等待异步请求完成
Wait for async request to complete
我想用boost编程一个HTTP客户端。如果我能做到以下几点,我可以使用异步模型:
- 在第一步中,将请求发送到服务器
- 在第二步中,要么读取已经到达的响应,要么同步等待,直到它到达
这是一个类似于我所生产的类:
class HTTPClient {
public:
void sendTheRequest(...) {
// Send the HTTP request
}
std::string getTheResponse(...) {
// return the already (asynchronously) received response, or wait for it
// in this function
}
}
有人能指出如何实现这一点吗?我担心我缺乏助推方面的知识。
编辑以进行澄清:方法sendTheRequest将在某个时刻被调用。也许在它获得响应后会直接调用,但这也可能在几毫秒后发生。这就是为什么我想异步发送请求,但也需要同步等待。
可能有点晚了,但我认为应该这样做。std::future::get
返回send_request_function的值,如果还没有返回,则等待,并在完成后返回。
class HTTPClient {
public:
void sendTheRequest(...) {
// Send the HTTP request
f = std::async(std::launch::async, send_request_function, args ...);
}
std::string getTheResponse(...) {
// return the already (asynchronously) received response, or wait for it
// in this function
return f.get();
}
private:
std::future<std::string> f;
}
为什么不使用boost附带的异步http客户端示例?
//
// async_client.cpp
// ~~~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2011 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;
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/1.0rn";
request_stream << "Host: " << server << "rn";
request_stream << "Accept: */*rn";
request_stream << "Connection: closernrn";
// Start an asynchronous resolve to translate the server and service names
// into a list of endpoints.
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.
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.
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_, "rn",
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 responsen";
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_, "rnrn",
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));
}
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.txtn";
return 1;
}
boost::asio::io_service io_service;
client c(io_service, argv[1], argv[2]);
io_service.run();
}
catch (std::exception& e)
{
std::cout << "Exception: " << e.what() << "n";
}
return 0;
}
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