Swift的闭包(二):捕获值

闭包可以从定义它的上下文中捕获常量和变量。

在Swift中，捕获值最简单的例子是嵌套函数，举个例子：

1 func makeIncrementer(forIncrement amount: Int) -> () -> Int {
2     var runningTotal = 0
3     func incrementer() -> Int {
4         runningTotal += amount
5         return runningTotal
6     }
7     return incrementer
8 }

在这个例子中incrementer()捕获两个值，分别是amount、runningTotal。可以运行一下，观察结果：

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

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

7 print(incrementByTen())     //30

注意：如果你把闭包赋值给一个类实例的一个属性，并且闭包通过指向（refer fo）实例或者实例的成员捕获值，那么，在闭包和这个实例间就会有一个强引用环。

闭包是引用类型（Reference Types）

闭包和函数都是引用类型。

Nonescaping Closures

当一个闭包作为参数传递给一个函数，但是在函数返回后调用的时候，我们说一个闭包是escaped的。当你声明一个有一个闭包作为参数的函数的时候，你可以在参数类型前写@nonescape来暗示这个closure不允许escape。如：

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

把一个闭包标记用@nonescape让你在闭包内隐式的引用（refer to）self，看下这个例子：

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可以延迟计算（delay evaluation），因为直到调用闭包时，闭包内的代码才被运行。延迟计算对于有副作用或者计算代价昂贵的代码非常有用，因为你可以控制什么时候代码进行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"

也可以传递给一个参数：

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：滥用autoclosure会使代码晦涩难懂。

@autoclosure属性隐含了@nonescape属性，如果你想要一个autoclosure允许esacpe，可以这样使用 @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!"