prefix(untilOutputFrom:) as filtering operator

PrefixUntilOutputFrom.swift

let on = PassthroughSubject<Void, Never>()
let strings = PassthroughSubject<String, Never>()

strings
    .prefix(untilOutputFrom: on)
    .sink(receiveValue: { print($0) })

["a1", "b1", "c3", "d1", "e5"]
    .forEach {
        strings.send($0)
        if $0.hasPrefix("c") {
            on.send()
        }
}

// Output:
//    a1
//    b1
//    c3