completion_source.cpp

#include <winrt/base.h>
#include <windows.h>

template <typename T>
struct completion_source
{
    completion_source()
    {
        m_signal.attach(CreateEvent(nullptr, true, false, nullptr));
    }

    void set(T const& value)
    {
        m_value = value;
        SetEvent(m_signal.get());
    }

    bool await_ready() const noexcept
    {
        return WaitForSingleObject(m_signal.get(), 0) == 0;
    }

    void await_suspend(std::experimental::coroutine_handle<> resume)
    {
        m_wait.attach(winrt::check_pointer(CreateThreadpoolWait(callback, resume.address(), nullptr)));
        SetThreadpoolWait(m_wait.get(), m_signal.get(), nullptr);
    }

    T await_resume() const noexcept
    {
        return m_value;
    }

private:

    static void __stdcall callback(PTP_CALLBACK_INSTANCE, void* context, PTP_WAIT, TP_WAIT_RESULT) noexcept
    {
        std::experimental::coroutine_handle<>::from_address(context)();
    }

    struct wait_traits
    {
        using type = PTP_WAIT;

        static void close(type value) noexcept
        {
            CloseThreadpoolWait(value);
        }

        static constexpr type invalid() noexcept
        {
            return nullptr;
        }
    };

    winrt::handle m_signal;
    winrt::handle_type<wait_traits> m_wait;
    T m_value{};
};

using namespace std::literals;
using namespace winrt;
using namespace Windows::Foundation;

fire_and_forget CompleteInFiveSeconds(completion_source<bool>& source)
{
    co_await 5s;
    source.set(true);
}

IAsyncAction Async()
{
    completion_source<bool> source;
    CompleteInFiveSeconds(source);
    co_await source;
}

struct App
{
    completion_source<hstring> m_source;

    IAsyncOperation<hstring> Async()
    {
        co_return co_await m_source;
    }

    fire_and_forget CompleteInFiveSeconds()
    {
        co_await 5s;
        m_source.set(L"Hello world");
    }
};

int main()
{
    {
        auto async = Async();
        puts("waiting");
        async.get();
        puts("done");
    }

    {
        App app;
        auto async = app.Async();
        app.CompleteInFiveSeconds();
        puts("waiting");
        auto message = async.get();
        printf("%ls\n", message.c_str());
    }
}