###callback的运用 ######struct里的运用

class FinishAlertView{    var buttonPressed : ((Int) -> Void)?    func getNextPress(){        buttonPressed?(2)    }}struct PressCounter{    var count = 0    mutating func buttonPressed(at Index : Int){        count += Index    }}let finish = FinishAlertView()var counter = PressCounter()finish.buttonPressed = {counter.buttonPressed(at : $0)}finish.getNextPress()finish.getNextPress()finish.getNextPress()finish.getNextPress()finish.getNextPress()counter.count  // 10复制代码

######callback在class里的运用

class FinishAlertView{    var buttonPressed : ((Int) -> Void)?    func getNextPress(){        buttonPressed?(2)    }}class PressCounterClass {    var count = 0    func buttonPressed(at Index : Int){        count += Index    }}let finish = FinishAlertView()var counterClass = PressCounterClass()finish.buttonPressed = {[weak counterClass] index in // 防止循环引用    counterClass?.buttonPressed(at: index)}finish.getNextPress()finish.getNextPress()finish.getNextPress()finish.getNextPress()finish.getNextPress()counterClass.count // 10复制代码

###Closure Expressions和Closure