What does $0 and $1 mean in Swift Closures?
It represents shorthanded arguments sent into a closure, this example breaks it down:
Swift 4:
var add = { (arg1: Int, arg2: Int) -> Int in
return arg1 + arg2
}
add = { (arg1, arg2) -> Int in
return arg1 + arg2
}
add = { arg1, arg2 in
arg1 + arg2
}
add = {
$0 + $1
}
let result = add(20, 20) // 40
TL;DR
Swift 5.7
$0 and $1 are closure’s first and second Shorthand Argument Names (SAN for short) or implicit parameter names, if you like. The shorthand argument names are automatically provided by Swift. The first argument is referenced by $0, the second argument is referenced by $1, the third one by $2, and so on.
As you know, a Closure is a self-contained block of functionality (a function without name) that can be passed around and used in your code. Closure has different names in other programming languages as well as slight differences in meaning – it's Lambda in Python and Kotlin, or it's Block in C and Objective-C.
Shortening a closure
let coffee: [String] = ["Cappuccino", "Espresso", "Latte", "Ristretto"]
1. Regular function
func backward(_ n1: String, _ n2: String) -> Bool {
return n1 > n2
}
var reverseOrder = coffee.sorted(by: backward)
/* RESULT: ["Ristretto", "Latte", "Espresso", "Cappuccino"] */
2. Inline closure expression
reverseOrder = coffee.sorted {
(n1: String, n2: String) -> Bool in return n1 > n2
}
3. Inferring Type from context in a closure with an implicit return
reverseOrder = coffee.sorted { n1, n2 in n1 > n2 }
4. Shorthand Argument Names
reverseOrder = coffee.sorted { $0 > $1 }
/* $0 and $1 are closure’s first and second String arguments. */
5. Operator methods
reverseOrder = coffee.sorted(by: >)
/* RESULT: ["Ristretto", "Latte", "Espresso", "Cappuccino"] */
Higher Order Function with closure
let companies = ["bmw", "kfc", "ibm", "htc"]
let uppercased = companies.map {
(item: String) -> String in return item.uppercased()
}
print(uppercased)
/* RESULT: ["BMW", "KFC", "IBM", "HTC"] */
Shorthand Argument Name $0
let uppercased = companies.map { $0.uppercased() }
print(uppercased)
/* RESULT: ["BMW", "KFC", "IBM", "HTC"] */
Full closure expression with remainder operator (a.k.a. modulo)
let numbers: [Int] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
let filteredNumbers = numbers.filter {
(arg: Int) -> Bool in return (arg % 2) == 0
}
print(filteredNumbers)
/* RESULT: [2, 4, 6, 8, 10] */
Shorthand Argument Name with remainder operator
let numbers: [Int] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
let filteredNumbers = numbers.filter { ($0 % 2) == 0 }
print(filteredNumbers)
/* RESULT: [2, 4, 6, 8, 10] */
SAN in variadic functions
Variadic functions are ones that accept any number of parameters. One of the most famous variadic methods is print(...). Shorthand Argument Names are perfect for variadic functions.
fileprivate func dessert(_ fruits: String...) -> Bool {
return fruits.contains { $0 == "Apple" }
}
let contains = dessert("Mango", "Durian", "Papaya")
print(contains)
/* RESULT: false */
$0 for existential Sequence type
Here, allSatisfy() method indicates whether every element of a sequence satisfies a given predicate or not. The Any type represents values of any type. Existential any type is used for storing an any kind of value which conforms to a specific protocol. Also, I used here a shorthand syntax for optional binding (shadowing an existing constant).
let inputData: [Any]? = [1, 2, 3, "Hello"]
func satisfiesOrNot(_ inputData: any Sequence) -> Bool {
inputData.allSatisfy { $0 is Int }
}
if let inputData {
satisfiesOrNot(inputData)
}
/* RESULT: false */
Repeating $0
let cubedNumber = { $0 * $0 * $0 } (25)
print(cubedNumber)
/* RESULT: 25^3 = 15625 */
$0 in closure capturing
If you use external values inside a closure, Swift captures them.
func trainer(_ said: String) -> (String) -> String {
return {
return "¡\(said) \($0)!"
}
}
let announcement = trainer("Bienvenido a mis cursos")
announcement("RealityKit 2022")
/* RESULT: "¡Bienvenido a mis cursos RealityKit 2022!" */
Shorthand Argument Names $0, $1, $2
let math: (Int8, Int8, Int8) -> Int8 = { $0 + $1 - $2 }
func feedClosure() -> (Int8, Int8, Int8) -> Int8 {
return math
}
feedClosure()(10, 20, 100)
/* RESULT: (10 + 20 - 100) = -70 */
SANs $0, $1, $2, $3, $4
let factorial = { $0 * $1 * $2 * $3 * $4 } (1, 2, 3, 4, 5)
print(factorial)
/* RESULT: 5! = 120 */
Key path expression
Starting from Swift 5.2+ you can access parameters of every instance via key path expression:
struct Lighter {
let manufacturer: String
let refillable: Bool
}
let zippo = Lighter(manufacturer: "Zippo", refillable: true)
let cricket = Lighter(manufacturer: "Cricket", refillable: false)
let lighters: [Lighter] = [zippo, cricket]
let refillableOnes = lighters.map(\.refillable)
print(refillableOnes)
/* RESULT: [true, false] */
Of course, you can alternatively use a familiar syntax:
Regular syntax – $0.property:
let refillableOnes = lighters.map { $0.refillable }
print(refillableOnes)
/* RESULT: [true, false] */
SAN with a subscript
(If you want to know how to implement negative indexing in arrays, read this post please.)
let arrays: [[String]] = [["Hello","Hola","你好"], ["world","mundo","世界"]]
let helloWorld = arrays.compactMap { $0[0] }
print(helloWorld)
/* RESULT: ["Hello", "world"] */
One more example with key shortcut (Swift 5.6) and a subscript:
let dictionaries: [[_ : Any?]] = [[1: "x"], [2: nil], [3: true]]
let values = dictionaries.compactMap { $0[$0.startIndex].value }
print(values)
/* RESULT: ["x", true] */
Or look at the example of unordered set:
let collection: Set<String> = ["One", "", "Three"]
collection.map {
switch $0.isEmpty {
case true:
print("Empty")
case false:
print("Element \($0) isn't empty")
}
}
/* RESULT: "Element Three isn't empty" */
/* "Empty" */
/* "Element One isn't empty" */
SAN in a completion handler
let completionHandler: ((Bool) -> Void)? = {
if $0 {
print("It is true, sister...")
} else {
print("False")
}
}
completionHandler?(true)
/* RESULT: It is true, sister... */
Regular syntax, however, is as following:
let completionHandler: ((Bool) -> Void)? = { sayTheTruth in
if sayTheTruth {
print("It is true, sister...")
} else {
print("False")
}
}
completionHandler?(false)
/* RESULT: False */
SAN in ForEach structure in SwiftUI
let columns: [GridItem] = Array(repeating: .init(.fixed(70)), count: 5)
var body: some View {
ScrollView {
LazyVGrid(columns: columns) {
ForEach((1...10), id: \.self) {
Text("\($0)").frame(maxWidth: .infinity)
}
}
}
}
/* RESULT: 1 2 3 4 5 */
/* 6 7 8 9 10 */
Operator method vs SAN
Operator Method:
let records: [Int] = [110, 108, 107, 109, 108]
public func averageSpeed(records: [Int]) throws -> Int {
let average = records.reduce(0, +) / records.count
return average
}
try averageSpeed(records: records)
/* RESULT: 108 */
Shorthand Argument Names $0 and $1:
public func averageSpeed(records: [Int]) throws -> Int {
let average = records.reduce(0) { $0 + $1 } / records.count
return average
}
try averageSpeed(records: records)
/* RESULT: 108 */
$0 as the resulting value
.onChanged and .onEnded modifiers perform an @escaping closure when a SwiftUI gesture changed or ended.
@State private var rotate: Angle = .zero
var myGesture: some Gesture {
RotationGesture()
.onChanged { rotate = $0 }
.onEnded { angle in rotate = angle }
}
var body: some View {
Rectangle()
.rotationEffect(rotate)
.gesture(myGesture)
}
Swift vs Kotlin vs Python
Also, let's see how Kotlin's lambda is similar to Swift's closure:
Swift
let element: [String] = ["Argentum","Aurum","Platinum"]
let characterCount = element.map { $0.count }
print(characterCount)
/* RESULT: [8, 5, 8] */
Kotlin
Kotlin's lambda expression has just one parameter with implicit name: it. In other words, if you have a function literal with exactly one parameter you don’t need to define that parameter explicitly, you can use it instead (like $0 in Swift).
val element = listOf("Argentum","Aurum","Platinum")
val characterCount = element.map { it.length }
println(characterCount)
/* RESULT: [8, 5, 8] */
But in Python there's no equivalent of Shorthand Argument Name.
Python
element = ["Argentum","Aurum","Platinum"]
characterCount = list(map(lambda x: len(x), element))
print(characterCount)
# RESULT: [8, 5, 8]
The refer to the first and second arguments of sort. Here, sort compares 2 elements and order them.
You can look up Swift official documentation for more info:
Swift automatically provides shorthand argument names to inline closures, which can be used to refer to the values of the closure’s arguments by the names $0, $1, $2, and so on.
$0 is the first parameter passed into the closure. $1 is the second parameter, etc. That closure you showed is shorthand for:
let sortedNumbers = numbers.sort { (firstObject, secondObject) in
return firstObject > secondObject
}