Eventsource support

[eaziz4]

I need to open a persistent connection with a server that will constantly feed me data using eventsource. Is this possible/documented anywhere with beast?

[ashtum]

Yes, it’s possible. You can use http::async_read_header and http::async_read_some to take control of the read loop.
However, Beast doesn't provide built-in support for EventSource. it's up to you to parse the body and extract the events.

#include <boost/asio.hpp>
#include <boost/asio/ssl.hpp>
#include <boost/beast.hpp>

#include <iostream>

using namespace boost;
namespace http = beast::http;
namespace ssl = asio::ssl;
using tcp = asio::ip::tcp;

asio::awaitable<void>
co_main()
{
    auto exec = co_await asio::this_coro::executor;
    ssl::context ssl_ctx(ssl::context::tlsv12);
    ssl::stream<tcp::socket> stream(exec, ssl_ctx);

    // connect to server
    {
        tcp::resolver resolver(exec);
        auto endpoints = co_await resolver.async_resolve("stream.wikimedia.org", "https");
        co_await asio::async_connect(stream.lowest_layer(), endpoints);
        co_await stream.async_handshake(ssl::stream_base::client);
    }

    // send request
    {
        http::request<http::empty_body> request(http::verb::get, "/v2/stream/recentchange", 11);
        request.set(http::field::host, "stream.wikimedia.org");
        request.set(http::field::accept, "text/event-stream");
        request.set(http::field::cache_control, "no-cache");
        co_await http::async_write(stream, request);
    }

    // read header
    http::response_parser<http::string_body> parser;
    beast::flat_buffer buffer;
    co_await http::async_read_header(stream, buffer, parser);

    // stream event
    for(;;)
    {
        co_await http::async_read_some(stream, buffer, parser);
        std::string& body = parser.get().body();
        std::cout << body;
        body.clear();
    }
}

int
main()
{
    asio::io_context ioc;
    asio::co_spawn(
        ioc,
        co_main(),
        [](auto eptr)
        {
            if(eptr)
                std::rethrow_exception(eptr);
        });
    ioc.run();
}