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：
func makeIncrementer(forIncrement amount: Int) -> () -> Int {
var runningTotal =
func incrementer() -> Int {
runningTotal += amount
return runningTotal
}
return incrementer
}
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：
let incrementByTen = makeIncrementer(forIncrement: )
print(incrementByTen()) //
print(incrementByTen()) //20
let incrementByNine = makeIncrementer(forIncrement: )
print(incrementByNine()) //
print(incrementByNine()) //18
print(incrementByTen()) //
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：
func someFunctionWithNonescapingClosure(closure: @noescape () -> Void) {
closure()
}
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：
class SomeClass {
var x =
func doSomething() {
someFunctionWithNonescapingClosure { x = }
someFunctionWithEscapingClosure { self.x = }
}
}
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。
var customersInLine = ["Chris", "Alex", "Ewa", "Barry", "Daniella"]
print(customersInLine.count)
// Prints "5" let customerProvider = { customersInLine.remove(at: ) }
print(customersInLine.count)
// Prints "5" print("Now serving \(customerProvider())!")
// Prints "Now serving Chris!"
print(customersInLine.count)
// Prints "4"
也可以传递给一个参数：
// customersInLine is ["Alex", "Ewa", "Barry", "Daniella"]
func serve(customer customerProvider: () -> String) {
print("Now serving \(customerProvider())!")
}
serve(customer: { customersInLine.remove(at: ) } )
// 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: ))
// 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：
// customersInLine is ["Barry", "Daniella"]
var customerProviders: [() -> String] = []
func collectCustomerProviders(_ customerProvider: @autoclosure(escaping) () -> String) {
customerProviders.append(customerProvider)
}
collectCustomerProviders(customersInLine.remove(at: ))
collectCustomerProviders(customersInLine.remove(at: )) 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!"