如何使用 boost beast websocket 客戶端收聽 websocket 提要？

Question

我是野獸新手，並試圖創建一個 websocket 客戶端，它將訂閱遠程服務器中的 websocket 事件並開始永遠監聽傳入的消息，除非我或服務器明確關閉連接。

現在，我使用的是野獸的 github 存儲庫中的 async_sll_websocket_client 示例。

#include "example/common/root_certificates.hpp"

#include <boost/beast/core.hpp>
#include <boost/beast/ssl.hpp>
#include <boost/beast/websocket.hpp>
#include <boost/beast/websocket/ssl.hpp>
#include <boost/asio/strand.hpp>
#include <cstdlib>
#include <functional>
#include <iostream>
#include <memory>
#include <string>

namespace beast = boost::beast;         // from <boost/beast.hpp>
namespace http = beast::http;           // from <boost/beast/http.hpp>
namespace websocket = beast::websocket; // from <boost/beast/websocket.hpp>
namespace net = boost::asio;            // from <boost/asio.hpp>
namespace ssl = boost::asio::ssl;       // from <boost/asio/ssl.hpp>
using tcp = boost::asio::ip::tcp;       // from <boost/asio/ip/tcp.hpp>

//------------------------------------------------------------------------------

// Report a failure
void
fail(beast::error_code ec, char const* what)
{
    std::cerr << what << ": " << ec.message() << "\n";
}

// Sends a WebSocket message and prints the response
class session : public std::enable_shared_from_this<session>
{
    tcp::resolver resolver_;
    websocket::stream<
        beast::ssl_stream<beast::tcp_stream>> ws_;
    beast::flat_buffer buffer_;
    std::string host_;
    std::string text_;

public:
    // Resolver and socket require an io_context
    explicit
    session(net::io_context& ioc, ssl::context& ctx)
        : resolver_(net::make_strand(ioc))
        , ws_(net::make_strand(ioc), ctx)
    {
    }

    // Start the asynchronous operation
    void
    run(
        char const* host,
        char const* port,
        char const* text)
    {
        // Save these for later
        host_ = host;
        text_ = text;

        // Look up the domain name
        resolver_.async_resolve(
            host,
            port,
            beast::bind_front_handler(
                &session::on_resolve,
                shared_from_this()));
    }

    void
    on_resolve(
        beast::error_code ec,
        tcp::resolver::results_type results)
    {
        if(ec)
            return fail(ec, "resolve");

        // Set a timeout on the operation
        beast::get_lowest_layer(ws_).expires_after(std::chrono::seconds(30));

        // Make the connection on the IP address we get from a lookup
        beast::get_lowest_layer(ws_).async_connect(
            results,
            beast::bind_front_handler(
                &session::on_connect,
                shared_from_this()));
    }

    void
    on_connect(beast::error_code ec, tcp::resolver::results_type::endpoint_type ep)
    {
        if(ec)
            return fail(ec, "connect");

        // Update the host_ string. This will provide the value of the
        // Host HTTP header during the WebSocket handshake.
        // See https://tools.ietf.org/html/rfc7230#section-5.4
        host_ += ':' + std::to_string(ep.port());

        // Set a timeout on the operation
        beast::get_lowest_layer(ws_).expires_after(std::chrono::seconds(30));

        // Perform the SSL handshake
        ws_.next_layer().async_handshake(
            ssl::stream_base::client,
            beast::bind_front_handler(
                &session::on_ssl_handshake,
                shared_from_this()));
    }

    void
    on_ssl_handshake(beast::error_code ec)
    {
        if(ec)
            return fail(ec, "ssl_handshake");

        // Turn off the timeout on the tcp_stream, because
        // the websocket stream has its own timeout system.
        beast::get_lowest_layer(ws_).expires_never();

        // Set suggested timeout settings for the websocket
        ws_.set_option(
            websocket::stream_base::timeout::suggested(
                beast::role_type::client));

        // Set a decorator to change the User-Agent of the handshake
        ws_.set_option(websocket::stream_base::decorator(
            [](websocket::request_type& req)
            {
                req.set(http::field::user_agent,
                    std::string(BOOST_BEAST_VERSION_STRING) +
                        " websocket-client-async-ssl");
            }));

        // Perform the websocket handshake
        ws_.async_handshake(host_, "/",
            beast::bind_front_handler(
                &session::on_handshake,
                shared_from_this()));
    }

    void
    on_handshake(beast::error_code ec)
    {
        if(ec)
            return fail(ec, "handshake");

        // Send the message
        ws_.async_write(
            net::buffer(text_),
            beast::bind_front_handler(
                &session::on_write,
                shared_from_this()));
    }

    void
    on_write(
        beast::error_code ec,
        std::size_t bytes_transferred)
    {
        boost::ignore_unused(bytes_transferred);

        if(ec)
            return fail(ec, "write");

        // Read a message into our buffer
        ws_.async_read(
            buffer_,
            beast::bind_front_handler(
                &session::on_read,
                shared_from_this()));
    }

    void
    on_read(
        beast::error_code ec,
        std::size_t bytes_transferred)
    {
        boost::ignore_unused(bytes_transferred);

        if(ec)
            return fail(ec, "read");

        //here I try to read the buffer again instead of ccalling the async_close.
        ws_.async_read(
            buffer_,
            beast::bind_front_handler(
                &session::on_read,
                shared_from_this()));


        // Close the WebSocket connection
        //ws_.async_close(websocket::close_code::normal,
        //    beast::bind_front_handler(
        //        &session::on_close,
        //        shared_from_this()));
    }

    void
    on_close(beast::error_code ec)
    {
        if(ec)
            return fail(ec, "close");

        // If we get here then the connection is closed gracefully

        // The make_printable() function helps print a ConstBufferSequence
        std::cout << beast::make_printable(buffer_.data()) << std::endl;
    }
};

//------------------------------------------------------------------------------

int main(int argc, char** argv)
{
    // Check command line arguments.
    if(argc != 4)
    {
        std::cerr <<
            "Usage: websocket-client-async-ssl <host> <port> <text>\n" <<
            "Example:\n" <<
            "    websocket-client-async-ssl echo.websocket.org 443 \"Hello, world!\"\n";
        return EXIT_FAILURE;
    }
    auto const host = argv[1];
    auto const port = argv[2];
    auto const text = argv[3];

    // The io_context is required for all I/O
    net::io_context ioc;

    // The SSL context is required, and holds certificates
    ssl::context ctx{ssl::context::tlsv12_client};

    // This holds the root certificate used for verification
    load_root_certificates(ctx);

    // Launch the asynchronous operation
    std::make_shared<session>(ioc, ctx)->run(host, port, text);

    // Run the I/O service. The call will return when
    // the socket is closed.
    ioc.run();

    return EXIT_SUCCESS;
}

到目前為止，我嘗試在on_read async_read中再次調用 async_read 並遞歸調用on_read 。 連接不會以這種方式關閉，但不會第二次調用“on_read”。 有沒有我可以應用的模式來持續收聽來自服務器的傳入消息？

編輯：顯然我的方法是正確的，我只需要在第一次on_read調用之后再發送一次訂閱消息。

Answer 1

這個 function 可能在第一次讀取后立即終止，這是對寫入（ 代碼）的響應。 試試這個代碼，只要套接字打開就繼續尋找消息。

        net::io_context ioc;
        auto const host = "localhost";
        auto const  port = "1234";
        auto const  port = "SOME TEXT";

        // Launch the asynchronous operation
        std::shared_ptr<session> p = std::make_shared<session>(ioc);
        p->run(host, port, text);
        ioc.run();

        while (p->is_socket_open()) {
            ioc.restart();              
            p->enable_async_read();
            ioc.run();
        }

enable_async_read & is_socket_open 的實現

void session::enable_async_read() {
ws_.async_read(
    buffer_,
    beast::bind_front_handler(
        &session::on_read,
        shared_from_this()));
    }

bool session::is_socket_open() {
return ws_.is_open();
}