如何在非GUI线程上正确创建QUdpSocket？ 已读未发出

Question

I'm writing a network library that wraps the QUdpSocket :

QAbstractSocket *UdpNetworkStreamer::addConnection()
{
    QUdpSocket *udpSocket = new QUdpSocket(this);
    udpSocket->bind(connection.port, QUdpSocket::ShareAddress);
    bool ret = udpSocket->joinMulticastGroup(QHostAddress(connection.ip));
    connect(udpSocket, SIGNAL(readyRead()), this, SLOT(readyRead()), Qt::QueuedConnection);

    return udpSocket;
}

1. create a new QUdpSocket .
2. connect to its readyRead signal.
3. call readDatagram when readyRead is raised.

All is working fine when I use the library from a Qt GUI application.

The problem starts when another user includes the library used outside of a Qt GUI application.

He calls the addConnection (which creates the socket and calls connect on the readyRead )

The thread on which the addConnection is called is non-Qt.

The addConnection seems to end successfully but the readyRead is never emitted.

Calling read (even though no readyRead was emitted) leads to a successful datagram read.

Fixes that did not work :

1. moving the the UDP socket thread to the this->thread

    QUdpSocket *udpSocket = new QUdpSocket(); udpSocket->moveToThread(this->thread()); udpSocket->setParent(this); 

2. I tried to simulate the problem by calling:void

    MainWindow::on__btnOpenMulticastReceiver_clicked() { QFuture<void> future = QtConcurrent::run(this, &MainWindow::CreateMulticastConnection, testHandle); } 

   This also led to same symptoms as the one the user had with my library, meaning the readyRead wasn't emitted.

3. QSignalSpy - I've activated a spy on the readyRead signal; the counter kept on being zero although I could read data directly from the socket. The spy gave valid results (ie progressed) when used the socket was initialized on the main thread.

My Questions:

1. What am I missing and doing wrong ?
2. What is the simplest way of having the readyRead emitted even though it is not created on the main GUI thread - I couldn't find any sample that works with no GUI or outside Qt threads.

Answer 1

My guess is that you need to add Q_OBJECT macro to the beginning of the class from where you need to emit the signal. Unless you do this, the signal-slot mechanism will not function properly.

Answer 2

I ended up solving the problem this way :

void MainWindow::OpenConnection()
{
  QThread *t = new QThread();
  t->start();

  SocketWrapper *w= new SocketWrapper();

  w->moveToThread(t);

  w->metaObject()->invokeMethod(w, "CreateSocket", Qt::QueuedConnection);
}

You must call invokeMethod() with the thread the socket wrapper was movedTo() upon creation of the socket, so that the thread that creates the socket will have a running event loop.

In addition to that, the CreateSocket() needs to be a slot in the SocketWrapper , something like that :

class SocketWrapper : public QObject
{
  Q_OBJECT
public:
  explicit SocketWrapper(QObject *parent = 0);


signals:

public slots:
  void readyRead();
  void CreateSocket();
private:
  QUdpSocket *_socket;
};