函数
函数的定义、参数、返回值
func sayHello(personName: String) -> String {
let greeting = "Hello," + personName + "!"
return greeting
}
可以省略参数和返回值
func sayGoodbye(personName: String) {
println("Goodbye,\(personName)!")
}
也可以包含多个参数和多个返回值
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)
}
多重返回值返回的是一个元祖，没有返回值时其实返回的是Void，一个空的元祖
参数名
参数名一般只能函数内部使用，但为了使语言表达更清晰，可以定义外部参数名，使用这些外部参数名使代码更易阅读
func join(string s1: String,toString s2: String,withJoiner joiner: String) -> String {
return s1 + joiner + s2
}
join(string: "hello",toString: "world",withJoiner: ",")
外部名可以与内部名保持一致
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")
函数参数可以提供默认值
func join(string s1: String,withJoiner joiner: String = " ") -> String {
return s1 + joiner + s2
}
join(string: "hello",toString:"world")
// returns "hello world"
join(string: "hello",withJoiner: "-")
// returns "hello-world"
对提供了默认值的参数，参数名自动表示为内部名和外部名
输入输出参数inout，表示参数可以被函数体修改，并保留给外部继续使用
func swapTwoInts(inout a: Int,inout b: Int) {
let temporaryA = a
a = b
b = temporaryA
}
调用时需要在参数前加&
var someInt = 3
var anotherInt = 107
swapTwoInts(&someInt,&anotherInt)
函数类型
函数类型由参数类型和返回值类型共同决定，类似c语言的函数指针
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 = addTwoInts
println("Result: \(mathFunction(2,3))")
mathFunction = multiplyTwoInts
println("Result: \(mathFunction(2,3))")
函数类型与其他类型一样，可以做参数，也可以做返回值
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 = 3
let moveNearerToZero = chooseStepFunction(currentValue > 0)
函数可以嵌套
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
}
闭包
闭包表达式
一般形式如下
{ (parameters) -> returnType in
statements
}
例如
reversed = sorted(names,{ (s1: String,s2: String) -> Bool in return s1 > s2 }) // 闭包可以类型推断，所以可以简化为 reversed = sorted(names,{ s1,s2 in return s1 > s2 } ) // 单行表达式可以省略返回的return reversed = sorted(names,s2 in s1 > s2 } ) // 还可以缩写参数 reversed = sorted(names,{ $0 > $1 } )
如果闭包函数很长，为了增强可读性，可以讲闭包函数放到（）外边紧跟函数调用，
func someFunctionThatTakesAClosure(closure: () -> ()) { // 函数体部分 } // 以下是使用尾随闭包进行函数调用 someFunctionThatTakesAClosure() { // 闭包主体部分 }
捕获值
闭包可以在其定义的上下文中捕获常量或变量。 即使定义这些常量和变量的原域已经不存在，闭包仍然可以在闭包函数体内引用和修改这些值。
func makeIncrementor(forIncrement amount: Int) -> () -> Int {
var runningTotal = 0
func incrementor() -> Int {
runningTotal += amount
return runningTotal
}
return incrementor
}
let incrementByTen = makeIncrementor(forIncrement: 10)
incrementByTen()
// 返回的值为10
incrementByTen()
// 返回的值为20
incrementByTen()
// 返回的值为30
