Boost.Asio C++ 网络编程之九：基于TCP的异步客户端

       现在，是比较有趣（也比较难）的异步实现！ 当查看流程图时，你需要知道Boost.Asio代表由Boost.Asio执行的一个异步调用。例如do_read()，Boost.Asio和on_read()代表了从do_read()到on_read()的逻辑流程，但是你永远不知道什么时候轮到on_read()被调用，你只是知道你最终会调用它。

基于TCP的异步客户端

1.流程图



2.实现

#ifdef WIN32
#define _WIN32_WINNT 0x0501
#include <stdio.h>
#endif

#include <boost/thread.hpp>
#include <boost/bind.hpp>
#include <boost/asio.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/enable_shared_from_this.hpp>
using namespace boost::asio;
io_service service;

#define MEM_FN(x)       boost::bind(&self_type::x, shared_from_this())
#define MEM_FN1(x,y)    boost::bind(&self_type::x, shared_from_this(),y)
#define MEM_FN2(x,y,z)  boost::bind(&self_type::x, shared_from_this(),y,z)

/** simple connection to server:
- logs in just with username (no password)
- all connections are initiated by the client: client asks, server answers
- server disconnects any client that hasn't pinged for 5 seconds

Possible requests:
- gets a list of all connected clients
- ping: the server answers either with "ping ok" or "ping client_list_changed"
*/
class talk_to_svr : public boost::enable_shared_from_this<talk_to_svr>
, boost::noncopyable {
typedef talk_to_svr self_type;
talk_to_svr(const std::string & username)
: sock_(service), started_(true), username_(username), timer_(service) {}
void start(ip::tcp::endpoint ep) {
sock_.async_connect(ep, MEM_FN1(on_connect, _1));
}
public:
typedef boost::system::error_code error_code;
typedef boost::shared_ptr<talk_to_svr> ptr;

static ptr start(ip::tcp::endpoint ep, const std::string & username) {
ptr new_(new talk_to_svr(username));
new_->start(ep);
return new_;
}
void stop() {
if (!started_) return;
std::cout << "stopping " << username_ << std::endl;
started_ = false;
sock_.close();
}
bool started() { return started_; }
private:
void on_connect(const error_code & err) {
if (!err)      do_write("login " + username_ + "\n");
else            stop();
}
void on_read(const error_code & err, size_t bytes) {
if (err) stop();
if (!started()) return;
// process the msg
std::string msg(read_buffer_, bytes);
if (msg.find("login ") == 0) on_login();
else if (msg.find("ping") == 0) on_ping(msg);
else if (msg.find("clients ") == 0) on_clients(msg);
else std::cerr << "invalid msg " << msg << std::endl;
}

void on_login() {
std::cout << username_ << " logged in" << std::endl;
do_ask_clients();
}
void on_ping(const std::string & msg) {
std::istringstream in(msg);
std::string answer;
in >> answer >> answer;
if (answer == "client_list_changed") do_ask_clients();
else postpone_ping();
}
void on_clients(const std::string & msg) {
std::string clients = msg.substr(8);
std::cout << username_ << ", new client list:" << clients;
postpone_ping();
}

void do_ping() {
do_write("ping\n");
}
void postpone_ping() {
// note: even though the server wants a ping every 5 secs, we randomly
// don't ping that fast - so that the server will randomly disconnect us
int millis = rand() % 7000;
std::cout << username_ << " postponing ping " << millis
<< " millis" << std::endl;
timer_.expires_from_now(boost::posix_time::millisec(millis));
timer_.async_wait(MEM_FN(do_ping));
}
void do_ask_clients() {
do_write("ask_clients\n");
}

void on_write(const error_code & err, size_t bytes) {
do_read();
}
void do_read() {
async_read(sock_, buffer(read_buffer_),
MEM_FN2(read_complete, _1, _2), MEM_FN2(on_read, _1, _2));
}
void do_write(const std::string & msg) {
if (!started()) return;
std::copy(msg.begin(), msg.end(), write_buffer_);
sock_.async_write_some(buffer(write_buffer_, msg.size()),
MEM_FN2(on_write, _1, _2));
}
size_t read_complete(const boost::system::error_code & err, size_t bytes) {
if (err) return 0;
bool found = std::find(read_buffer_, read_buffer_ + bytes, '\n') < read_buffer_ + bytes;
// we read one-by-one until we get to enter, no buffering
return found ? 0 : 1;
}

private:
ip::tcp::socket sock_;
enum { max_msg = 1024 };
char read_buffer_[max_msg];
char write_buffer_[max_msg];
bool started_;
std::string username_;
deadline_timer timer_;
};

int main(int argc, char* argv[]) {
// connect several clients
ip::tcp::endpoint ep(ip::address::from_string("127.0.0.1"), 8001);
char* names[] = { "John", "James", "Lucy", "Tracy", "Frank", "Abby", 0 };
for (char ** name = names; *name; ++name) {
talk_to_svr::start(ep, *name);
boost::this_thread::sleep(boost::posix_time::millisec(100));
}
service.run();
system("pause");
}

注意每一个read操作都会触发一个ping操作

1.当read操作结束时，on_read()被调用

2.on_read()调用on_login()，on_ping()或者on_clients()

3.每一个方法要么发出一个ping，要么请求客户端列表

4.如果我们请求客户端列表，当read操作接收到它们时，它会发出一个ping操作。

还需注意的是这里的postpone ping操作，是基于boost::asio::deadline_timer实现的异步定时。

关于boost::asio::deadline_timer详见：Boost.Asio C++ 网络编程之十一：deadline_timer使用说明

参考链接：http://download.csdn.net/download/caoshangpa/10229882