首先：您可以广播 UDP，但这不适用于已连接的客户端。那只是... UDP。
其次，该链接显示了如何在 Asio 中拥有类似条件变量（事件）的接口。这只是你问题的一小部分。您忘记了大局：您需要以一种或另一种方式了解打开的连接集：
- 例如为每个连接保留一个包含会话指针 (
weak_ptr) 的容器
- 每个连接订阅一个信号槽（例如Boost Signals）。
选项 1. 非常适合性能，选项 2. 更适合灵活性（将事件源与订阅者分离，从而可以拥有异构订阅者，例如不来自连接）。
因为我认为选项 1. 相对于线程来说更简单，更好的 w.r.t.效率（例如，您可以从一个缓冲区为所有客户端提供服务而无需复制）并且您可能不需要双重解耦信号/插槽，让我参考一个答案，我已经为纯 Asio（没有 Beast）展示了同样多的内容：
它展示了“连接池”的概念——它本质上是weak_ptr<connection> 对象的线程安全容器，带有一些垃圾收集逻辑。
演示：介绍 Echo 服务器
在chatting about things 之后，我想花时间实际演示这两种方法，所以我在说什么就完全清楚了。
首先让我们展示一个简单的、普通的异步 TCP 服务器与
- 具有多个并发连接
- 每个连接的会话从客户端逐行读取，并将相同的内容回显给客户端
- 3 秒后停止接受，最后一个客户端断开连接后退出
master branch on github
#include <boost/asio.hpp>
#include <memory>
#include <list>
#include <iostream>
namespace ba = boost::asio;
using ba::ip::tcp;
using boost::system::error_code;
using namespace std::chrono_literals;
using namespace std::string_literals;
static bool s_verbose = false;
struct connection : std::enable_shared_from_this<connection> {
connection(ba::io_context& ioc) : _s(ioc) {}
void start() { read_loop(); }
void send(std::string msg, bool at_front = false) {
post(_s.get_io_service(), [=] { // _s.get_executor() for newest Asio
if (enqueue(std::move(msg), at_front))
write_loop();
});
}
private:
void do_echo() {
std::string line;
if (getline(std::istream(&_rx), line)) {
send(std::move(line) + '\n');
}
}
bool enqueue(std::string msg, bool at_front)
{ // returns true if need to start write loop
at_front &= !_tx.empty(); // no difference
if (at_front)
_tx.insert(std::next(begin(_tx)), std::move(msg));
else
_tx.push_back(std::move(msg));
return (_tx.size() == 1);
}
bool dequeue()
{ // returns true if more messages pending after dequeue
assert(!_tx.empty());
_tx.pop_front();
return !_tx.empty();
}
void write_loop() {
ba::async_write(_s, ba::buffer(_tx.front()), [this,self=shared_from_this()](error_code ec, size_t n) {
if (s_verbose) std::cout << "Tx: " << n << " bytes (" << ec.message() << ")" << std::endl;
if (!ec && dequeue()) write_loop();
});
}
void read_loop() {
ba::async_read_until(_s, _rx, "\n", [this,self=shared_from_this()](error_code ec, size_t n) {
if (s_verbose) std::cout << "Rx: " << n << " bytes (" << ec.message() << ")" << std::endl;
do_echo();
if (!ec)
read_loop();
});
}
friend struct server;
ba::streambuf _rx;
std::list<std::string> _tx;
tcp::socket _s;
};
struct server {
server(ba::io_context& ioc) : _ioc(ioc) {
_acc.bind({{}, 6767});
_acc.set_option(tcp::acceptor::reuse_address());
_acc.listen();
accept_loop();
}
void stop() {
_ioc.post([=] {
_acc.cancel();
_acc.close();
});
}
private:
void accept_loop() {
auto session = std::make_shared<connection>(_acc.get_io_context());
_acc.async_accept(session->_s, [this,session](error_code ec) {
auto ep = ec? tcp::endpoint{} : session->_s.remote_endpoint();
std::cout << "Accept from " << ep << " (" << ec.message() << ")" << std::endl;
session->start();
if (!ec)
accept_loop();
});
}
ba::io_context& _ioc;
tcp::acceptor _acc{_ioc, tcp::v4()};
};
int main(int argc, char** argv) {
s_verbose = argc>1 && argv[1] == "-v"s;
ba::io_context ioc;
server s(ioc);
std::thread th([&ioc] { ioc.run(); }); // todo exception handling
std::this_thread::sleep_for(3s);
s.stop(); // active connections will continue
th.join();
}
方法一、添加广播消息
所以，让我们添加同时发送到所有活动连接的“广播消息”。我们添加两个：
- 每个新连接一个（说“玩家 ## 已进入游戏”）
-
一个模拟全局“服务器事件”的事件，就像您在问题中描述的那样）。它从 main 内部触发：
std::this_thread::sleep_for(1s);
auto n = s.broadcast("random global event broadcast\n");
std::cout << "Global event broadcast reached " << n << " active connections\n";
注意我们如何通过注册一个指向每个接受的连接的弱指针并在每个连接上进行操作来做到这一点：
_acc.async_accept(session->_s, [this,session](error_code ec) {
auto ep = ec? tcp::endpoint{} : session->_s.remote_endpoint();
std::cout << "Accept from " << ep << " (" << ec.message() << ")" << std::endl;
if (!ec) {
auto n = reg_connection(session);
session->start();
accept_loop();
broadcast("player #" + std::to_string(n) + " has entered the game\n");
}
});
broadcast 也直接从main 使用，很简单：
size_t broadcast(std::string const& msg) {
return for_each_active([msg](connection& c) { c.send(msg, true); });
}
using-asio-post branch on github
#include <boost/asio.hpp>
#include <memory>
#include <list>
#include <iostream>
namespace ba = boost::asio;
using ba::ip::tcp;
using boost::system::error_code;
using namespace std::chrono_literals;
using namespace std::string_literals;
static bool s_verbose = false;
struct connection : std::enable_shared_from_this<connection> {
connection(ba::io_context& ioc) : _s(ioc) {}
void start() { read_loop(); }
void send(std::string msg, bool at_front = false) {
post(_s.get_io_service(), [=] { // _s.get_executor() for newest Asio
if (enqueue(std::move(msg), at_front))
write_loop();
});
}
private:
void do_echo() {
std::string line;
if (getline(std::istream(&_rx), line)) {
send(std::move(line) + '\n');
}
}
bool enqueue(std::string msg, bool at_front)
{ // returns true if need to start write loop
at_front &= !_tx.empty(); // no difference
if (at_front)
_tx.insert(std::next(begin(_tx)), std::move(msg));
else
_tx.push_back(std::move(msg));
return (_tx.size() == 1);
}
bool dequeue()
{ // returns true if more messages pending after dequeue
assert(!_tx.empty());
_tx.pop_front();
return !_tx.empty();
}
void write_loop() {
ba::async_write(_s, ba::buffer(_tx.front()), [this,self=shared_from_this()](error_code ec, size_t n) {
if (s_verbose) std::cout << "Tx: " << n << " bytes (" << ec.message() << ")" << std::endl;
if (!ec && dequeue()) write_loop();
});
}
void read_loop() {
ba::async_read_until(_s, _rx, "\n", [this,self=shared_from_this()](error_code ec, size_t n) {
if (s_verbose) std::cout << "Rx: " << n << " bytes (" << ec.message() << ")" << std::endl;
do_echo();
if (!ec)
read_loop();
});
}
friend struct server;
ba::streambuf _rx;
std::list<std::string> _tx;
tcp::socket _s;
};
struct server {
server(ba::io_context& ioc) : _ioc(ioc) {
_acc.bind({{}, 6767});
_acc.set_option(tcp::acceptor::reuse_address());
_acc.listen();
accept_loop();
}
void stop() {
_ioc.post([=] {
_acc.cancel();
_acc.close();
});
}
size_t broadcast(std::string const& msg) {
return for_each_active([msg](connection& c) { c.send(msg, true); });
}
private:
using connptr = std::shared_ptr<connection>;
using weakptr = std::weak_ptr<connection>;
std::mutex _mx;
std::vector<weakptr> _registered;
size_t reg_connection(weakptr wp) {
std::lock_guard<std::mutex> lk(_mx);
_registered.push_back(wp);
return _registered.size();
}
template <typename F>
size_t for_each_active(F f) {
std::vector<connptr> active;
{
std::lock_guard<std::mutex> lk(_mx);
for (auto& w : _registered)
if (auto c = w.lock())
active.push_back(c);
}
for (auto& c : active) {
std::cout << "(running action for " << c->_s.remote_endpoint() << ")" << std::endl;
f(*c);
}
return active.size();
}
void accept_loop() {
auto session = std::make_shared<connection>(_acc.get_io_context());
_acc.async_accept(session->_s, [this,session](error_code ec) {
auto ep = ec? tcp::endpoint{} : session->_s.remote_endpoint();
std::cout << "Accept from " << ep << " (" << ec.message() << ")" << std::endl;
if (!ec) {
auto n = reg_connection(session);
session->start();
accept_loop();
broadcast("player #" + std::to_string(n) + " has entered the game\n");
}
});
}
ba::io_context& _ioc;
tcp::acceptor _acc{_ioc, tcp::v4()};
};
int main(int argc, char** argv) {
s_verbose = argc>1 && argv[1] == "-v"s;
ba::io_context ioc;
server s(ioc);
std::thread th([&ioc] { ioc.run(); }); // todo exception handling
std::this_thread::sleep_for(1s);
auto n = s.broadcast("random global event broadcast\n");
std::cout << "Global event broadcast reached " << n << " active connections\n";
std::this_thread::sleep_for(2s);
s.stop(); // active connections will continue
th.join();
}
方法 2：那些广播但使用 Boost Signals2
信号方法是Dependency Inversion 的一个很好的例子。
最突出的注释：
区别是理解增加的灵活性的关键：信号所有者/调用者对订阅者一无所知。这就是我们所说的解耦/依赖倒置
using-signals2 branch on github
#include <boost/asio.hpp>
#include <memory>
#include <list>
#include <iostream>
#include <boost/signals2.hpp>
namespace ba = boost::asio;
using ba::ip::tcp;
using boost::system::error_code;
using namespace std::chrono_literals;
using namespace std::string_literals;
static bool s_verbose = false;
struct connection : std::enable_shared_from_this<connection> {
connection(ba::io_context& ioc) : _s(ioc) {}
void start() { read_loop(); }
void send(std::string msg, bool at_front = false) {
post(_s.get_io_service(), [=] { // _s.get_executor() for newest Asio
if (enqueue(std::move(msg), at_front))
write_loop();
});
}
private:
void do_echo() {
std::string line;
if (getline(std::istream(&_rx), line)) {
send(std::move(line) + '\n');
}
}
bool enqueue(std::string msg, bool at_front)
{ // returns true if need to start write loop
at_front &= !_tx.empty(); // no difference
if (at_front)
_tx.insert(std::next(begin(_tx)), std::move(msg));
else
_tx.push_back(std::move(msg));
return (_tx.size() == 1);
}
bool dequeue()
{ // returns true if more messages pending after dequeue
assert(!_tx.empty());
_tx.pop_front();
return !_tx.empty();
}
void write_loop() {
ba::async_write(_s, ba::buffer(_tx.front()), [this,self=shared_from_this()](error_code ec, size_t n) {
if (s_verbose) std::cout << "Tx: " << n << " bytes (" << ec.message() << ")" << std::endl;
if (!ec && dequeue()) write_loop();
});
}
void read_loop() {
ba::async_read_until(_s, _rx, "\n", [this,self=shared_from_this()](error_code ec, size_t n) {
if (s_verbose) std::cout << "Rx: " << n << " bytes (" << ec.message() << ")" << std::endl;
do_echo();
if (!ec)
read_loop();
});
}
friend struct server;
ba::streambuf _rx;
std::list<std::string> _tx;
tcp::socket _s;
boost::signals2::scoped_connection _subscription;
};
struct server {
server(ba::io_context& ioc) : _ioc(ioc) {
_acc.bind({{}, 6767});
_acc.set_option(tcp::acceptor::reuse_address());
_acc.listen();
accept_loop();
}
void stop() {
_ioc.post([=] {
_acc.cancel();
_acc.close();
});
}
size_t broadcast(std::string const& msg) {
_broadcast_event(msg);
return _broadcast_event.num_slots();
}
private:
boost::signals2::signal<void(std::string const& msg)> _broadcast_event;
size_t reg_connection(connection& c) {
c._subscription = _broadcast_event.connect(
[&c](std::string msg){ c.send(msg, true); }
);
return _broadcast_event.num_slots();
}
void accept_loop() {
auto session = std::make_shared<connection>(_acc.get_io_context());
_acc.async_accept(session->_s, [this,session](error_code ec) {
auto ep = ec? tcp::endpoint{} : session->_s.remote_endpoint();
std::cout << "Accept from " << ep << " (" << ec.message() << ")" << std::endl;
if (!ec) {
auto n = reg_connection(*session);
session->start();
accept_loop();
broadcast("player #" + std::to_string(n) + " has entered the game\n");
}
});
}
ba::io_context& _ioc;
tcp::acceptor _acc{_ioc, tcp::v4()};
};
int main(int argc, char** argv) {
s_verbose = argc>1 && argv[1] == "-v"s;
ba::io_context ioc;
server s(ioc);
std::thread th([&ioc] { ioc.run(); }); // todo exception handling
std::this_thread::sleep_for(1s);
auto n = s.broadcast("random global event broadcast\n");
std::cout << "Global event broadcast reached " << n << " active subscribers\n";
std::this_thread::sleep_for(2s);
s.stop(); // active connections will continue
th.join();
}
查看方法 1. 和 2. 之间的差异：Compare View on github
针对 3 个并发客户端运行时的输出示例：
(for a in {1..3}; do netcat localhost 6767 < /etc/dictionaries-common/words > echoed.$a& sleep .1; done; time wait)