函數(shù)作為參數(shù)

以乘法為例

1.multiply為定義的方法，作為參數(shù)傳入performOperation

@IBAction func operate(sender: UIButton) {
        let operation = sender.currentTitle!
        if userIsInTheMiddleOfTypingNumber {
            enter()
        }
        switch operation{
        case "*": performOperation(multiply)
            
        default:break
        }
    }
    
    func multiply(opt1:Double,opt2:Double)->Double{
        return opt1 * opt2
    }
    
    func performOperation(operation:(Double,Double)->Double){
        if operandStack.count >= 2 {
            displayValue = operation(operandStack.removeLast(), operandStack.removeLast())
            enter()
        }
    }

2.把multiply函數(shù)作為一個(gè)塊，需要改變書(shū)寫(xiě)的語(yǔ)法

    @IBAction func operate(sender: UIButton) {
        let operation = sender.currentTitle!
        if userIsInTheMiddleOfTypingNumber {
            enter()
        }
        switch operation{
        case "*": performOperation({(opt1:Double,opt2:Double)->Double in
            return opt1 * opt2
            })
            
        default:break
        }
    }

3.因?yàn)閟wift可以做到類型推斷，因此可以進(jìn)一步將塊簡(jiǎn)化

    @IBAction func operate(sender: UIButton) {
        let operation = sender.currentTitle!
        if userIsInTheMiddleOfTypingNumber {
            enter()
        }
        switch operation{
        case "*": performOperation({(opt1,opt2) in
            return opt1 * opt2
            })
            
        default:break
        }
    }

4.由于performOperation方法知道調(diào)用的方法將會(huì)返回一個(gè)Double值，而且，此處僅且返回一個(gè)值，因此可以進(jìn)一步簡(jiǎn)化

    @IBAction func operate(sender: UIButton) {
        let operation = sender.currentTitle!
        if userIsInTheMiddleOfTypingNumber {
            enter()
        }
        switch operation{
        case "*": performOperation({(opt1,opt2) in opt1 * opt2})
            
        default:break
        }
    }

5.Swift 不強(qiáng)制要求給參數(shù)命名，如果參數(shù)沒(méi)有命名，默認(rèn)為$0,$1...以此類推，因此，參數(shù)名也可以省略

    @IBAction func operate(sender: UIButton) {
        let operation = sender.currentTitle!
        if userIsInTheMiddleOfTypingNumber {
            enter()
        }
        switch operation{
        case "*": performOperation({$0 * $1})
            
        default:break
        }
    }

6.若此參數(shù)為函數(shù)的最后一個(gè)參數(shù)，且為operation類型，則可以將此參數(shù)移到括號(hào)的外面，若之前有其他參數(shù)，依然可以放在括號(hào)內(nèi)，若此時(shí)除了此參數(shù)沒(méi)有其他參數(shù)，則括號(hào)也可以省略，得到最終精簡(jiǎn)版

    @IBAction func operate(sender: UIButton) {
        let operation = sender.currentTitle!
        if userIsInTheMiddleOfTypingNumber {
            enter()
        }
        switch operation{
        case "*": performOperation{$0 * $1}
            
        default:break
        }
    }