ReplaySubject

import Combine

func pipeReplaySubject<Output>(
    of type: Output.Type
) -> (
    input: (Output) -> Void,
    output: some Publisher<Output, Never>
) {
    let subject = CurrentValueSubject<Optional<Output>, Never>(nil)
    return (
        input: subject.send,
        output: subject.compactMap { $0 }
    )
}

let (input, output) = pipeReplaySubject(of: Int.self)
var subscriptions: Set<AnyCancellable> = []
output.sink { print("Subscribe before first send \($0)") }.store(in: &subscriptions)
input(1)
output.sink { print("Subscribe after first send \($0)")}.store(in: &subscriptions)
input(2)
output.sink { print("Subscribe after second send \($0)")}.store(in: &subscriptions)