Swift的閉包(二):捕獲值

閉包可以從定義它的上下文中捕獲常量和變量。

在Swift中，捕獲值最簡單的例子是嵌套函數，舉個例子：

func makeIncrementer(forIncrement amount: Int) -> () -> Int {
    var runningTotal = 0
    func incrementer() -> Int {
        runningTotal += amount
        return runningTotal
    }
    return incrementer
}

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：

let incrementByTen = makeIncrementer(forIncrement: 10)
print(incrementByTen())     //10
print(incrementByTen())     //20

let incrementByNine = makeIncrementer(forIncrement: 9)
print(incrementByNine())    //9
print(incrementByNine())    //18

print(incrementByTen())     //30

注意：如果你把閉包賦值給一個類實例的一個屬性，並且閉包通過指向（refer fo）實例或者實例的成員捕獲值，那么，在閉包和這個實例間就會有一個強引用環。

閉包是引用類型（Reference Types）

閉包和函數都是引用類型。

Nonescaping Closures

當一個閉包作為參數傳遞給一個函數，但是在函數返回后調用的時候，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：

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

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：

class SomeClass {
    var x = 10
    func doSomething() {
        someFunctionWithNonescapingClosure { x = 200 }
        someFunctionWithEscapingClosure { self.x = 100 }
    }
}
 
let instance = SomeClass()
instance.doSomething()
print(instance.x)
// Prints "200"
 
completionHandlers.first?()
print(instance.x)
// 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，因為你可以控制什么時候代碼進行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"

也可以傳遞給一個參數：

// 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!"

使用@autoclosure：

// 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!" 

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：

// 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!"