在开始之前，先来看一段代码
void pop3::operator() ( const boost::system::error_code& ec, std::size_t length ) { using namespace boost::asio; ip::tcp::endpoint endpoint; std::string status; std::string maillength; std::istream inbuffer ( m_streambuf.get() ); std::string msg; reenter ( this ) { restart: m_socket.reset( new ip::tcp::socket(io_service) ); do { #ifndef DEBUG // 延时 60s _yield ::boost::delayedcallsec( io_service, 60, boost::bind(*this, ec, 0) ); #endif // dns 解析并连接. _yield boost::async_avconnect( boost::proxychain(io_service).add_proxy()(boost::proxy_tcp(*m_socket, ip::tcp::resolver::query(m_mailserver, "110"))), *this); // 失败了延时 10s if ( ec ) _yield ::boost::delayedcallsec ( io_service, 10, boost::bind(*this, ec, 0) ); } while ( ec ); // 尝试到连接成功为止! // 好了，连接上了. m_streambuf.reset ( new streambuf ); // "+OK QQMail POP3 Server v1.0 Service Ready(QQMail v2.0)" _yield async_read_until ( *m_socket, *m_streambuf, "\n", *this ); inbuffer >> status; if ( status != "+OK" ) { // 失败，重试. goto restart; } // 发送用户名. _yield m_socket->async_write_some ( buffer ( std::string ( "user " ) + m_mailaddr + "\n" ), *this ); if(ec) goto restart; // 接受返回状态. m_streambuf.reset ( new streambuf ); _yield async_read_until ( *m_socket, *m_streambuf, "\n", *this ); inbuffer >> status; // 解析是不是 OK. if ( status != "+OK" ) { // 失败，重试. goto restart; } // 发送密码. _yield m_socket->async_write_some ( buffer ( std::string ( "pass " ) + m_passwd + "\n" ), *this ); // 接受返回状态. m_streambuf.reset ( new streambuf ); _yield async_read_until ( *m_socket, *m_streambuf, "\n", *this ); inbuffer >> status; // 解析是不是 OK. if ( status != "+OK" ) { // 失败，重试. goto restart; } // 完成登录. 开始接收邮件. // 发送 list 命令. _yield m_socket->async_write_some ( buffer ( std::string ( "list\n" ) ), *this ); // 接受返回的邮件. m_streambuf.reset ( new streambuf ); _yield async_read_until ( *m_socket, *m_streambuf, "\n", *this ); inbuffer >> status; // 解析是不是 OK. if ( status != "+OK" ) { // 失败，重试. goto restart; } // 开始进入循环处理邮件. maillist.clear(); _yield m_socket->async_read_some ( m_streambuf->prepare ( 8192 ), *this ); m_streambuf->commit ( length ); while ( status != "." ) { maillength.clear(); status.clear(); inbuffer >> status; inbuffer >> maillength; // 把邮件的编号push到容器里. if ( maillength.length() ) maillist.push_back ( status ); if ( inbuffer.eof() && status != "." ) _yield m_socket->async_read_some ( m_streambuf->prepare ( 8192 ), *this ); } // 获取邮件. while ( !maillist.empty() ) { // 发送 retr #number 命令. msg = boost::str ( boost::format ( "retr %s\r\n" ) % maillist[0] ); _yield m_socket->async_write_some ( buffer ( msg ), *this ); // 获得 +OK m_streambuf.reset ( new streambuf ); _yield async_read_until ( *m_socket, *m_streambuf, "\n", *this ); inbuffer >> status; // 解析是不是 OK. if ( status != "+OK" ) { // 失败，重试. goto restart; } // 获取邮件内容，邮件一单行的 . 结束. _yield async_read_until ( *m_socket, *m_streambuf, "\r\n.\r\n", *this ); // 然后将邮件内容给处理. process_mail ( inbuffer ); // 删除邮件啦. msg = boost::str ( boost::format ( "dele %s\r\n" ) % maillist[0] ); _yield m_socket->async_write_some ( buffer ( msg ), *this ); maillist.erase ( maillist.begin() ); // 获得 +OK m_streambuf.reset ( new streambuf ); _yield async_read_until ( *m_socket, *m_streambuf, "\n", *this ); inbuffer >> status; // 解析是不是 OK. if ( status != "+OK" ) { // 失败，但是并不是啥大问题. std::cout << "deleting mail failed" << std::endl; // but 如果是连接出问题那还是要重启的. if(ec) goto restart; } } // 处理完毕. _yield async_write ( *m_socket, buffer ( "quit\n" ), *this ); _yield ::boost::delayedcallsec ( io_service, 1, boost::bind ( *this, ec, 0 ) ); if(m_socket->is_open()) m_socket->shutdown ( ip::tcp::socket::shutdown_both ); _yield ::boost::delayedcallsec ( io_service, 1, boost::bind ( *this, ec, 0 ) ); m_socket.reset(); std::cout << "邮件处理完毕" << std::endl; _yield ::boost::delayedcallsec ( io_service, 30, boost::bind ( *this, ec, 0 ) ); goto restart; } }
这个代码，乍一看就是同步代码嘛！而事实上它是异步的
在这个代码里，使用了 _yield 前缀再配合 async_* 异步函数，使用异步实现了同步的pop3登录算法。
这个神奇的代码，神奇之处就是 reenter(this) 和 _yield。这2个地方就是实现的全部的关键。
我在群课程里有简单的提到过协程，有兴趣的可以到 avplayer社区讲座：协程 围观。
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