Swift的閉包(二):捕獲值

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閉包可以從定義它的上下文中捕獲常量和變數。

在Swift中，捕獲值最簡單的例子是嵌套函數，舉個例子：

1 func makeIncrementer(forIncrement amount: Int) -> () -> Int {2     var runningTotal = 03     func incrementer() -> Int {4         runningTotal += amount5         return runningTotal6     }7     return incrementer8 }

在這個例子中incrementer()捕獲兩個值，分別是amount、runningTotal。可以運行一下，觀察結果：

1 let incrementByTen = makeIncrementer(forIncrement: 10)2 print(incrementByTen())     //103 print(incrementByTen())     //20
4 let incrementByNine = makeIncrementer(forIncrement: 9)5 print(incrementByNine())    //96 print(incrementByNine())    //18
7 print(incrementByTen())     //30

注意：如果你把閉包賦值給一個類執行個體的一個屬性，並且閉包通過指向（refer fo）執行個體或者執行個體的成員捕獲值，那麼，在閉包和這個執行個體間就會有一個強引用環。

閉包是參考型別（Reference Types）

閉包和函數都是參考型別。

Nonescaping Closures

當一個閉包作為參數傳遞給一個函數，但是在函數返回後調用的時候，我們說一個閉包是escaped的。當你聲明一個有一個閉包作為參數的函數的時候，你可以在參數類型前寫@nonescape來暗示這個closure不允許escape。如：

1 func someFunctionWithNonescapingClosure(closure: @noescape () -> Void) {2     closure()3 }

把一個閉包標記用@nonescape讓你在閉包內隱式的引用（refer to）self，看下這個例子：

 1 class SomeClass { 2     var x = 10 3     func doSomething() { 4         someFunctionWithNonescapingClosure { x = 200 } 5         someFunctionWithEscapingClosure { self.x = 100 } 6     } 7 } 8   9 let instance = SomeClass()10 instance.doSomething()11 print(instance.x)12 // Prints "200"13  14 completionHandlers.first?()15 print(instance.x)16 // Prints "100"

Autoclosures

An autoclosure is a closure that is 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.

Autoclosures可以延遲計算（delay evaluation），因為直到調用閉包時，閉包內的代碼才被運行。延遲計算對於有副作用或者計算代價昂貴的代碼非常有用，因為你可以控制什麼時候代碼進行evaluation。

 1 var customersInLine = ["Chris", "Alex", "Ewa", "Barry", "Daniella"] 2 print(customersInLine.count) 3 // Prints "5" 4   5 let customerProvider = { customersInLine.remove(at: 0) } 6 print(customersInLine.count) 7 // Prints "5" 8   9 print("Now serving \(customerProvider())!")10 // Prints "Now serving Chris!"11 print(customersInLine.count)12 // Prints "4"

也可以傳遞給一個參數：

1 // customersInLine is ["Alex", "Ewa", "Barry", "Daniella"]2 func serve(customer customerProvider: () -> String) {3     print("Now serving \(customerProvider())!")4 }5 serve(customer: { customersInLine.remove(at: 0) } )6 // Prints "Now serving Alex!"

使用@autoclosure：

1 // customersInLine is ["Ewa", "Barry", "Daniella"]2 func serve(customer customerProvider: @autoclosure () -> String) {3     print("Now serving \(customerProvider())!")4 }5 serve(customer: customersInLine.remove(at: 0))6 // Prints "Now serving Ewa!" 

注意：濫用autoclosure會使代碼晦澀難懂。

@autoclosure屬性隱含了@nonescape屬性，如果你想要一個autoclosure允許esacpe，可以這樣使用 @autoclosure(escaping) ，如：

 1 // customersInLine is ["Barry", "Daniella"] 2 var customerProviders: [() -> String] = [] 3 func collectCustomerProviders(_ customerProvider: @autoclosure(escaping) () -> String) { 4     customerProviders.append(customerProvider) 5 } 6 collectCustomerProviders(customersInLine.remove(at: 0)) 7 collectCustomerProviders(customersInLine.remove(at: 0)) 8   9 print("Collected \(customerProviders.count) closures.")10 // Prints "Collected 2 closures."11 for customerProvider in customerProviders {12     print("Now serving \(customerProvider())!")13 }14 // Prints "Now serving Barry!"15 // Prints "Now serving Daniella!"

 

Swift的閉包(二):捕獲值