Swift8 - 闭包

// ## 尾随闭包
func someFunctionThatTakesAClosure(closure: () -> Void) {
    // 函数体部分
}

someFunctionThatTakesAClosure(closure: {
    
})

someFunctionThatTakesAClosure {
    
}

// ## 值捕获
func makeIncrementer(forIncrement amount: Int) -> () -> Int {
    var runningTotal = 0
    func incrementer() -> Int {
        runningTotal += amount
        return runningTotal
    }
    return incrementer
}
let incrementByTen = makeIncrementer(forIncrement: 10)
var d = incrementByTen() //10
d = incrementByTen() //20
d = incrementByTen() //30

let incrementBySeven = makeIncrementer(forIncrement: 7)
d = incrementBySeven() //7
d = incrementByTen() //40
let alsoIncrementByTen = incrementByTen
alsoIncrementByTen() //50


// ## 逃逸闭包
//一种能使闭包“逃逸”出函数的方法是，将这个闭包保存在一个函数外部定义的变量中
var completionHandlers: [ ()->Void ] = []
func someFunctionWithEscapingClosure(completionHandler: @escaping ()->Void) {
    completionHandlers.append(completionHandler)
}
//将一个闭包标记为 @escaping 意味着你必须在闭包中显式地引用 self。
func someFunctionWithNonescapingClosure(closure: () -> Void) {
    closure()
}
class SomeClass {
    var x = 10
    func doSomething() {
        someFunctionWithEscapingClosure {
            self.x = 100
        }
        someFunctionWithNonescapingClosure { x = 200 }
    }
}
let instance = SomeClass()
instance.doSomething()
print(instance.x)
// 打印出“200”

completionHandlers.first?()
print(instance.x)
// 打印出“100”


// ## 自动闭包让你能够延迟求值，因为直到你调用这个闭包，代码段才会被执行
var customersInLine = ["Chris", "Alex", "Ewa", "Barry", "Daniella"]
print(customersInLine.count)
// 打印出“5”

let customerProvider = { customersInLine.remove(at: 0) }
print(customersInLine.count)
// 打印出“5”

print("现在才移除第一个元素 \(customerProvider())")
// 现在才移除第一个元素 Chris
print(customersInLine.count)
// 打印出“4”

//将闭包作为参数传递给函数时，你能获得同样的延时求值行为。
func serve(customer customerProvider: ()->String) {
    print("函数被调用时才移除第一个元素 \(customerProvider())")
}
serve(customer: { customersInLine.remove(at: 0) })

//通过将参数标记为 @autoclosure 来接收一个自动闭包
func serve(customer customerProvider: @autoclosure () -> String) {
    print("Now serving \(customerProvider())!")
}
serve(customer: customersInLine.remove(at: 0))
// 打印“Now serving Ewa!”

// ## 自动闭包和逃逸闭包
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")// 打印“Collected 2 closures.”
for customerProvider in customerProviders {
    print("Now serving \(customerProvider())!")
}
// 打印“Now serving Barry!”
// 打印“Now serving Daniella!”