How to use RxCpp operators on a list of observables?

Question

I want to use zip operator on a list / array of RxCpp observables, but RxCpp operators only accept variadic arguments, and the size of my list is dynamic (known at runtime).

Is there a way to use these operators on a list of observables? Thanks in advance.

Answer 1

Here is a naive attempt which iteratively zips each observable in a vector and accumulates the result:

template <typename T>
rxcpp::observable<std::shared_ptr<std::vector<T>>> zip_v(const std::vector<rxcpp::observable<T>>& observables) {
    // map the first observable to convert values to a single-element vector
    auto it = observables.cbegin();
    rxcpp::observable<std::shared_ptr<std::vector<T>>> acc = it->map([](T t) {
        return std::make_shared<std::vector<T>>(std::initializer_list<T>{ t });
    });

    // fold each observable into accumulator by zipping, and pushing back value
    while (++it != observables.cend()) {
        acc = acc.zip([](std::shared_ptr<std::vector<T>> acc, T next) { 
            acc->push_back(next);
            return acc;
        }, *it);
    }
    return acc;
}

Usage example:

    std::vector<rxcpp::observable<int>> sources{
        rxcpp::observable<>::range(1, 5),
        rxcpp::observable<>::range(6, 10),
        rxcpp::observable<>::range(11, 15)
    };
    zip_v(sources).
        take(3).
        subscribe(
            [](auto vs){
                for (auto& v : *vs) {
                    printf("%d ", v);
                }
                printf("\n");
            },
            [](){ printf("complete\n"); });

Output:

    1 6 11
    2 7 12
    3 8 13
    OnCompleted