Lambdas with Qt's QNetworkAccessManager

implementation.cpp

class RemoteCloud {
  QNetworkAccessManger m_nam;
  
  QString URL(QString url); // left as an exercise to the reader, this just maps a short name to a server/versioned endpoint
  QNetworkReplay *get(const QString& url);
  QNetworkReply *post(const QString& url, const QVariantMap& object);
  
public:
  QNetworkReply *get(const QString& endpoint, std::function<void(QNetworkReply*)> lambda,  bool sync=false);
  QNetworkReply *post(const QString& endpoint,
					std::function<void(QNetworkReply*, const QVariantMap&)> lambda,
          bool sync=false);
};


QNetworkReply *RemoteCloud::get(const QString& endpoint, const QVariantMap& item) {
	QString url = URL(endpoint);
	if (item.size()) {
		QStringList params;

		for(const QString& key : item.keys()) {
			params.append(key + "=" + item[key].toString());
		}

		QString queryParams = params.join("&");
		url += "?" + queryParams;
	}

	QNetworkRequest request = QNetworkRequest(url);
	QNetworkReply* reply = m_nam.get(request);
	return reply;
}

QNetworkReply *RemoteCloud::post(const QString& endpoint, const QVariantMap& item) {
	QNetworkRequest request = QNetworkRequest(URL(endpoint));
	request.setHeader(QNetworkRequest::ContentTypeHeader, "application/json");

	QNetworkReply* reply = m_nam.post(request, body);
	return reply;
}


QNetworkReply *RemoteCloud::get(
    const QVariantMap& params,
    const QString& endpoint, std::function<void(QNetworkReply*)> lambda,
    bool sync, 
  ) {
	QEventLoop loop;

	QNetworkReply *reply = get(endpoint, params);
	QObject::connect(reply, &QNetworkReply::finished, &loop, &QEventLoop::quit);
	QObject::connect(reply, &QNetworkReply::finished, reply, [=]{
		lambda(reply);
		reply->deleteLater();
	});
	if (sync)
    loop.exec(); // wait for finished
	return reply;
}


QNetworkReply* RemoteCloud::post(const QString& endpoint,
					std::function<void(QNetworkReply*, const QVariantMap&)> lambda,
          bool sync) {
	QEventLoop loop;
	QNetworkReply* reply = nullptr;
  reply = post(URL(endpoint), object);
  QObject::connect(reply, &QNetworkReply::finished, &loop, &QEventLoop::quit);
  QObject::connect(reply, &QNetworkReply::finished, reply, [=]{
    lambda(reply, object);
    reply->deleteLater();
  });
  if (sync)
    loop.exec(); // wait for finished
	return reply;
}

invocation.cpp

remoteCloud->get(
  "users",
  QVariantMap(),
	[=] (QNetworkReply* reply){
				if (reply->error() == QNetworkReply::NoError) {
					/* save users: write reply->body() */
				} else {
					qDebug() << Q_FUNC_INFO << reply->request().url() << reply->request().attribute(QNetworkRequest::CustomVerbAttribute) << reply->errorString();
				}
        / /will be deleted automatically
  },
  false // will return immediately
);

// sequential object upload
for (const QVariantMap& event: QList<QVariantMap> {...} ) {
  remoteCloud->post(
    "events",
    event,
  	[=] (QNetworkReply* reply){
				if (reply->error() == QNetworkReply::NoError) {
					/* file upload complete */
				} else {
					qDebug() << Q_FUNC_INFO << reply->request().url() << reply->request().attribute(QNetworkRequest::CustomVerbAttribute) << reply->errorString();
				}
    }, //lambda
    true // sync
  ); // post
} // for file