Swift4 - 学习笔记：闭包 | 闭包应用 | 闭包作为函数参数 | 捕获特性 | 枚举

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最近在学习移动端开发，记录一下以前端视角学习 IOS 开发的过程。这是我整理了 Swift4 的学习笔记。

闭包 Closure

闭包的基本使用与简写

• 语法优化

var myClosure:() -> Void = {
    print("Hello world")
}

myClosure()

var mySecondClosure:(Int,Int) -> Int = {
    (a:Int,b:Int) -> Int in //闭包标准写法
    return a*b
}

mySecondClosure = {
    (a,b) in //闭包简写，类型自动推导 可以根据参数推断
    return a*b
}

mySecondClosure = {
    (a,b) in //进一步闭包简写，如果函数体只包含一句 return 代码，可省略 return
    a*b
}
mySecondClosure = {
    $0 * $1 
//再简写：被捕获的参数列表中，含有a、b，下标从0开始，可通过"$"获取。省略参数列表 （a, b）和 关键字 in
}

闭包应用

• 排序

var arr:[Int] = [1,3,5,6,7,2,4,6,8]

arr.sorted() //[1, 2, 3, 4, 5, 6, 6, 7, 8]

arr.sorted {
    $0 > $1  //[8, 7, 6, 6, 5, 4, 3, 2, 1]
}

var arr = [0,1,2,3,4,5,6,7,8,9,10]
arr.sorted {
    return fabs(Float($0-5)) < fabs(Float($1-5))  //离5越近排在越前面 [5, 4, 6, 3, 7, 2, 8, 1, 9, 0, 10]
}

• 遍历

//forEach
var arr2 = [5,6,7,8,9]

arr2.forEach {
    print("\($0)")  //[5, 6, 7, 8, 9]
}

var arr3 = [1,2,3,4]
//filter
arr3.filter {
    return $0 > 3   //[4]
}

var arr4 = ["0","1","imagine"]
//map
var num = arr4.map {
    Int($0) //[0, 1, nil] 类型转换可能失败，所以返回的是可选型
}
var num2 = arr4.compactMap {
    Int($0) //[0, 1] //compactMap : 解包->展开并合并
}
//reduce
var arr5 = [0,1,2]

var sum = arr5.reduce(0) {
    return $0 + $1  //3
}

闭包作为函数参数

func handler(_ a: Int, _ b: Int,operation:(Int,Int)->Int) ->Int {
    let res = operation(a,b)
    return res
}

let multipyClosure = { //实现一个闭包
    (a:Int,b:Int) in
    a * b
}

handler(2, 3, operation: multipyClosure) //将闭包作为参数传递

捕获

闭包可以从上下文环境中捕获常量、变量，并在自己的作用域内使用。

//捕获
var num = 3
arr.sorted {
    return fabs(Float($0-num)) < fabs(Float($1-num))  //向外层找变量 num
}

枚举基本使用

表述一组值

枚举相当于创建了一种新的数据类型，而类型的取值由里面的case值进行表征

enum CompassPoint { // 大写开头
    case north,west,east,south
}

enum GameEnding {
    case Win
    case Lose
    case Draw
}

var yourScore:Int  = 100
var enemyScore:Int = 100

var thisGameEnding:GameEnding
if yourScore > enemyScore {thisGameEnding = GameEnding.Win}
else if yourScore == enemyScore {thisGameEnding = GameEnding.Draw}
else {thisGameEnding = .Lose}  //可省略GameEnding
switch thisGameEnding
{
case .Win: print("win") 
case .Draw: print("Draw")
case .Lose: print("Lose")
}

enum VowleCharacter:Character {
    case A = "a"
    case E = "e"
    case I = "i"
    case O = "o"
    case U = "u"
}

let vowelA = VowleCharacter.A

var userInputCharacter:Character = "a"
if userInputCharacter == vowelA.rawValue
{
    print("it is an 'a'")   //"it is an 'a'\n"
}else {
    print("it is not an 'a'")
}

灵活使用

enum Barcode {
    case UPCA(Int,Int,Int,Int)
    case QRCode(String)   //将枚举变量QRCode关联为String类型
}

let productCodeA = Barcode.UPCA(4, 102, 306, 8)
let productCodeB = Barcode.QRCode("This is a infomation")

switch productCodeA {
case .UPCA(let numberSystem,let manufacture,let identifier,let check):
    print("UPC-A with value of \(numberSystem), \(manufacture), \(identifier),\(check).")   //"UPC-A with value of 4, 102, 306,8.\n"
case .QRCode(let productCode):
    print("QRCode with value of \(productCode).")
}