AnyCancellableBuilder.swift

import Foundation
import Combine

@_functionBuilder
public struct AnyCancellableBuilder {
    public static func buildBlock () -> [AnyCancellable] {
        return []
    }
    
    public static func buildBlock (_ child: AnyCancellable) -> [AnyCancellable] {
        return [child]
    }
    
    public static func buildBlock (_ children: AnyCancellable...) -> [AnyCancellable] {
        return children
    }
    
    public static func buildIf (_ children: AnyCancellable?) -> [AnyCancellable] {
        guard let children = children else { return [] }
        return [children]
    }
    
    public static func buildEither (first: AnyCancellable) -> [AnyCancellable] {
        return [first]
    }
    
    public static func buildEither (second: AnyCancellable) -> [AnyCancellable] {
        return [second]
    }
}


extension Set where Element == AnyCancellable {
    public mutating func collect (@AnyCancellableBuilder _ make: () -> AnyCancellable) {
        insert(make())
    }
    
    public mutating func collect (@AnyCancellableBuilder _ make: () -> [AnyCancellable]) {
        make()
            .forEach { self.insert($0) }
    }
}

func test () {
    var bag = Set<AnyCancellable>()
    
    bag.collect {
        Just(1234)
            .sink { print($0) }
    }
}