Swift Extensions

Add functionality to existing types without subclassing, including methods, computed properties, and protocol conformances.

Computed Properties and Methods

Extend any type to add helpers that are scoped to that type.

Syntax: extension Type { var computed: T { ... }; func util() { ... } }

Example

extension String {
  var isBlank: Bool { trimmingCharacters(in: .whitespacesAndNewlines).isEmpty }
  func repeated(_ n: Int) -> String { String(repeating: self, count: n) }
}

print("  ".isBlank)    // true
print("Hi".repeated(3)) // HiHiHi

This example adds a computed property and a helper method to String.

Protocol Conformance in Extensions

Adopt protocols outside the original type declaration to separate concerns.

Syntax: extension Type: Protocol { ... }

Example

protocol Describable { func describe() -> String }

struct User { let name: String }

extension User: Describable {
  func describe() -> String { "User(\(name))" }
}

print(User(name: "Morgan").describe())

This example adds Describable conformance to User via an extension.

Nesting by Responsibility

Organize large types using multiple extensions grouped by feature (e.g., networking, formatting).

Syntax:

extension Type { /* Feature A */ }
extension Type { /* Feature B */ }

Example

struct Article { let title: String; let body: String }

extension Article { // Formatting helpers
  var preview: String { String(body.prefix(40)) + "..." }
}

extension Article { // Networking stub
  static func fetchAll() -> [Article] { [] }
}

Swift Extensions