如何使用从处理程序调度的最终回调将响应异步返回给调用方on_read?

但是有什么方法可以通过 Asio 的io_context调用 thie _callback？我想以异步方式调用此回调，因为由用户提供的此回调可能会阻止，从而也阻止io_context的线程？类似于io_context中调度 on_read((、on_write(( 等其他处理程序的方式？

是的。您所追求的是async_result协议。我在其他答案中有一些例子(例如，我如何从boost：：asio：:p ost获得未来？

下面是构建基块：

存储处理程序

在您的"会话"中(让我们将其重命名http_request_op并将其隐藏在某个详细的命名空间中(，您需要记住一个完成处理程序。

别担心，没有人必须想出这样的处理程序。我们将添加一个初始化函数async_http_request，以适合您。

最终用户可能使用未来或协程 (yield_context(。当然，如果他们愿意，他们可以提供一个普通的香草回调。

using Response = http::response<http::string_body>;
template <typename Handler>
class http_request_op : public std::enable_shared_from_this<http_request_op<Handler> > {
// ...
Response res_;
Handler handler_;
// ...
public:
template <typename Executor>
explicit http_request_op(Executor ex, Handler handler)
: resolver_(ex),
stream_(ex),
handler_(std::move(handler))
{ }

现在在最后一步中，您调用该handler_。为了简单起见，我将fail助手变成了一个成员函数，并将其命名为complete：

void complete(beast::error_code ec, char const* what) {
if (ec && what) {
// TODO: A better idea would to make a custom `Response` type that
// has room for "fail stage"
res_.reason(what);
}
post(stream_.get_executor(), [this, ec, self=this->shared_from_this()] {
handler_(ec, std::move(res_));
});
}

所有检查ec和以前使用过fail的地方现在都使用相同的ec调用complete。此外，在on_read中，我们添加了一个无条件完成：

void on_read(beast::error_code ec, size_t /*bytes_transferred*/) {
if (ec)
return complete(ec, "read");
stream_.socket().shutdown(tcp::socket::shutdown_both, ec);
// unconditional complete here
return complete(ec, "shutdown");
}

启动功能 (async_http_request(

template <typename Context, typename Token>
auto async_http_request(Context& ctx, beast::string_view host, beast::string_view port, beast::string_view target, int version, Token&& token) {
using result_type = typename net::async_result<std::decay_t<Token>, void(beast::error_code, Response)>;
using handler_type = typename result_type::completion_handler_type;
handler_type handler(std::forward<Token>(token));
result_type result(handler);
std::make_shared<detail::http_request_op<handler_type> >
(make_strand(ctx), std::move(handler))
->start(host, port, target, version);
return result.get();
}

你会看到这会创建一个异步结果，它从传递的令牌中制作一个"处理程序"，踢http_request_op并返回异步结果。

返回的内容取决于传递的令牌。查看用法：

用法

我将展示最终用户可以选择使用此async_http_request启动函数的各种方式：

使用未来

auto future = async_http_request(ioc.get_executor(), host, port, target, version, net::use_future);
ioc.run();
std::cout << future.get() << "n";

返回类型为std::future<Response>。

承诺的创建和设置返回值/异常信息由 Asio 神奇地处理。

使用协程/屈服上下文：

net::spawn(ioc, [&ioc,args](net::yield_context yield) {
try {
auto host   = args[0];
auto port   = args[1];
auto target = args[2];
int version = args[3]=="1.0"? 10 : 11;
Response res = async_http_request(
ioc,
host, port, target, version,
yield);
std::cout << res << std::endl;
} catch (boost::system::system_error const& se) {
// no way to get at response here
std::cout << "There was an error: " << se.code().message() << std::endl;
}
});
ioc.run();

返回类型在这里Response。请注意，如果报告错误条件，则会引发异常。或者，传递一个error_code变量：

beast::error_code ec;
Response res = async_http_request(
ioc,
host, port, target, version,
yield[ec]);
std::cout << ec.message() << "n" << res << std::endl;

仍在使用回调

/*void*/ async_http_request(ioc, host, port, target, version, 
[](beast::error_code ec, Response const& res) {
std::cout << ec.message() << "b" << res << "n";
});

返回值最终只是void.

完整演示代码

没有现场演示，因为没有在线编译器支持网络请求，而且它超出了编译限制(例如这里(

#include <boost/asio.hpp>
#include <boost/asio/spawn.hpp>
#include <boost/asio/use_future.hpp>
#include <boost/beast/core.hpp>
#include <boost/beast/http.hpp>
#include <boost/beast/version.hpp>
#include <iostream>
#include <memory>
namespace beast = boost::beast;
namespace http = beast::http;
namespace net = boost::asio;
using tcp = boost::asio::ip::tcp;
using Response = http::response<http::string_body>;
namespace detail {
template <typename Handler>
class http_request_op : public std::enable_shared_from_this<http_request_op<Handler> > {
tcp::resolver resolver_;
beast::tcp_stream stream_;
beast::flat_buffer buffer_;
http::request<http::empty_body> req_;
Response res_;
Handler handler_;
template <typename F>
auto bind(F ptmf) { return beast::bind_front_handler(ptmf, this->shared_from_this()); }
void complete(beast::error_code ec, char const* what) {
if (ec && what) {
// TODO: A better idea would to make a custom `Response` type that
// has room for "fail stage"
res_.reason(what);
}
post(stream_.get_executor(), [this, ec, self=this->shared_from_this()] {
handler_(ec, std::move(res_));
});
}
public:
template <typename Executor>
explicit http_request_op(Executor ex, Handler handler)
: resolver_(ex),
stream_(ex),
handler_(std::move(handler))
{ }
void start(beast::string_view host, beast::string_view port, beast::string_view target, int version) {
req_.version(version);
req_.method(http::verb::get);
req_.target(target);
req_.set(http::field::host, host);
req_.set(http::field::user_agent, BOOST_BEAST_VERSION_STRING);
resolver_.async_resolve(host.to_string(), port.to_string(), 
bind_executor(stream_.get_executor(), bind(&http_request_op::on_resolve)));
}
private:
void on_resolve(beast::error_code ec, tcp::resolver::results_type results) {
if (ec)
return complete(ec, "resolve");
stream_.expires_after(std::chrono::seconds(30));
stream_.async_connect(results, bind(&http_request_op::on_connect));
}
void on_connect(beast::error_code const& ec, tcp::endpoint const&) {
if (ec)
return complete(ec, "connect");
stream_.expires_after(std::chrono::seconds(30));
http::async_write(stream_, req_, bind(&http_request_op::on_write));
}
void on_read(beast::error_code ec, size_t /*bytes_transferred*/) {
if (ec)
return complete(ec, "read");
stream_.socket().shutdown(tcp::socket::shutdown_both, ec);
// unconditional complete here
return complete(ec, "shutdown");
}
void on_write(beast::error_code ec, size_t /*bytes_transferred*/) {
if (ec)
return complete(ec, "write");
http::async_read(stream_, buffer_, res_, bind(&http_request_op::on_read));
}
};
}
template <typename Context, typename Token>
auto async_http_request(Context& ctx, beast::string_view host, beast::string_view port, beast::string_view target, int version, Token&& token) {
using result_type = typename net::async_result<std::decay_t<Token>, void(beast::error_code, Response)>;
using handler_type = typename result_type::completion_handler_type;
handler_type handler(std::forward<Token>(token));
result_type result(handler);
std::make_shared<detail::http_request_op<handler_type> >
(make_strand(ctx), std::move(handler))
->start(host, port, target, version);
return result.get();
}
int main(int argc, char** argv) {
std::vector<beast::string_view> args{argv+1, argv+argc};
if (args.size() == 3) args.push_back("1.1");
if (args.size() != 4) {
std::cerr << "Usage: http-client-async <host> <port> <target> [<HTTP "
"version: 1.0 or 1.1(default)>]n"
<< "Example:n"
<< "    http-client-async www.example.com 80 /n"
<< "    http-client-async www.example.com 80 / 1.0n";
return 255;
}
auto host   = args[0];
auto port   = args[1];
auto target = args[2];
int version = args[3]=="1.0"? 10 : 11;
net::io_context ioc;
net::spawn(ioc, [=,&ioc](net::yield_context yield) {
try {
Response res = async_http_request(
ioc,
host, port, target, version,
yield);
std::cout << "From coro (try/catch): " << res.reason() << std::endl;
} catch (boost::system::system_error const& se) {
// no way to get at response here
std::cout << "coro exception: " << se.code().message() << std::endl;
}
});
net::spawn(ioc, [=,&ioc](net::yield_context yield) {
beast::error_code ec;
Response res = async_http_request(
ioc,
host, port, target, version,
yield[ec]);
std::cout << "From coro: " << ec.message() << ", " << res.reason() << "n";
});
/*void*/ async_http_request(ioc, host, port, target, version, 
[](beast::error_code ec, Response const& res) {
std::cout << "From callback: " << ec.message() << ", " << res.reason() << "n";
});
auto future = async_http_request(ioc, host, port, target, version, net::use_future);
ioc.run();
try {
std::cout << "From future: " << future.get().reason() << "n";
} catch (boost::system::system_error const& se) {
std::cout << "future exception: " << se.code().message() << std::endl;
}
}

成功和失败请求的输出：

$ ./sotest www.example.com 80 / 1.1
From callback: Success, OK
From coro: Success, OK
From coro (try/catch): OK
From future: OK
$ ./sotest www.example.com 81 / 1.1
From callback: The socket was closed due to a timeout, connect
coro exception: The socket was closed due to a timeout
From coro: The socket was closed due to a timeout, connect
From future: future exception: The socket was closed due to a timeout
$ ./sotest www.example.cough 80 / 1.1
From callback: Host not found (authoritative), resolve
coro exception: Host not found (authoritative)
From coro: Host not found (authoritative), resolve
From future: future exception: Host not found (authoritative)
$ ./sotest www.example.com rhubarb / 1.1
From callback: Service not found, resolve
coro exception: Service not found
From coro: Service not found, resolve
From future: future exception: Service not found

请注意，超时示例当然总共运行 ~30 秒，因为一切都异步运行。

在调用时引用此示例：

修改构造函数session采用接受 http 状态整数和正文字符串的回调。

typedef std::function<void(unsigned int, const std::string&)> CALLBACK;
CALLBACK callback_;
explicit
session(net::io_context& ioc, CALLBACK& callback)
: resolver_(net::make_strand(ioc))
, stream_(net::make_strand(ioc))
, _callback(callback)
{
}

修改session::on_read以调用回调。

void
on_read(
beast::error_code ec,
std::size_t bytes_transferred)
{
if(ec)
{
_callback(0, "");
}
else
{
_callback(_res.result_int(), _res.body());
}
}