Errors facing while running QWebSocket in different thread than the main thread.

gully

All I want to do is create a websocket client connection using Qt on a thread. Create a processing thread,so that when data comes to the websocket handler, the data is passed(through slots and signals) to the processor ->then the processor processes the data and passes it to the main-thread(again via signals and slots) to render the data.I am doing this because I want to keep the main thread ublocked because processing and websocket data would take lot of time.
First of all,is this the right approach?
Second:when I try to run QWebsockets on a different thread I am getting the error .

QObject: Cannot create children for a parent that is in a different thread.
(Parent is QWebSocket(0x1a493f30), parent's thread is QThread(0x187fc0e8), current thread is QThread(0xe3fe08)

Not sure where to include this

(Make sure 'QAbstractSocket::SocketState' is registered using qRegisterMetaType().)

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
 
    QUrl url("ws://localhost:8080");
    websocketObj=new websocketclientimpl(url);
    websocketObj->moveToThread(&workerThread_websocket);
    connect(&workerThread_websocket, &QThread::started, websocketObj, &websocketclientimpl::startConnection);
    workerThread_websocket.start();
    QMainWindow::showMaximized();
 
}
 
MainWindow::~MainWindow()
{
    delete ui;
    workerThread_websocket.quit();
    workerThread_websocket.wait();
}

using namespace std;
namespace Ui {
class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
 
private:
    Ui::MainWindow *ui;
    websocketclientimpl *websocketObj;
    calculation *calculationObj;
 
    QThread workerThread_websocket;
    QThread workerThread_calculator;
 
};

class websocketclientimpl : public QObject
{
    Q_OBJECT
public:
    QWebSocket m_webSocket;
    websocketclientimpl(const QUrl &url);
    void startConnection();
    void stopConnection();
 
private Q_SLOTS:
    void onConnected();
    void onClosed();
    void onTextMessageReceived(QString message);
 
Q_SIGNALS:
    void processIncomingMessage(QString message);
 
private:
 
    QUrl m_url;
};

websocketclientimpl::websocketclientimpl(const QUrl &url){
this->m_url=url;
}
 
void websocketclientimpl::startConnection()
{
    qDebug() << "WebSocket client:" << m_url;
    connect(&m_webSocket, &QWebSocket::connected, this, &websocketclientimpl::onConnected);
    connect(&m_webSocket, &QWebSocket::disconnected, this, &websocketclientimpl::onClosed);
    QNetworkRequest request=QNetworkRequest(QUrl(m_url));
    m_webSocket.open(request);
 
}
 
void websocketclientimpl::stopConnection(){
    m_webSocket.disconnect();
     qDebug() << "WebSocket stopped";
}
 
void websocketclientimpl::onConnected()
{
    qDebug() << "WebSocket connected..";
}
 
void websocketclientimpl::onClosed()
{
 
}
 
void websocketclientimpl::onTextMessageReceived(QString message)
{
    qDebug() << "message received in websocket class"<<message;
    emit processIncomingMessage(message);
}

Can anyone help me out here? Stuck very badly...

karlheinzreichel

The problem is that the m_websocket is allocated during creation of websocketclientimpl.
At that state websocketclientimpl and m_websocket is in ownership of the mainthread.
After that you shift websocketclientimpl to workerthread.
The m_websocket object remains in ownership of mainthread.
Result: you get the noted error message.

One possible workaround:
Use m_webSocket as as QWebsocket pointer.
Instantiate the m_webSocket in start connection (delete it in stopConnection)

void websocketclientimpl::startConnection()
{
    m_webSocket = new QWebSocket;
    qDebug() << "WebSocket client:" << m_url;
    connect(m_webSocket, &QWebSocket::connected, this, &websocketclientimpl::onConnected);
    connect(m_webSocket, &QWebSocket::disconnected, this, &websocketclientimpl::onClosed);
    QNetworkRequest request=QNetworkRequest(QUrl(m_url));
    m_webSocket->open(request);

}

void websocketclientimpl::stopConnection(){
    if (m_webSocket != nullptr) {
    m_webSocket->disconnect();
    qDebug() << "WebSocket stopped";
    m_webSocket->deleteLater();
    }
}

regards
Karl-Heinz

VRonin

@karlheinzreichel is correct, just one minor point, you can pass a parent to the constructor of m_webSocket instead of calling deleteLater explicitly as it might even be safer (deleteLater would leak if the event loop in the worker thread is stopped before it gets called)

gully

@karlheinzreichel thanks a lot..yes that was the problem and now it is solved..Thanks a ton for pointing it out and for the solution.