Swift Function & Closure

Functions

Functionsare 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.

函数是以一个用来执行特殊任务的自包含代码块

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

Closures:

Closuresare 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.

和函数一样也就是代码块，和C，OC里面的block相似，和其他语言的匿名函数相似

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

表达式如下:

```
{ (parameters) -> return type in
```
```
    statements
```
```
}
```

Global 和nested函数实际上是closure的特例:

Global函数:一个有名字但不获取任何值的closure

nested函数:有名字，同时能够获取到他们enclosing function里的值

Closure 式子:一个轻量级语法书写的没有名字的closure，同时能够获取到他所处的 context中的值

这边有个例子

//csdn 竟然没有swift这个语言的标签＝＝

    // 点击操作按钮
    @IBAction func operate(sender: UIButton) {
        let operation = sender.currentTitle!
        if isUserInMiddleOfTypingNumber {
            enter()
        }
        switch operation {
        case "×":performOperation { $0 * $1 } // Trailing closure 这边传入的其实是 function type 这个nested function就可以直接这么写了
        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()
        }
    }

之前一直看不懂这语法，今天来解释下

function 和closure分别有Trailing functions 和 Trailing Closures

//含义为如果你要传一个closure expression或 function type给function，当作它最后的一个参数时，你可以这么写

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
}

// 如果啊你要传的这个closure expression是这个function唯一的argument，你连这a pair ofparentheses （）都不用写！！！

Swift Function & Closure

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