Use timeout for connection in QT

In this article, we will try to understand about using timeout for our common problem in communicating between client and server. In QT, this bug will not resolve. So, we always look forward solution in a next update of QT.

Context of problem
Solution for timeout problem
Wrapping up

Context of problem

Normally, when we want to get data from server based on SOAP or RESTful API, in QT, we will use some classes to do this such as QNetworkRequest, QNetworkReply, and QEventLoop.

QNetworkRequest req(QUrl("http://www.google.com"));
QNetworkReply *reply = nam.get(req);
QEventLoop loop;
connect(reply, SIGNAL(finished()), &loop, SLOT(quit()));
loop.exec();

But we will have problem in QEventLoop, mainly because this loop will work until the client receives data. So, when our computer have no internet, this loop is unlimited.

Therefore, normal solution for this problem is to set timeout for GET method in http. But in QT, it do not support the timeout. So, we have to manually implement timeout.

Solution for timeout problem

Use QTimer to set timeout. Here are two ways to implement:

Use QTimer with QEventLoop

  void useReplyTimeout(QNetworkReply* reply) {
      QTimer timer;    
      timer.setSingleShot(true);

      QEventLoop loop;
      connect(&timer, SIGNAL(timeout()), &loop, SLOT(quit()));
      connect(reply, SIGNAL(finished()), &loop, SLOT(quit()));
      timer.start(30000);  // use miliseconds
      loop.exec();

      if(timer.isActive()) {
          timer.stop();

          if(m_reply->error() > 0) {
              ... // handle error
          }
          else {      
              int v = reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();

              if (v >= 200 && v < 300) {  // Success
                  ...
              }
          }
      } else {
          // timeout
          disconnect(reply, SIGNAL(finished()), &loop, SLOT(quit()));
          reply->abort();
      }
  }

Using useReplyTimeout() function will make our program to work like a charm.

Wrap all above things into ReplyTimeout class

  class ReplyTimeout : public QObject {
      Q_OBJECT
      QBasicTimer m_timer;
      HandleMethod m_method;

      public:
          enum HandleMethod { Abort, Close };

          QReplyTimeout(QNetworkReply* reply, const int timeout, HandleMethod method = Abort) :  
              QObject(reply), m_method(method)
          {
              Q_ASSERT(reply);
              if (reply && reply->isRunning()) {
              m_timer.start(timeout, this);
              connect(reply, &QNetworkReply::finished, this, &QObject::deleteLater);
          }

          static void set(QNetworkReply* reply, const int timeout, HandleMethod method = Abort) {
              new ReplyTimeout(reply, timeout, method);
          }
      protected:
          void timerEvent(QTimerEvent * ev) {
              if (!m_timer.isActive() || ev->timerId() != m_timer.timerId())
              return;
              auto reply = static_cast<QNetworkReply*>(parent());
              if (reply->isRunning()) {
              if (m_method == Close)
                  reply->close();
              else if (m_method == Abort)
                  reply->abort();
              m_timer.stop();
          }
  };

When using ReplyTimeout class, I do not get data from server, although, I set timeout with a huge number. And I still do not understand why it is not working.

Wrapping up

We know about how to send request to server and get response from server.
Using QTimer class to set timeout for our program.

Refer:

https://stackoverflow.com/questions/13207493/qnetworkreply-and-qnetworkaccessmanager-timeout-in-http-request

https://stackoverflow.com/questions/37444539/how-to-set-qnetworkreply-timeout-without-external-timer

http://amin-ahmadi.com/2016/04/05/check-internet-connection-availability-status-qt/

Table of contents

Context of problem

Solution for timeout problem

Wrapping up