Swift入門（二）

標籤：swift

函數函數的定義、參數、傳回值

func sayHello(personName: String) -> String {    let greeting = "Hello, " + personName + "!"    return greeting}

可以省略參數和傳回值

func sayGoodbye(personName: String) {    println("Goodbye, \(personName)!")}

也可以包含多個參數和多個傳回值

func count(string1: String, string2: String) -> (vowels: Int, consonants: Int, others: Int) {    var vowels = 0, consonants = 0, others = 0    for character in string1 {        switch String(character).lowercaseString {        case "a", "e", "i", "o", "u":            ++vowels        case "b", "c", "d", "f", "g", "h", "j", "k", "l", "m",          "n", "p", "q", "r", "s", "t", "v", "w", "x", "y", "z":            ++consonants        default:            ++others        }    }    return (vowels, consonants, others)}

多重傳回值返回的是一個元祖，沒有傳回值時其實返回的是Void，一個空的元祖

參數名

參數名一般只能函數內部使用，但為了使語言表達更清晰，可以定義外部參數名，使用這些外部參數名使代碼更易閱讀

func join(string s1: String, toString s2: String, withJoiner joiner: String) -> String {    return s1 + joiner + s2}join(string: "hello", toString: "world", withJoiner: ", ")

外部名可以與內部名保持一致

func containsCharacter(#string: String, #characterToFind: Character) -> Bool {    for character in string {        if character == characterToFind {            return true        }    }    return false}let containsAVee = containsCharacter(string: "aardvark", characterToFind: "v")

函數參數可以提供預設值

func join(string s1: String, toString s2: String, withJoiner joiner: String = " ") -> String {    return s1 + joiner + s2}join(string: "hello", toString:"world")// returns "hello world"join(string: "hello", toString: "world", withJoiner: "-")// returns "hello-world"

對提供了預設值的參數，參數名自動表示為內部名和外部名
輸入輸出參數inout，表示參數可以被函數體修改，並保留給外部繼續使用

func swapTwoInts(inout a: Int, inout b: Int) {    let temporaryA = a    a = b    b = temporaryA}

調用時需要在參數前加&

var someInt = 3var anotherInt = 107swapTwoInts(&someInt, &anotherInt)

函數類型

函數類型由參數類型和傳回值類型共同決定，類似c語言的函數指標

func addTwoInts(a: Int, b: Int) -> Int {    return a + b}func multiplyTwoInts(a: Int, b: Int) -> Int {    return a * b}var mathFunction: (Int, Int) -> Int = addTwoIntsprintln("Result: \(mathFunction(2, 3))")mathFunction = multiplyTwoIntsprintln("Result: \(mathFunction(2, 3))")

函數類型與其他類型一樣，可以做參數，也可以做傳回值

func stepForward(input: Int) -> Int {    return input + 1}func stepBackward(input: Int) -> Int {    return input - 1}func chooseStepFunction(backwards: Bool) -> (Int) -> Int {    return backwards ? stepBackward : stepForward}var currentValue = 3let moveNearerToZero = chooseStepFunction(currentValue > 0)

函數可以嵌套

func chooseStepFunction(backwards: Bool) -> (Int) -> Int {    func stepForward(input: Int) -> Int { return input + 1 }    func stepBackward(input: Int) -> Int { return input - 1 }    return backwards ? stepBackward : stepForward}

閉包

閉包特性應用在嵌套函數和閉包運算式中，是一種函數式編程特性

閉包運算式

一般形式如下

{ (parameters) -> returnType in    statements}

例如

reversed = sorted(names, { (s1: String, s2: String) -> Bool in    return s1 > s2})// 閉包可以類型推斷，所以可以簡化為reversed = sorted(names, { s1, s2 in return s1 > s2 } )// 單行運算式可以省略返回的returnreversed = sorted(names, { s1, s2 in s1 > s2 } )// 還可以縮寫參數reversed = sorted(names, { $0 > $1 } )

如果閉包函數很長，為了增強可讀性，可以講閉包函數放到（）外邊緊跟函數調用，

func someFunctionThatTakesAClosure(closure: () -> ()) {    // 函數體部分}// 以下是使用尾隨閉包進行函數調用someFunctionThatTakesAClosure() {  // 閉包主體部分}

捕獲值

閉包可以在其定義的上下文中捕獲常量或變數。 即使定義這些常量和變數的原域已經不存在，閉包仍然可以在閉包函數體內引用和修改這些值。

func makeIncrementor(forIncrement amount: Int) -> () -> Int {    var runningTotal = 0    func incrementor() -> Int {        runningTotal += amount        return runningTotal    }    return incrementor}let incrementByTen = makeIncrementor(forIncrement: 10)incrementByTen()// 返回的值為10incrementByTen()// 返回的值為20incrementByTen()// 返回的值為30

Swift入門（二）