add promises example

untests/curly_tests.cpp

@@ -57,6 +57,26 @@ namespace
             throw std::exception();
         }
     };
+
+    netex::promise<net::content_t> download(std::string url) {
+        return netex::make_promise<net::content_t>([
+            url = std::move(url)
+        ](auto resolve, auto reject){
+            net::request_builder(std::move(url))
+                .callback([resolve,reject](net::request request) mutable {
+                    if ( !request.is_done() ) {
+                        reject(net::exception("network error"));
+                        return;
+                    }
+                    net::response response = request.get();
+                    if ( response.is_http_error() ) {
+                        reject(net::exception("server error"));
+                        return;
+                    }
+                    resolve(std::move(response.content));
+                }).send();
+        });
+    }
 }
 
 TEST_CASE("curly") {
@@ -706,7 +726,7 @@ TEST_CASE("curly_examples") {
         // Error message: Couldn't resolve host name
     }
 
-    SECTION("Request Callback") {
+    SECTION("Request Callbacks") {
         auto request = net::request_builder("http://www.httpbin.org/get")
             .callback([](net::request request){
                 if ( request.is_done() ) {
@@ -771,4 +791,20 @@ TEST_CASE("curly_examples") {
                 .send().get();
         }
     }
+
+    SECTION("Promised Requests") {
+        auto promise = download("https://httpbin.org/image/png")
+            .then([](const net::content_t& content){
+                std::cout << content.size() << " bytes downloaded" << std::endl;
+            }).except([](std::exception_ptr e){
+                try {
+                    std::rethrow_exception(e);
+                } catch (const std::exception& ee) {
+                    std::cerr << "Failed to download: " << ee.what() << std::endl;
+                }
+            });
+
+        promise.wait();
+        // 8090 bytes downloaded
+    }
 }