闭包

闭包表达式(Closure Expression)

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

var fn = {
    (v1: Int, v2: Int) -> Int in
    return v1 + v2
}
fn(10, 20)

{
    (v1: Int, v2: Int) -> Int in
    return v1 + v2
}(10, 20)
    
{
    (参数列表) -> 返回值类型 in
    函数体代码
}

闭包表达式的简写

func exec(v1: Int, v2: Int, fn: (Int, Int) -> Int) {
    print(fn(v1, v2))
}

exec(v1: 10, v2: 20, fn: {
    (v1: Int, v2: Int) -> Int in
    return v1 + v2
})

exec(v1: 10, v2: 20, fn: { $0 + $1 })

exec(v1: 10, v2: 20, fn: +)

exec(v1: 10, v2: 20, fn: {
    v1, v2 in return v1 + v2
})

exec(v1: 10, v2: 20, fn: {
    v1, v2 in v1 + v2
})

尾随闭包

func exec(v1: Int, v2: Int, fn: (Int, Int) -> Int) {
    print(fn(v1, v2))
}

exec(v1: 10, v2: 20) {
    $0 + $1
}

func exec(fn: (Int, Int) -> Int) {
    print(fn(1, 2))
}

exec(fn: { $0 + $1 })
exec() { $0 + $1 }
exec { $0 + $1 }

示例 – 数组的排序

func sort(by areInIncreasingOrder: (Element, Element) -> Bool)
/// 返回true: i1排在i2前面
/// 返回false: i1排在i2后面
func cmp(i1: Int, i2: Int) -> Bool {
    // 大的排在前面
    return i1 > i2
}

var nums = [11, 2, 18, 6, 5, 68, 45]
nums.sort(by: cmp)
// [68, 45, 18, 11, 6, 5, 2]
nums.sort(by: {
    (i1: Int, i2: Int) -> Bool in return i1 < i2
})
nums.sort(by: { i1, i2 in return i1 < i2 })
nums.sort(by: { i1, i2 in i1 < i2 })
nums.sort(by: { $0 < $1 })
nums.sort(by: <)
nums.sort() { $0 < $1 }
nums.sort { $0 < $1 }
// [2, 5, 6, 11, 18, 45, 68]

忽略参数

func exec(fn: (Int, Int) -> Int) {
    print(fn(1, 2))
}
exec { _,_ in 10 } // 10

闭包(Closure)

typealias Fn = (Int) -> Int
func getFn() -> Fn {
    var num = 0
    func plus(_ i: Int) -> Int {
        num += i
        return num
    }
    return plus
} // 返回的plus和num形成了闭包

func getFn() -> Fn {
    var num = 0
    return {
        num += $0
        return num
    }
}

var fn1 = getFn()
var fn2 = getFn()
fn1(1) // 1
fn2(2) // 2
fn1(3) // 4
fn2(4) // 6
fn1(5) // 9
fn2(6) // 12
//思考:如果num是全局变量呢?

class Closure {
    var num = 0
    func plus(_ i: Int) -> Int {
        num += i
        return num
    }
}
var cs1 = Closure()
var cs2 = Closure()
cs1.plus(1) // 1
cs2.plus(2) // 2
cs1.plus(3) // 4
cs2.plus(4) // 6
cs1.plus(5) // 9
cs2.plus(6) // 12

练习

typealias Fn = (Int) -> (Int, Int)
func getFns() -> (Fn, Fn) {
    var num1 = 0
    var num2 = 0
    func plus(_ i: Int) -> (Int, Int) {
        num1 += i
        num2 += i << 1
        return (num1, num2)
    }
    func minus(_ i: Int) -> (Int, Int) {
        num1 -= i
        num2 -= i << 1
        return (num1, num2)
    }
    return (plus, minus)
}


let (p, m) = getFns()
p(5) // (5, 10)
m(4) // (1, 2)
p(3) // (4, 8)
m(2) // (2, 4)

class Closure {
    var num1 = 0
    var num2 = 0
    func plus(_ i: Int) -> (Int, Int) {
        num1 += i
        num2 += i << 1
        return (num1, num2)
    }
    func minus(_ i: Int) -> (Int, Int) {
        num1 -= i
        num2 -= i << 1
        return (num1, num2)
    }
}

var cs = Closure()
cs.plus(5) // (5, 10)
cs.minus(4) // (1, 2)
cs.plus(3) // (4, 8)
cs.minus(2) // (2, 4)

var functions: [() -> Int] = []
for i in 1...3 {
    functions.append { i }
}
for f in functions {
    print(f())
}
// 1
// 2
// 3

class Closure {
    var i: Int
    init(_ i: Int) {
        self.i = i
    }
    func get() -> Int {
        return i
    }
}
var clses: [Closure] = []
for i in 1...3 {
    clses.append(Closure(i)) }
for cls in clses {
    print(cls.get())
}

注意

func add(_ num: Int) -> (inout Int) -> Void {
    func plus(v: inout Int) {
        v += num
    }
    return plus
}
var num = 5
add(20)(&num)
print(num)

自动闭包

// 如果第1个数大于0，返回第一个数。否则返回第2个数
func getFirstPositive(_ v1: Int, _ v2: Int) -> Int {
    return v1 > 0 ? v1 : v2
}
getFirstPositive(10, 20) // 10
getFirstPositive(-2, 20) // 20
getFirstPositive(0, -4) // -4

// 改成函数类型的参数，可以让v2延迟加载
func getFirstPositive(_ v1: Int, _ v2: () -> Int) -> Int? {
    return v1 > 0 ? v1 : v2()
}
getFirstPositive(-4) { 20 }

func getFirstPositive(_ v1: Int, _ v2: @autoclosure () -> Int) -> Int? { return v1 > 0 ? v1 : v2()
}
getFirstPositive(-4, 20)