combineLatestOnce.png

combineLatestOnce.swift

extension Publisher {
    func combineLatestOnce<P>(_ other: P) -> AnyPublisher<(Output?, P.Output?), Failure> where P: Publisher, P.Failure == Failure {
        return prepare()
            .combineLatest(other.prepare())
            .filter { $0.0 != nil || $0.1 != nil }
            .eraseToAnyPublisher()
    }
    
    /// helper function for `combineLatestOnce`
    /// immediately starts the stream with `nil` and follows each upstream value with `nil`
    private func prepare() -> AnyPublisher<Output?, Failure> {
        flatMap { value in
            [value, nil].publisher
                .setFailureType(to: Failure.self)
        }
        .prepend(nil)
        .eraseToAnyPublisher()
    }
}

example.swift

let arrSub = PassthroughSubject<[Int], Never>()
let intSub = PassthroughSubject<Int, Never>()

let c = arrSub.combineLatestOnce(intSub)
    .scan([]) { previous, newValues in
        // new arrays get passed on, replacing the previous array
        if let arr = newValues.0 {
            return arr
        } 
        // new integers get appended to previous array
        else if let int = newValues.1 {
            var arr = previous
            arr.append(int)
            return arr
        } 
        // we shouldn't have both `nil` because of the `filter` in `combineLatestOnce`,
        // but just in case, republish the previous array
        else {
            return previous
        }
    }
    .sink {
        print($0)
    }


DispatchQueue.main.async {
    // command                  output
    arrSub.send([1, 2, 3])   // [1, 2, 3]
    intSub.send(4)           // [1, 2, 3, 4]
    intSub.send(5)           // [1, 2, 3, 4, 5]

    arrSub.send([4, 5, 6])   // [4, 5, 6]
    intSub.send(7)           // [4, 5, 6, 7]
    intSub.send(8)           // [4, 5, 6, 7, 8]
}

RunLoop.main.run()