Swift Q & A


What is a closure in Swift?

In Swift, a closure is a self-contained block of code that can be stored and executed within your code. Closures are similar to functions, but they are defined in a more compact form and can capture and store references to variables and constants from the surrounding context in which they are defined. Closures are a powerful feature in Swift and are used extensively in various scenarios, such as handling asynchronous tasks, sorting data, and defining concise inline operations.


Here are the key components and characteristics of closures in Swift:


  1. Syntax: Closures are defined within braces `{}` and can take parameters, provide a return type, and include the actual code to be executed. The basic structure is `{ (parameters) -> returnType in code`.


  1. Capture Values: Closures can capture and store references to variables and constants from their surrounding context, even if those values no longer exist in that context. This allows closures to retain and use values even after the surrounding function has finished executing.


  1. Types of Closures: Swift supports three main types of closures:

   – Global Functions: Closures that have a name but do not capture any values.

   – Nested Functions: Closures defined within a function that can capture values from their containing function.

   – Closure Expressions: Anonymous closures defined in a concise form, often used as arguments to higher-order functions like `map`, `filter`, and `sort`.


  1. Usage: Closures are commonly used in Swift for tasks like sorting arrays, performing asynchronous operations with completion handlers, and implementing concise code blocks for tasks such as mapping and filtering collections.


Here’s an example of a closure that adds two integers:

let additionClosure: (Int, Int) -> Int = { (a, b) in
    return a + b

let result = additionClosure(5, 3)  // result is 8

Closures in Swift provide a flexible and powerful way to encapsulate functionality, making your code more concise and expressive. They are a crucial part of Swift’s functional programming capabilities and are used extensively in modern Swift development.


Previously at
Flag Argentina
time icon
Experienced iOS Engineer with 7+ years mastering Swift. Created fintech solutions, enhanced biopharma apps, and transformed retail experiences.