Code to pick a random winner for the "Practical Combine" book by Donny Wals written in Combine; this is a giveaway from Twitter user: @onurgenes

onurgenes-combine-book-giveaway

import Combine

typealias TwitterUserName = String

enum BookGiveawayError: Error {
    case noUserCommented
}

final class BookGiveaway: ObservableObject {

    @Published var onDecisionTriggered = PassthroughSubject<Void, Error>()

    let twitterUsers: [TwitterUserName]
    var cancellable: AnyCancellable?

    init(twitterUsers: [TwitterUserName]) {

        self.twitterUsers = twitterUsers

        cancellable = $onDecisionTriggered
            .tryMap { _ -> TwitterUserName in
                guard let bookWinner = self.twitterUsers.randomElement() else {
                    throw BookGiveawayError.noUserCommented
                }

                return bookWinner
            }
            .mapError({ $0 })
            .sink(receiveCompletion: { error in
                print(error)
            }, receiveValue: { bookWinner in
                print(#"Congratulations \#(bookWinner)! You won a free copy of the "Practical Combine" book by @DonnyWals!"#)
            })

    }

}

private func loadUsersWhoCommented() -> [TwitterUserName] {
    [ /* List of all the users who commented on @onurgenes's tweet */ ]
}

let bg = BookGiveaway(twitterUsers: loadUsersWhoCommented())
let signal: Void = Void()
bg.onDecisionTriggered.send(signal)