函數(shù)的定義

func pi() -> Double {
    return 3.14
}

//形參默認是let, 也只能是let常量
func sum(v1: Int, v2: Int) -> Int {
    return v1 + v2
}

//無返回值
func sayHello() -> Void {
    print("hello")
}

func sayHello() -> () {
    print("hello")
}

func sayHello() {
    print("hello")
}

隱式返回(Implicit Return)

如果整個函數(shù)體是一個單一表達式, 那么函數(shù)會隱式返回這個表達式:

func sum(v1: Int, v2: Int) -> Int {
    v1 + v2
}
sum(v1: 10, v2: 20)   //30

返回元祖: 實現(xiàn)多返回值

func calculate(v1: Int, v2: Int) -> (sum: Int, difference: Int, average: Int){
    let sum = v1 + v2
    return (sum, v1 - v2, sum >> 1)
}
let result = calculate(v1: 20, v2: 10)
result.sum  //30
result.difference  //10
result.average  //15

參數(shù)標(biāo)簽(Argument Label)

可以修改參數(shù)標(biāo)簽:

func goToWork(at time: String) {
    print("This time is \(time)")
}
goToWork(at: "08:00")
// This time is 08:00

可以使用下劃線 _ 省略參數(shù)標(biāo)簽

func sum(_ v1: Int, _ v2: Int) -> Int {
    v1 + v2
}
sum(10,20)

默認參數(shù)值(Default Parameter Value)

func check(name: String = "nobody", age: Int, job: String = "none") {
    print("name=\(name), age=\(age), job=\(job)")
}
check(name: "Jack", age: 20, job: "Doctor") //name=Jack, age=20, job=Doctor
check(name: "Rose", age: 18) //name=Rose, age=18, job=none
check(age: 10, job: "Batman") //name=nobody, age=10, job=Batman
check(age: 15) //name=nobody, age=15, job=none

C++的默認參數(shù)值有個限制: 必須從右往左設(shè)置, 由于Swift擁有參數(shù)標(biāo)簽, 因此沒有類似限制

但是在省略參數(shù)標(biāo)簽時, 需要特別注意, 避免出錯

//這里的middle不可以省略參數(shù)標(biāo)簽
func test(_ first: Int = 10, middle: Int, _ last: Int = 30) {}
test(middle: 20)

可變參數(shù)(Variadic Parameter)

func sum(_ numbers: Int...) -> Int {
    var total = 0
    for number in numbers {
        total += number
    }
    return total
}
sum(10, 20, 30, 40) //100

一個函數(shù)最多只能有1個可變參數(shù)

緊跟在可變參數(shù)后面的參數(shù)不能省略參數(shù)標(biāo)簽

//參數(shù)string不能省略標(biāo)簽
func test(_ numbers: Int..., string: String, _ other: String) { }
test(10, 20, 30, string: "Jack", "Rose")

輸入輸出參數(shù)(In-Out Parameter)

可以用inout定義一個輸入輸出參數(shù):可以在函數(shù)內(nèi)部修改外部實參的值

可變參數(shù)不能標(biāo)記為inout

inout參數(shù)不能有默認值

inout參數(shù)只能傳入可以被多次賦值的

inout參數(shù)的本質(zhì)是地址傳遞(引用傳遞)

 func swapValues(_ v1: inout Int, _ v2: inout Int) {
     let tmp = v1
     v1 = v2
     v2 = tmp
 }
var num1 = 10
var num2 = 20
swapValues(&num1, &num2)
func swapValues(_ v1: inout Int, _ v2: inout Int) {
    (v1, v2) = (v2, v1)
}

函數(shù)重載(Function Overload)

規(guī)則

函數(shù)名相同

參數(shù)個數(shù)不同 || 參數(shù)類型不同 || 參數(shù)標(biāo)簽不同

func sum(v1: Int, v2: Int) -> Int { v1 + v2 }
func sum(v1: Int, v2: Int, v3: Int) -> Int { v1 + v2 + v3 } // 參數(shù)個數(shù)不同

sum(v1: 10, v2: 20) // 30
sum(v1: 10, v2: 20, v3: 30) // 60
sum(v1: 10, v2: 20.0) // 30.0
sum(v1: 10.0, v2: 20) // 30.0
sum(10, 20) // 30
sum(a: 10, b: 20) // 30

func sum(v1: Int, v2: Double) -> Double { Double(v1) + v2 } // 參數(shù)類型不同
func sum(v1: Double, v2: Int) -> Double { v1 + Double(v2) } // 參數(shù)類型不同
 func sum(_ v1: Int, _ v2: Int) -> Int { v1 + v2 } // 參數(shù)標(biāo)簽不同
func sum(a: Int, b: Int) -> Int { a + b } // 參數(shù)標(biāo)簽不同

函數(shù)重載注意點

返回值類型與函數(shù)重載無關(guān)

func sum(v1: Int, v2: Int) -> Int { v1 + v2 }
func sum(v1: Int, v2: Int) {}
sum(v1: 10, v2: 20)  //報錯 有歧義

默認參數(shù)值和函數(shù)重載一起使用產(chǎn)生二義性時，編譯器并不會報錯(在C++中會報錯)

func sum(v1: Int, v2: Int) -> Int {
    v1 + v2
}
func sum(v1:  Int, v2: Int, v3: Int = 10) -> Int {
    v1 + v2 + v3 
}
// 會調(diào)用sum(v1: Int, v2: Int) sum(v1: 10, v2: 20)

函數(shù)重載注意點

可變參數(shù)、省略參數(shù)標(biāo)簽、函數(shù)重載一起使用產(chǎn)生二義性時，編譯器有可能會報錯

func sum(v1: Int, v2: Int) -> Int {
    v1 + v2
}
func sum(_ v1: Int, _ v2: Int) -> Int {
    v1 + v2 
}
func sum(_ numbers: Int...) -> Int {
    var total = 0
    for number in numbers {
        total += number
}
    return total
}
// error: ambiguous use of 'sum'
sum(10, 20)

函數(shù)類型(Function Type)

每一個函數(shù)都是有類型的，函數(shù)類型由形式參數(shù)類型、返回值類型組成

func test() { } // () -> Void 或者 () -> ()
func sum(a: Int, b: Int) -> Int { a+b } // (Int, Int) -> Int
 // 定義變量
var fn: (Int, Int) -> Int = sum fn(2, 3) // 5，調(diào)用時不需要參數(shù)標(biāo)簽

函數(shù)類型作為函數(shù)參數(shù)

func sum(v1: Int, v2: Int) -> Int {
    v1 + v2
}
func difference(v1: Int, v2: Int) -> Int { v1 - v2 }
func printResult(_ mathFn: (Int, Int) -> Int, _ a: Int, _ b: Int) {
    print("Result: \(mathFn(a, b))")
}
printResult(sum, 5, 2) // Result: 7
printResult(difference, 5, 2) // Result: 3

函數(shù)類型作為函數(shù)返回值

func next(_ input: Int) -> Int {
    input + 1
}
func previous(_ input: Int) -> Int { input - 1 }
func forward(_ forward: Bool) -> (Int) -> Int {
    forward ? next : previous
}
forward(true)(3) // 4
forward(false)(3) // 2

返回值是函數(shù)類型的函數(shù)，叫做高階函數(shù)(Higher-Order Function)

typealias

typealias用來給類型起別名

typealias Byte = Int8
typealias Short = Int16
typealias Long = Int64

typealias Date = (year: Int, month: Int, day: Int)
func test(_ date: Date) {
    print(date.0)
    print(date.year)
}
test((2011, 9, 10))

按照Swift標(biāo)準(zhǔn)庫的定義，Void就是空元組()

public typealias Void = ()

typealias IntFn = (Int, Int) -> Int

func difference(v1: Int, v2: Int) -> Int {
    v1 - v2
}
let fn: IntFn = difference
fn(20, 10) // 10

func setFn(_ fn: IntFn) { }
setFn(difference)

func getFn() -> IntFn { difference }
public typealias Void = ()

嵌套函數(shù)(Nested Function)

將函數(shù)定義在函數(shù)內(nèi)部

func forward(_ forward: Bool) -> (Int) -> Int {
    func next(_ input: Int) -> Int {
        input + 1 
    }
    func previous(_ input: Int) -> Int {
        input - 1
    }
    return forward ? next : previous
}
forward(true)(3) // 4
forward(false)(3) // 2