//// async_client.cpp// ~~~~~~~~~~~~~~~~//异步的HTTP客户端，与同步的非常类似。较不同的是最后一点同步循环的地方// Copyright (c) 2003-2013 Christopher M. Kohlhoff (chris at kohlhoff dot com)//// Distributed under the Boost Software License, Version 1.0. (See accompanying// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)//#include <iostream>#include <istream>#include <ostream>#include <string>#include <boost/asio.hpp>#include <boost/bind.hpp>using boost::asio::ip::tcp;//client客户端类class client{public:  client(boost::asio::io_service& io_service,      const std::string& server, const std::string& path)    : resolver_(io_service),      socket_(io_service)  {    // Form the request. We specify the "Connection: close" header so that the    // server will close the socket after transmitting the response. This will    // allow us to treat all data up until the EOF as the content.    std::ostream request_stream(&request_);    request_stream << "GET " << path << " HTTP/1.0\r\n";    request_stream << "Host: " << server << "\r\n";    request_stream << "Accept: */*\r\n";    request_stream << "Connection: close\r\n\r\n";    // Start an asynchronous resolve to translate the server and service names    // into a list of endpoints.    //异步解析DNS，并返回列表    tcp::resolver::query query(server, "http");    resolver_.async_resolve(query,        boost::bind(&client::handle_resolve, this,          boost::asio::placeholders::error,          boost::asio::placeholders::iterator));  }private:  void handle_resolve(const boost::system::error_code& err,      tcp::resolver::iterator endpoint_iterator)  {    if (!err)    {      // Attempt a connection to each endpoint in the list until we      // successfully establish a connection.        //这是每个都尝试连接吗？      boost::asio::async_connect(socket_, endpoint_iterator,          boost::bind(&client::handle_connect, this,            boost::asio::placeholders::error));    }    else    {      std::cout << "Error: " << err.message() << "\n";    }  }  void handle_connect(const boost::system::error_code& err)  {    if (!err)    {      // The connection was successful. Send the request.        //连接成功后尝试连接      boost::asio::async_write(socket_, request_,          boost::bind(&client::handle_write_request, this,            boost::asio::placeholders::error));    }    else    {      std::cout << "Error: " << err.message() << "\n";    }  }  void handle_write_request(const boost::system::error_code& err)  {    if (!err)    {      // Read the response status line. The response_ streambuf will      // automatically grow to accommodate the entire line. The growth may be      // limited by passing a maximum size to the streambuf constructor.      boost::asio::async_read_until(socket_, response_, "\r\n",          boost::bind(&client::handle_read_status_line, this,            boost::asio::placeholders::error));    }    else    {      std::cout << "Error: " << err.message() << "\n";    }  }  void handle_read_status_line(const boost::system::error_code& err)  {    if (!err)    {      // Check that response is OK.      std::istream response_stream(&response_);      std::string http_version;      response_stream >> http_version;      unsigned int status_code;      response_stream >> status_code;      std::string status_message;      std::getline(response_stream, status_message);      if (!response_stream || http_version.substr(0, 5) != "HTTP/")      {        std::cout << "Invalid response\n";        return;      }      if (status_code != 200)      {        std::cout << "Response returned with status code ";        std::cout << status_code << "\n";        return;      }      // Read the response headers, which are terminated by a blank line.      boost::asio::async_read_until(socket_, response_, "\r\n\r\n",          boost::bind(&client::handle_read_headers, this,            boost::asio::placeholders::error));    }    else    {      std::cout << "Error: " << err << "\n";    }  }  void handle_read_headers(const boost::system::error_code& err)  {    if (!err)    {      // Process the response headers.      std::istream response_stream(&response_);      std::string header;      while (std::getline(response_stream, header) && header != "\r")        std::cout << header << "\n";      std::cout << "\n";      // Write whatever content we already have to output.      if (response_.size() > 0)        std::cout << &response_;      // Start reading remaining data until EOF.      boost::asio::async_read(socket_, response_,          boost::asio::transfer_at_least(1),          boost::bind(&client::handle_read_content, this,            boost::asio::placeholders::error));    }    else    {      std::cout << "Error: " << err << "\n";    }  }  void handle_read_content(const boost::system::error_code& err)  {    if (!err)    {      // Write all of the data that has been read so far.      std::cout << &response_;      // Continue reading remaining data until EOF.      boost::asio::async_read(socket_, response_,          boost::asio::transfer_at_least(1),          boost::bind(&client::handle_read_content, this,            boost::asio::placeholders::error));    }    //读到eof就退出？    else if (err != boost::asio::error::eof)    {      std::cout << "Error: " << err << "\n";    }  }  tcp::resolver resolver_;  tcp::socket socket_;  boost::asio::streambuf request_;  boost::asio::streambuf response_;};int main(int argc, char* argv[]){  try  {    if (argc != 3)    {      std::cout << "Usage: async_client <server> <path>\n";      std::cout << "Example:\n";      std::cout << "  async_client www.boost.org /LICENSE_1_0.txt\n";      return 1;    }    boost::asio::io_service io_service;    client c(io_service, argv[1], argv[2]);    //会停止结束吗？没有异步任务就结束    io_service.run();  }  catch (std::exception& e)  {    std::cout << "Exception: " << e.what() << "\n";  }  return 0;}