ViewBuilderを理解するためresultBuilderを独自実装

playground.swift

@resultBuilder
public struct AdderBuilder {
    //supports DSL in block
    public static func buildBlock(_ components: Int...) -> Int {
        components.reduce(0, +)
    }
    
    //supports "if" statement without "else"
    public static func buildOptional(_ component: Int?) -> Int{
        return component ?? 0
    }
    
    //supports "if else" statement
    public static func buildEither(first component: Int) -> Int {
        return component
    }
    public static func buildEither(second component : Int) -> Int{
        return component
    }
    
    //supports "for in" statement
    public static func buildArray(_ components: [Int]) -> Int{
        return components.reduce(0, +)
    }

}

func blockAdder(@AdderBuilder block:()->Int)->Int{
    return block()
}

        
let sum = blockAdder{//buildBlock()
    0
    1
    if 0<1 {//buildOptional()
        2
    }
    
    if 0<1{//buildEither(first:)
        3
    }else{//buildEither(first:)
        4
    }
    
    for _ in 0...2 {//buildArray
        5
    }
}