Connect to WebSocket server from Qt UI with Echo Client Example - c++

Question

I'm using Websocket Echo Server Example in CLI and it works fine. I'm trying to connect to this server from my Qt GUI project. I have MainWindow class with an appropriate slot
void MainWindow::on_pushButton_clicked()
{
qDebug() << "Push btn clicked";
EchoClient client(QUrl(QStringLiteral("ws://localhost:1234")));
}
and EchoClient files from CLI Websocket Echo Client Example.
The main problem is that that I can't connect to the server when I push button on the form. However, I see debug message "Push btn clicked". It is supposed to be printed "Hello, world!". But nothing happens, no errors. Even signal void EchoClient::onConnected() is not fired.
But if I move EchoClient client(QUrl(QStringLiteral("ws://localhost:1234"))); to main.cpp it connects:
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
EchoClient client(QUrl(QStringLiteral("ws://localhost:1234")));
return a.exec();
}
I am completely new in C++ and Qt.
Why this happens? Is it something with processing threads in UI? What should I know?
Qt 5.4.

Answer 1

inside your on_pushButton_clicked() function you are creating the EchoClient (which is asynchronous) in the stack of the function. As soon as the function exits, the EchoClient object is destroyed from the stack.
You can think about different solutions like creating a private field in the MainWindow class
private EchoClient *client;
then set it to null in the MainWindow constructor:
this->client = NULL;
and only at this point doing something like this in your click() routine:
void MainWindow::on_pushButton_clicked()
{
qDebug() << "Push btn clicked";
if (this->client == NULL)
{
this->client = new EchoClient(QUrl(QStringLiteral("ws://localhost:1234")));
}
else
qWarning() << "Carefull, the client is already running";
}
Then it is up to you taking care of the like cycle of the client object you have created.
Probably you have to think when you want to destroy it, probably via a "reset" button routine.
I suggest this reading: http://doc.qt.io/qt-4.8/qapplication.html#details