Closures

Version: Swift 5.5 Source: swift-book: Closures 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

  • Overview

  • Closure Expressions

    • The Sorted Method

    • Closure Expression Syntax

    • Inferring Type From Context

    • Implicit Returns from Single-Expression Closures

    • Shorthand Argument Names

    • Operator Methods

  • Trailing Closures

  • Capturing Values

  • Closures Are Reference Types

  • Escaping Closures

  • 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:

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:

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:

• 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:

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:

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:

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:

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:

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:

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.

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 section above can be written outside of the sorted(by:) method’s parentheses as a trailing closure:

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:

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 Int values into an array of String values. The array [16, 58, 510] is used to create the new array ["OneSix", "FiveEight", "FiveOneZero"]:

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:

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:

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.

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.

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:

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:

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:

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:

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:

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.

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:

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.

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:

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.

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.

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.

// 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.

// 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.

// 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.