# Closures > Version: *Swift 5.5*\ > Source: [*swift-book: Closures*](https://docs.swift.org/swift-book/LanguageGuide/Closures.html)\ > Digest Date: *January 16, 2022* *Closures* in Swift are similar to *blocks* in C and Objective-C and to *lambdas* in other programming languages. * [Closures](#closures) * [Overview](#overview) * [Closure Expressions](#closure-expressions) * [The Sorted Method](#the-sorted-method) * [Closure Expression Syntax](#closure-expression-syntax) * [Inferring Type From Context](#inferring-type-from-context) * [Implicit Returns from Single-Expression Closures](#implicit-returns-from-single-expression-closures) * [Shorthand Argument Names](#shorthand-argument-names) * [Operator Methods](#operator-methods) * [Trailing Closures](#trailing-closures) * [Capturing Values](#capturing-values) * [Closures Are Reference Types](#closures-are-reference-types) * [Escaping Closures](#escaping-closures) * [Autoclosures](#autoclosures) ## Overview Closures can *capture* and *store* references to any constants and variables from the *context* in which they’re defined. This is known as *closing over* those constants and variables. Swift handles all of the memory management of capturing for you. **Global and nested functions, actually special cases of closures**. Closures take one of three forms: * *Global functions* are closures that have a name and don’t capture any values. * *Nested functions* are closures that have a name and can capture values from their enclosing function. * *Closure expressions* are *unnamed* closures written in a lightweight syntax that can capture values from their surrounding context. Swift’s closure expressions have a clean, clear style, with optimizations that encourage brief, clutter-free syntax in common scenarios. These optimizations include: * Inferring parameter and return value types from context * Implicit returns from single-expression closures * Shorthand argument names * Trailing closure syntax ## Closure Expressions *Closure expressions* are a way to write inline closures in a brief, focused syntax. ### The Sorted Method Swift’s standard library provides a method called `sorted(by:)`, which sorts an array of values of a known type, based on the output of a sorting closure that you provide. Once it completes the sorting process, the `sorted(by:)` method returns a *new* array of the same type and size as the old one, with its elements in the correct sorted order. The original array *isn’t* modified by the `sorted(by:)` method. The closure expression examples below use the `sorted(by:)` method to sort an array of String values in reverse alphabetical order. Here’s the initial array to be sorted: ```swift let names = ["Chris", "Alex", "Ewa", "Barry", "Daniella"] ``` The `sorted(by:)` method accepts a closure that takes two arguments of the same type as the array’s contents, and returns a `Bool` value to say whether the first value should appear before or after the second value once the values are sorted. **The sorting closure needs to return true if the first value should appear before the second value**, and false otherwise. This example is sorting an array of `String` values, and so the sorting closure needs to be a function of type `(String, String) -> Bool`. One way to provide the sorting closure is to write a normal function of the correct type, and to pass it in as an argument to the `sorted(by:)` method: ```swift func backward(_ s1: String, _ s2: String) -> Bool { return s1 > s2 } var reversedNames = names.sorted(by: backward) // reversedNames is equal to ["Ewa", "Daniella", "Chris", "Barry", "Alex"] ``` If the first string (`s1`) is greater than the second string (`s2`), the `backward(_:_:)` function will return `true`, indicating that `s1` should appear before `s2` in the sorted array. ### Closure Expression Syntax Closure expression syntax has the following general form: ![](https://883042997-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-Mi1EHMi0dOhf5YbiyN_%2Fuploads%2Fgit-blob-d9fc7c4e70f25f8b6471420d8a2c76207acbf5dd%2FclosureExpressionSyntax.jpg?alt=media) * The *parameters* in closure expression syntax can be `in-out` parameters, but they can’t have a default value. * *Variadic parameters*（可变参数）can be used if you name the variadic parameter. * *Tuples* can also be used as parameter types and return types. The example below shows a closure expression version of the `backward(_:_:)` function from above: ```swift reversedNames = names.sorted(by: { (s1: String, s2: String) -> Bool in return s1 > s2 }) ``` The start of the closure’s body is introduced by the `in` keyword. This keyword indicates that the definition of the closure’s *parameters* and *return type* has finished, and the *body* of the closure is about to begin. Because the body of the closure is so short, it can even be written on a single line: ```swift reversedNames = names.sorted(by: { (s1: String, s2: String) -> Bool in return s1 > s2 } ) ``` ### Inferring Type From Context Because all of the types can be inferred, the return arrow (`->`) and the parentheses around the names of the parameters can also be omitted: ```swift reversedNames = names.sorted(by: { s1, s2 in return s1 > s2 } ) ``` It’s always possible to infer the parameter types and return type when passing a closure to a function or method as an inline closure expression. As a result, you *never* need to write an inline closure in its fullest form when the closure is used as a function or method argument. ### Implicit Returns from Single-Expression Closures Single-expression closures can implicitly return the result of their single expression by omitting the `return` keyword from their declaration, as in this version of the previous example: ```swift reversedNames = names.sorted(by: { s1, s2 in s1 > s2 } ) ``` ### Shorthand Argument Names 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. If you use these shorthand argument names within your closure expression, you can omit the closure’s *argument list* from its definition. The type of the shorthand argument names is inferred from the expected function type, and the highest numbered shorthand argument you use determines the number of arguments that the closure takes. The `in` keyword can also be omitted, because the closure expression is made up entirely of its body: ```swift reversedNames = names.sorted(by: { $0 > $1 } ) ``` * Here, `$0` and `$1` refer to the closure’s first and second `String` arguments. Because `$1` is the shorthand argument with highest number, the closure is understood to take two arguments. * Because the `sorted(by:)` function here expects a closure whose arguments are both strings, the shorthand arguments `$0` and `$1` are both of type `String`. ### Operator Methods There’s actually an even shorter way to write the closure expression above. Swift’s `String` type defines its string-specific implementation of the greater-than operator (`>`) as a method that has two parameters of type `String`, and returns a value of type `Bool`. This exactly matches the method type needed by the `sorted(by:)` method. Therefore, you can simply pass in the greater-than operator, and Swift will infer that you want to use its string-specific implementation: ```swift reversedNames = names.sorted(by: >) ``` ## Trailing Closures If you need to pass a closure expression to a function as the function’s final argument and the closure expression is long, it can be useful to write it as a `trailing closure` instead. When you use the trailing closure syntax, you *don’t* write the *argument label* for the first closure as part of the function call. A function call can include multiple trailing closures; however, the first few examples below use a single trailing closure. ```swift func someFunctionThatTakesAClosure(closure: () -> Void) { // function body goes here } // Here's how you call this function without using a trailing closure: someFunctionThatTakesAClosure(closure: { // closure's body goes here }) // Here's how you call this function with a trailing closure instead: someFunctionThatTakesAClosure() { // trailing closure's body goes here } ``` The string-sorting closure from the [Closure Expression Syntax](#closure-expression-syntax) section above can be written outside of the `sorted(by:)` method’s parentheses as a trailing closure: ```swift reversedNames = names.sorted() { $0 > $1 } ``` If a closure expression is provided as the function’s or method’s only argument and you provide that expression as a trailing closure, you don’t need to write a pair of parentheses `()` after the function or method’s name when you call the function: ```swift reversedNames = names.sorted { $0 > $1 } ``` Trailing closures are most useful when the closure is sufficiently long that it isn’t possible to write it inline on a single line. As an example, Swift’s `Array` type has a `map(_:)` method, which takes a closure expression as its single argument. The closure is called once for each item in the array, and returns an alternative mapped value (possibly of some other type) for that item. You specify the nature of the mapping and the type of the returned value by writing code in the closure that you pass to `map(_:)`. After applying the provided closure to each array element, the `map(_:)` method returns a new array containing all of the new mapped values, in the same order as their corresponding values in the original array. Here’s how you can use the `map(_:)` method with a trailing closure to convert an array of `I`nt values into an array of `String` values. The array `[16, 58, 510]` is used to create the new array `["OneSix", "FiveEight", "FiveOneZero"]`: ```swift let digitNames = [ 0: "Zero", 1: "One", 2: "Two", 3: "Three", 4: "Four", 5: "Five", 6: "Six", 7: "Seven", 8: "Eight", 9: "Nine" ] let numbers = [16, 58, 510] ``` The code above creates a dictionary of mappings between the integer digits and English-language versions of their names. It also defines an array of integers, ready to be converted into strings. You can now use the `numbers` array to create an array of `String` values, by passing a closure expression to the array’s `map(_:)` method as a trailing closure: ```swift let strings = numbers.map { (number) -> String in var number = number var output = "" repeat { output = digitNames[number % 10]! + output number /= 10 } while number > 0 return output } // strings is inferred to be of type [String] // its value is ["OneSix", "FiveEight", "FiveOneZero"] ``` In this example, the variable `number` is initialized with the value of the closure’s `number` parameter, so that the value can be modified within the closure body. (**The parameters to functions and closures are always constants.**) If a function takes multiple closures, you *omit* the *argument label* for the first trailing closure and you label the remaining trailing closures. For example, the function below loads a picture for a photo gallery: ```swift func loadPicture(from server: Server, completion: (Picture) -> Void, onFailure: () -> Void) { if let picture = download("photo.jpg", from: server) { completion(picture) } else { onFailure() } } ``` When you call this function to load a picture, you provide two closures. The first closure is a completion handler that displays a picture after a successful download. The second closure is an error handler that displays an error to the user. ```swift loadPicture(from: someServer) { picture in someView.currentPicture = picture } onFailure: { print("Couldn't download the next picture.") } ``` In this example, the `loadPicture(from:completion:onFailure:)` function dispatches its network task into the background, and calls one of the two completion handlers when the network task finishes. Writing the function this way lets you cleanly separate the code that’s responsible for handling a network failure from the code that updates the user interface after a successful download, instead of using just one closure that handles both circumstances. ## Capturing Values A closure can *capture* constants and variables from the surrounding context in which it’s defined. The closure can then refer to and modify the values of those constants and variables from within its body, even if the original scope that defined the constants and variables no longer exists. In Swift, the simplest form of a closure that can capture values is a *nested function*, written within the body of another function. A nested function can capture any of its outer function’s arguments and can also capture any constants and variables defined within the outer function. Here’s an example of a function called `makeIncrementer`, which contains a *nested function* called `incrementer`. The nested `incrementer()` function captures two values, `runningTotal` and `amount`, from its surrounding context. After capturing these values, `incrementer` is returned by `makeIncrementer` as a closure that increments `runningTotal` by `amount` each time it’s called. ```swift func makeIncrementer(forIncrement amount: Int) -> () -> Int { var runningTotal = 0 func incrementer() -> Int { runningTotal += amount return runningTotal } return incrementer } ``` When considered in isolation, the nested incrementer() function might seem unusual: ```swift func incrementer() -> Int { runningTotal += amount return runningTotal } ``` The `incrementer()` function doesn’t have any parameters, and yet it refers to `runningTotal` and `amount` from within its function body. It does this by capturing a *reference* to `runningTotal` and `amount` from the surrounding function and using them within its own function body. Capturing by reference ensures that `runningTotal` and amount don’t disappear when the call to `makeIncrementer` ends, and also ensures that `runningTotal` is available the next time the incrementer function is called. > **NOTE**: > > As an optimization, Swift may instead capture and store a *copy* of a value if that value isn’t mutated by a closure, and if the value isn’t mutated after the closure is created. > > Swift also handles all memory management involved in disposing of variables when they’re no longer needed. Here’s an example of makeIncrementer in action: ```swift let incrementByTen = makeIncrementer(forIncrement: 10) ``` This example sets a constant called incrementByTen to refer to an incrementer function that adds 10 to its runningTotal variable each time it’s called. Calling the function multiple times shows this behavior in action: ```swift incrementByTen() // returns a value of 10 incrementByTen() // returns a value of 20 incrementByTen() // returns a value of 30 ``` If you create a second incrementer, it will have its own stored reference to a new, separate `runningTotal` variable: ```swift let incrementBySeven = makeIncrementer(forIncrement: 7) incrementBySeven() // returns a value of 7 ``` Calling the original incrementer (`incrementByTen`) again continues to increment its own `runningTotal` variable, and doesn’t affect the variable captured by `incrementBySeven`: ```swift incrementByTen() // returns a value of 40 ``` ## Closures Are Reference Types In the example above, incrementBySeven and incrementByTen are constants, but the closures these constants refer to are still able to increment the runningTotal variables that they have captured. This is because functions and closures are *reference types*. Whenever you assign a function or a closure to a constant or a variable, you are actually setting that constant or variable to be a reference to the function or closure. This also means that if you assign a closure to two different constants or variables, both of those constants or variables refer to the same closure. ```swift let alsoIncrementByTen = incrementByTen alsoIncrementByTen() // returns a value of 50 incrementByTen() // returns a value of 60 ``` ## Escaping Closures A closure is said to *escape* a function when the closure is passed as an argument to the function, but is called *after* the function returns. When you declare a function that takes a closure as one of its parameters, you can write `@escaping` before the parameter’s type to indicate that the closure is allowed to escape. One way that a closure can escape is by being stored in a variable that’s defined *outside* the function. As an example, many functions that start an asynchronous operation take a closure argument as a completion handler. The function returns after it starts the operation, but the closure isn’t called until the operation is completed, the closure needs to escape, to be called later. For example: ```swift var completionHandlers: [() -> Void] = [] func someFunctionWithEscapingClosure(completionHandler: @escaping () -> Void) { completionHandlers.append(completionHandler) } ``` The `someFunctionWithEscapingClosure(_:)` function takes a closure as its argument and adds it to an array that’s declared outside the function. If you didn’t mark the parameter of this function with `@escaping`, you would get a compile-time error. Normally, a closure captures variables implicitly by using them in the body of the closure, but in this case you need to be *explicit*. If you want to capture `self`, write `self` explicitly when you use it, or include `self` in the closure’s *capture list*. Writing `self` explicitly lets you express your intent, and reminds you to confirm that there isn’t a reference cycle. For example, in the code below, * the closure passed to `someFunctionWithEscapingClosure(_:)` refers to `self` explicitly. * In contrast, the closure passed to `someFunctionWithNonescapingClosure(_:)` is a *nonescaping* closure, which means it can refer to `self` implicitly. ```swift func someFunctionWithNonescapingClosure(closure: () -> Void) { closure() } class SomeClass { var x = 10 func doSomething() { someFunctionWithEscapingClosure { self.x = 100 } someFunctionWithNonescapingClosure { x = 200 } } } let instance = SomeClass() instance.doSomething() print(instance.x) // Prints "200" completionHandlers.first?() print(instance.x) // Prints "100" ``` Here’s a version of `doSomething()` that captures `self` by including it in the closure’s capture list, and then refers to `self` implicitly: ```swift class SomeOtherClass { var x = 10 func doSomething() { someFunctionWithEscapingClosure { [self] in x = 100 } someFunctionWithNonescapingClosure { x = 200 } } } ``` If `self` is an instance of a structure or an enumeration, you can always refer to `self` implicitly. However, an escaping closure can’t capture a mutable reference to `self` when `self` is an instance of a structure or an enumeration. Structures and enumerations don’t allow shared mutability, as discussed in [Structures and Enumerations Are Value Types](https://huanglibo.gitbook.io/notebook/swift/languageguide/classesandstructures). ```swift struct SomeStruct { var x = 10 mutating func doSomething() { someFunctionWithNonescapingClosure { x = 200 } // Ok someFunctionWithEscapingClosure { x = 100 } // Error } } ``` The call to the `someFunctionWithEscapingClosure` function in the example above is an error because it’s inside a mutating method, so `self` is mutable. That violates the rule that escaping closures can’t capture a mutable reference to `self` for structures. ## Autoclosures An *autoclosure* is a closure that’s automatically created to wrap an expression that’s being passed as an argument to a function. **It doesn’t take any arguments**, and **when it’s called, it returns the value of the expression that’s wrapped inside of it**. This syntactic convenience lets you omit braces around a function’s parameter by writing a normal expression instead of an explicit closure. It’s common to *call* functions that take autoclosures, but it’s not common to implement that kind of function. For example, the `assert(condition:message:file:line:)` function takes an autoclosure for its `condition` and `message` parameters; its `condition` parameter is evaluated only in debug builds and its `message` parameter is evaluated only if `condition` is `false`. An autoclosure lets you *delay* evaluation, because the code inside isn’t run until you call the closure. Delaying evaluation is useful for code that has side effects or is computationally expensive, because it lets you control when that code is evaluated. The code below shows how a closure delays evaluation. ```swift var customersInLine = ["Chris", "Alex", "Ewa", "Barry", "Daniella"] print(customersInLine.count) // Prints "5" let customerProvider = { customersInLine.remove(at: 0) } print(customersInLine.count) // Prints "5" print("Now serving \(customerProvider())!") // Prints "Now serving Chris!" print(customersInLine.count) // Prints "4" ``` Even though the first element of the `customersInLine` array is removed by the code inside the closure, the array element isn’t removed until the closure is actually called. If the closure is never called, the expression inside the closure is never evaluated, which means the array element is never removed. Note that the type of `customerProvider` isn’t `String` but `() -> String`, a function with no parameters that returns a string. You get the same behavior of delayed evaluation when you pass a closure as an argument to a function. ```swift // customersInLine is ["Alex", "Ewa", "Barry", "Daniella"] func serve(customer customerProvider: () -> String) { print("Now serving \(customerProvider())!") } serve(customer: { customersInLine.remove(at: 0) } ) // Prints "Now serving Alex!" ``` The `serve(customer:)` function in the listing *above* takes an explicit closure that returns a customer’s name. The version of `serve(customer:)` *below* performs the same operation but, instead of taking an explicit closure, it takes an *autoclosure* by marking its parameter’s type with the `@autoclosure` attribute. Now you can call the function as if it took a `String` argument instead of a closure. The argument is automatically converted to a closure, because the `customerProvider` parameter’s type is marked with the `@autoclosure` attribute. ```swift // customersInLine is ["Ewa", "Barry", "Daniella"] func serve(customer customerProvider: @autoclosure () -> String) { print("Now serving \(customerProvider())!") } serve(customer: customersInLine.remove(at: 0)) // Prints "Now serving Ewa!" ``` > **NOTE**: **Overusing autoclosures can make your code hard to understand.** The context and function name should make it clear that evaluation is being deferred. If you want an autoclosure that’s allowed to escape, use both the `@autoclosure` and `@escaping` attributes. ```swift // customersInLine is ["Barry", "Daniella"] var customerProviders: [() -> String] = [] func collectCustomerProviders(_ customerProvider: @autoclosure @escaping () -> String) { customerProviders.append(customerProvider) } collectCustomerProviders(customersInLine.remove(at: 0)) collectCustomerProviders(customersInLine.remove(at: 0)) print("Collected \(customerProviders.count) closures.") // Prints "Collected 2 closures." for customerProvider in customerProviders { print("Now serving \(customerProvider())!") } // Prints "Now serving Barry!" // Prints "Now serving Daniella!" ``` In the code above, instead of calling the closure passed to it as its `customerProvider` argument, the `collectCustomerProviders(_:)` function appends the closure to the `customerProviders` array. The array is declared *outside* the scope of the function, which means the closures in the array can be executed after the function returns. As a result, the value of the `customerProvider` argument must be allowed to escape the function’s scope.