Swift Extensions
Swift extensions let you add new functionality — methods, computed properties, initializers, and protocol conformances — to existing types, even ones you don't own.
What Extensions Do
An extension adds capability to an existing type — including types from Apple's frameworks or third-party libraries — without subclassing or modifying the original source. This keeps related behavior organized and lets you extend types you can't otherwise touch, like Int or String.
Extending Int
extension Int {
var isEven: Bool {
return self % 2 == 0
}
var squared: Int {
return self * self
}
func repeated(_ action: () -> Void) {
for _ in 0..<self {
action()
}
}
}
print(4.isEven) // true
print(5.squared) // 25
3.repeated { print("Hi") } // prints Hi three timesExtending With Computed Properties
Extensions frequently add computed properties that express a value in a different form, such as unit conversions. Because they're computed rather than stored, they don't change the type's memory layout.
Extending Double
extension Double {
var km: Double { return self * 1000.0 }
var m: Double { return self }
var cm: Double { return self / 100.0 }
func rounded(toPlaces places: Int) -> Double {
let multiplier = pow(10.0, Double(places))
return (self * multiplier).rounded() / multiplier
}
}
let distance = 3.5.km
print(distance) // 3500.0
let value = 3.14159.rounded(toPlaces: 2)
print(value) // 3.14Adding Initializers and Protocol Conformance
Extensions can add convenience initializers and make an existing type conform to a new protocol. This is the standard way to satisfy a protocol requirement without touching the type's original declaration.
Conforming to a Protocol
protocol Summarizable {
func summary() -> String
}
struct Article {
let title: String
let wordCount: Int
}
extension Article: Summarizable {
func summary() -> String {
return "\(title) (\(wordCount) words)"
}
}
let post = Article(title: "Swift Extensions", wordCount: 850)
print(post.summary()) // Swift Extensions (850 words)What Extensions Cannot Do
- Cannot add stored properties — only computed ones
- Cannot override existing methods or properties
- Cannot add designated initializers to classes (only convenience initializers)
- Cannot add deinitializers
Exercise: Swift Extensions
What can a Swift extension add to an existing type?
