Optional Required
Inline use ✅ ❌
Capture variables ✅ ❌
Used as value ✅ Limited

1️⃣4️⃣ Common Mistakes ❌

❌ Overusing shorthand syntax (hurts readability)
❌ Forgetting @escaping
❌ Retain cycles with self
❌ Using map for side effects

📌 Interview Questions (Swift Closures)

Q1. What is a closure?
👉 An anonymous function that can capture values

Q2. What is an escaping closure?
👉 Executed after function returns

Q3. Why use [weak self]?
👉 To avoid retain cycles

Q4. What is trailing closure syntax?
👉 Writing closure outside function call

✅ Summary

✔ Closures are anonymous functions
✔ Can capture surrounding variables
✔ Support shorthand & trailing syntax
✔ @escaping for async behavior
✔ [weak self] prevents memory leaks
✔ Essential for Swift & iOS interviews

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