【Swift4】(6) 闭包 | 闭包应用 | 闭包作为函数参数 | 捕获特性

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
}

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]
}

//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
}

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