标签：最简 types led enter 参数传递 exp first ica pes
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。
闭包和函数都是引用类型。
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"
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!"
标签：最简 types led enter 参数传递 exp first ica pes
原文地址：http://www.cnblogs.com/xjshi/p/6044701.html
