我正在试验 Boost beast::websocket websocket_client_async.cpp例如,结合 websocket_server_async.cpp .
正如给定的那样,客户端 示例只是建立一个连接,向服务器发送一个字符串(它只是回显),打印回复,关闭并存在。
我正在尝试修改客户端以使 session 保持事件状态,以便我可以重复发送/接收字符串。因此,虽然示例代码的 on_handshake
函数会立即通过 ws_.async_write(...)
发送字符串,但我将其分离到它自己的 write(.. .)
函数。
这是我修改过的 session
类:
using tcp = boost::asio::ip::tcp;
namespace websocket = boost::beast::websocket;
void fail(boost::system::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<tcp::socket> ws_;
std::atomic<bool> io_in_progress_;
boost::beast::multi_buffer buffer_;
std::string host_;
public:
// Resolver and socket require an io_context
explicit session(boost::asio::io_context& ioc) : resolver_(ioc), ws_(ioc) {
io_in_progress_ = false;
}
bool io_in_progress() const {
return io_in_progress_;
}
// +---------------------+
// | The "open" sequence |
// +---------------------+
void open(char const* host, char const* port)
{
host_ = host;
// Look up the domain name
resolver_.async_resolve(host, port,
std::bind( &session::on_resolve, shared_from_this(),
std::placeholders::_1, std::placeholders::_2 )
);
}
void on_resolve(boost::system::error_code ec, tcp::resolver::results_type results)
{
if (ec)
return fail(ec, "resolve");
boost::asio::async_connect(
ws_.next_layer(), results.begin(), results.end(),
std::bind( &session::on_connect, shared_from_this(),
std::placeholders::_1 )
);
}
void on_connect(boost::system::error_code ec)
{
if (ec)
return fail(ec, "connect");
ws_.async_handshake(host_, "/",
std::bind( &session::on_handshake, shared_from_this(),
std::placeholders::_1 )
);
}
void on_handshake(boost::system::error_code ec)
{
if (ec)
return fail(ec, "handshake");
else {
std::cout << "Successful handshake with server.\n";
}
}
// +---------------------------+
// | The "write/read" sequence |
// +---------------------------+
void write(const std::string &text)
{
io_in_progress_ = true;
ws_.async_write(boost::asio::buffer(text),
std::bind( &session::on_write, shared_from_this(),
std::placeholders::_1, std::placeholders::_2 )
);
}
void on_write(boost::system::error_code ec, std::size_t bytes_transferred)
{
boost::ignore_unused(bytes_transferred);
if (ec)
return fail(ec, "write");
ws_.async_read(buffer_,
std::bind( &session::on_read, shared_from_this(),
std::placeholders::_1, std::placeholders::_2 )
);
}
void on_read(boost::system::error_code ec, std::size_t bytes_transferred)
{
io_in_progress_ = false; // end of write/read sequence
boost::ignore_unused(bytes_transferred);
if (ec)
return fail(ec, "read");
std::cout << boost::beast::buffers(buffer_.data()) << std::endl;
}
// +----------------------+
// | The "close" sequence |
// +----------------------+
void close()
{
io_in_progress_ = true;
ws_.async_close(websocket::close_code::normal,
std::bind( &session::on_close, shared_from_this(),
std::placeholders::_1)
);
}
void on_close(boost::system::error_code ec)
{
io_in_progress_ = false; // end of close sequence
if (ec)
return fail(ec, "close");
std::cout << "Socket closed successfully.\n";
}
};
问题是,虽然连接工作正常并且我可以发送一个字符串,但永远不会命中 on_read
回调(除非我做了下面描述的丑陋的 hack)。
我的 main
看起来像这样:
void wait_for_io(std::shared_ptr<session> psession, boost::asio::io_context &ioc)
{
// Continually try to run the ioc until the callbacks are finally
// triggered (as indicated by the session::io_in_progress_ flag)
while (psession->io_in_progress()) {
std::this_thread::sleep_for(std::chrono::milliseconds(1));
ioc.run();
}
}
int main(int argc, char** argv)
{
// Check command line arguments.
if (argc != 3) {
std::cerr << "usage info goes here...\n";
return EXIT_FAILURE;
}
const char *host = argv[1], *port = argv[2];
boost::asio::io_context ioc;
std::shared_ptr<session> p = std::make_shared<session>(ioc);
p->open(host, port);
ioc.run(); // This works. Connection is established and all callbacks are executed.
p->write("Hello world"); // String is sent & received by server,
// even before calling ioc.run()
// However, session::on_read callback is never
// reached.
ioc.run(); // This seems to be ignored and returns immediately, so
wait_for_io(p, ioc); // <-- so this hack is necessary
p->close(); // session::on_close is never reached
ioc.run(); // Again, this seems to be ignored and returns immediately, so
wait_for_io(p, ioc); // <-- this is necessary
return EXIT_SUCCESS;
}
如果我这样做:
p->write("Hello world");
while(1) {
std::this_thread::sleep_for(std::chrono::milliseconds(1));
}
我可以确认服务器发送和接收了字符串1,session::on_read
回调未到达.
p->close()
也会发生同样的事情。
但是,如果我添加奇怪的 wait_for_io()
函数,一切正常。我很肯定这是一个可怕的黑客攻击,但我无法弄清楚发生了什么。
1 注意:我可以确认消息确实到达了服务器,因为我修改了服务器示例以将任何接收到的字符串打印到控制台。这是我所做的唯一修改。回显到客户端的功能没有改变。
最佳答案
io_context::run
只会在没有待处理的工作时返回。如果您只是确保一直有一个对 websocket::stream::async_read
的挂起调用处于事件状态,那么 run
将永远不会返回,并且不需要 hack .此外,您将收到服务器发送的所有消息。
关于c++ - Boost beast::websocket 回调函数,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/50030897/