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Boost.Asio C++ 网络编程之九:基于TCP的异步客户端

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       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 deadline_timer使用说明