Add a base class for http async clients; refine examples.

example/http_async_client/CMakeLists.txt

@@ -0,0 +1,4 @@
+add_executable(http_async_client main.cc)
+
+target_link_libraries(http_async_client webcc ${Boost_LIBRARIES})
+target_link_libraries(http_async_client "${CMAKE_THREAD_LIBS_INIT}")

example/http_async_client/main.cc

@@ -0,0 +1,49 @@
+#include <iostream>
+
+#include "boost/asio/io_context.hpp"
+
+#include "webcc/http_async_client.h"
+#include "webcc/logger.h"
+
+// TODO: The program blocks during read response.
+//   Only HttpBin.org has this issue.
+
+static void Test(boost::asio::io_context& io_context) {
+  auto request = webcc::HttpRequest::Make(webcc::kHttpGet, "/get",
+                                          "httpbin.org");
+
+  webcc::HttpAsyncClientPtr client{
+    new webcc::HttpAsyncClient(io_context)
+  };
+
+  client->SetTimeout(3);
+
+  // Response callback.
+  auto callback = [](webcc::HttpResponsePtr response, webcc::Error error,
+                     bool timed_out) {
+    if (error == webcc::kNoError) {
+      std::cout << response->content() << std::endl;
+    } else {
+      std::cout << DescribeError(error);
+      if (timed_out) {
+        std::cout << " (timed out)";
+      }
+      std::cout << std::endl;
+    }
+  };
+
+  client->Request(request, callback);
+}
+
+int main() {
+  WEBCC_LOG_INIT("", webcc::LOG_CONSOLE);
+
+  boost::asio::io_context io_context;
+
+  //Test(io_context);
+  Test(io_context);
+
+  io_context.run();
+
+  return 0;
+}