Swift Tutorial · Closures, Generics and Errors · Lesson 18 of 30

Swift Closures

Closure syntax and its shorthand forms, trailing closures, capturing values, and escaping closures.

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Closure syntax

A closure is a function without a name: { (a: Int, b: Int) -> Int in a + b }. Swift lets you shorten it a lot: types are inferred, a single expression doesn't need return, $0 and $1 name the arguments, and an operator such as + can stand in for a closure. When a closure is the last argument, it goes after the parentheses (a trailing closure).

From full syntax to shorthand

let numbers = [5, 2, 9, 1]

let full = numbers.sorted(by: { (a: Int, b: Int) -> Bool in return a < b })
let short = numbers.sorted { $0 < $1 }
let shortest = numbers.sorted(by: <)
print(full, short, shortest)

let add: (Int, Int) -> Int = { $0 + $1 }
print(add(2, 3))
print(numbers.reduce(0, +))

Output

[1, 2, 5, 9] [1, 2, 5, 9] [1, 2, 5, 9]
5
17

Run this on your computer — see “Get Started” for setup.

Capturing values

Closures capture variables from the surrounding scope and keep them alive. A closure stored for later use (for example a completion handler) is marked @escaping.

Captures and escaping closures

func makeCounter() -> () -> Int {
    var count = 0
    return {
        count += 1               // captured and kept alive
        return count
    }
}

let next = makeCounter()
print(next(), next(), next())

final class Uploader {
    private var handlers: [() -> Void] = []
    func onComplete(_ handler: @escaping () -> Void) {
        handlers.append(handler)     // stored to be called later
    }
    func finish() { handlers.forEach { $0() } }
}

let uploader = Uploader()
uploader.onComplete { print("upload finished") }
uploader.onComplete { print("cache cleared") }
uploader.finish()

Output

1 2 3
upload finished
cache cleared

Run this on your computer — see “Get Started” for setup.

Test yourself

In a closure, what does $0 refer to?

The return value
The first argument
self
The closure itself

Show answer

Answer: The first argument

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