@resultBuilder
struct RBS {
    // 基本闭包支持
    static func buildBlock(_ components: Int...) -> Int {
        components.reduce(0) { partialResult, i in
            partialResult + i
        }
    }
    // 支持条件判断
    static func buildEither(first component: Int) -> Int {
        component
    }
    static func buildEither(second component: Int) -> Int {
        component
    }
    // 支持循环
    static func buildArray(_ components: [Int]) -> Int {
        components.reduce(0) { partialResult, i in
            partialResult + i
        }
    }
}

let a = RBS.buildBlock(
    1,
    2,
    3
)
print(a) // 6

// 应用到函数中
@RBS func f1() -> Int {
    1
    2
    3
}
print(f1()) // 6

// 设置了 buildEither 就可以在闭包中进行条件判断。
@RBS func f2(stopAtThree: Bool) -> Int {
    1
    2
    3
    if stopAtThree == true {
        0
    } else {
        4
        5
        6
    }
}
print(f2(stopAtThree: false)) // 21

// 设置了 buildArray 就可以在闭包内使用循环了
@RBS func f3() -> Int {
    for i in 1...3 {
        i * 2
    }
}
print(f3()) // 12

@dynamicMemberLookup
struct D {
    // 找字符串
    subscript(dynamicMember m: String) -> String {
        let p = ["one": "first", "two": "second"]
        return p[m, default: ""]
    }
    // 找整型
    subscript(dynamicMember m: String) -> Int {
        let p = ["one": 1, "two": 2]
        return p[m, default: 0]
    }
    // 找闭包
    subscript(dynamicMember m: String) -> (_ s: String) -> Void {
        return {
            print("show \($0)")
        }
    }
    // 静态数组成员
    var p = ["This is a member"]
    // 动态数组成员
    subscript(dynamicMember m: String) -> [String] {
        return ["This is a dynamic member"]
    }
}

let d = D()
let s1: String = d.one
print(s1) // first
let i1: Int = d.one
print(i1) // 1
d.show("something") // show something
print(d.p) // ["This is a member"]
let dynamicP:[String] = d.dp
print(dynamicP) // ["This is a dynamic member"]

@dynamicCallable
struct D {
    // 带参数说明
    func dynamicallyCall(withKeywordArguments args: KeyValuePairs<String, Int>) -> Int {
        let firstArg = args.first?.value ?? 0
        return firstArg * 2
    }
    
    // 无参数说明
    func dynamicallyCall(withArguments args: [String]) -> String {
        var firstArg = ""
        if args.count > 0 {
            firstArg = args[0]
        }
        return "show \(firstArg)"
    }
}

let d = D()
let i = d(numberIs: 2)
print(i) // 4
let s = d("hi")
print(s) // show hi