Swift Function & Closure

Functions

Functions are self-contained chunks of code that perform a specific task. You give a function a name that identifies what it does, and this name is used to “call” the function to perform its task when needed.

函數是以一個用來執行特殊任務的自包含代碼塊

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Functions.html#//apple_ref/doc/uid/TP40014097-CH10-ID158

Closures:

Closures are self-contained blocks of functionality that can be passed around and used in your code. Closures in Swift are similar to blocks in C and Objective-C and to lambdas in other programming languages.

和函數一樣也就是代碼塊，和C，OC裏面的block相似，和其他語言的匿名函數相似

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html#//apple_ref/doc/uid/TP40014097-CH11-ID94

表達式如下:

• { (parameters) -> return type in

•     statements

• }

Global 和nested函數實際上是closure的特例:

Global函數:一個有名字但不獲取任何值的closure

Nested函數:有名字，同時能夠獲取到他們enclosing function裏的值

Closure 式子:一個輕量級語法書寫的沒有名字的closure，同時能夠獲取到他所處的 context中的值

這邊有個例子

//csdn 竟然沒有swift這個語言的標籤＝＝

    // 點擊操作按鈕
    @IBAction func operate(sender: UIButton) {
        let operation = sender.currentTitle!
        if isUserInMiddleOfTypingNumber {
            enter()
        }
        switch operation {
        case "×":performOperation { $0 * $1 } // Trailing closure 這邊傳入的其實是 function type 這個nested function就可以直接這麼寫了
        case "÷":performOperation { $0 / $1 }
        case "+":performOperation { $0 + $1 }
        case "−":performOperation { $0 - $1 }
        case "√":performOperationSqrt { sqrt($0)}
        default: break
        }
    }
    
    //  function type as Parameter Type
    func performOperation(operation: (Double, Double) -> Double) {
        if operandStack.count >= 2 {
            displayValue = operation(operandStack.removeLast() , operandStack.removeLast())
            enter()
        }
    }

之前一直看不懂這語法，今天來解釋下

function 和closure分別有Trailing functions 和 Trailing Closures  

 //含義爲 如果你要傳一個closure expression或 function type給function，當作它最後的一個參數時，你可以這麼寫

func someFunctionThatTakesAClosure(closure: () -> ()) {
    // function body goes here
}
 
// here's how you call this function without using a trailing closure:
 
someFunctionThatTakesAClosure({
    // closure's body goes here
})
 
// here's how you call this function with a trailing closure instead:
 
someFunctionThatTakesAClosure() {
    // trailing closure's body goes here
}

// 如果啊 你要傳的這個closure expression是這個function唯一的argument，你連這a pair of parentheses （）都不用寫！！！