Publisherの配列を先頭からひとつずつ順次実行する。

SequencialPublisher.swift

import Foundation
import Combine

extension Combine.Publishers {
    struct Sequencial<Sequence, Output, Failure>: Publisher
    where
        Sequence: Swift.Sequence,
        Sequence.Element: Publisher,
        Sequence.Element.Output == Output,
        Sequence.Element.Failure == Failure
    {
        var publishers: Sequence
        
        init(publishers: Sequence) {
            self.publishers = publishers
        }
        
        func receive<S>(
            subscriber: S
        ) where S : Subscriber, Failure == S.Failure, Output == S.Input {
            publishers.publisher.setFailureType(to: Sequence.Element.Failure.self)
                .flatMap(maxPublishers: .max(1)) { $0 }
                .receive(subscriber: subscriber)
        }
    }
}

extension Array where Element: Publisher {
    var sequentialPublisher: Publishers.Sequencial<Self, Element.Output, Element.Failure> {
        Publishers.Sequencial(publishers: self)
    }
}