Swift Extensions

Swift Extensions

Extensions add new functionality to an existing class, structure, enumeration, or protocol type.

This includes the ability to extend types for which you do not have access to the original source code!

Extensions are incredibly useful for organizing code and keeping your base objects clean and focused.

Extending Built-in Types

You can use an extension to add custom methods or computed properties to Apple's built-in types, such as Int, String, or Double.

You declare an extension using the extension keyword.

extension Int {
    // Adding a computed property to Int
    var squared: Int {
        return self * self
    }
}

let number = 5
print(number.squared) // Outputs 25

Because of the extension, every single Integer in your entire project now has a .squared property!

Adding Methods via Extensions

Extensions can also add new instance methods and type methods to existing types.

extension String {
    func shout() {
        print("\(self.uppercased())!!!")
    }
}

let greeting = "hello"
greeting.shout()

Organizing Code with Extensions

A very common and highly recommended pattern in Swift development is to use extensions to group related functionality together.

Instead of cluttering a single Class definition with dozens of methods, you can split logical chunks into separate extensions.

This is especially useful when conforming to multiple protocols.

struct User {
    var name: String
}

// We use an extension strictly to handle protocol conformance!
extension User: CustomStringConvertible {
    var description: String {
        return "User(name: \(name))"
    }
}
let admin = User(name: "Akash")
print(admin)

What Extensions Cannot Do

It is important to note that extensions cannot add new stored properties (variables) to a type. They can only add computed properties.