qt signals/slots in a plugin

Question

I have an app with such structure: all the datatypes (class INode) are stored in plugins (DLLs). Some of the datatypes can be drawn (if they're IDrawable).

To load an object of, e.g. class PointCloudNode: public INode I have a special input plugin (DLL) which is called class PointCloudParser: public IIOPlugin and IIOPlugin is a thread with some specific functionality: class IIOPlugin: public QThread.

All the objects are created by NodeFactory class which is a singleton stored in separate DLL.

And here's the problem:

void PointCloudNode::update()
{
QObject::connect (this,SIGNAL(tmptmp()),this,SLOT(drawObject()));
emit tmptmp();
}

If I do this from any thread (main thread or the Input Plugin thread)

NodeFactory* fab = NodeFactory::getInstance();
boost::shared_ptr<INode> pc(fab->createNode("pointCloud","myPC"));
boost::shared_ptr<IDrawable> dr = boost::dynamic_pointer_cast<IDrawable>(pc);
dr->update();

The update launches, the tmptmp() signal is emitted, and the slot (drawObject()) executes correctly.

BUT if do just the same, but create the object in my Input Plugin, pass over the shared pointer and execute dr->update() in another function, the slot drawObject() is never entered though all the code is executed (including connect, etc.).

To be more precise, here's the Input Plugin:

 void PointCloudParserPlugin::doLoad(const QString& inputName, boost::shared_ptr<INode> container)
 {
   NodeFactory* factory = NodeFactory::getInstance();
   boost::shared_ptr<INode> node = factory->createNode("pointCloud", inputName);

   // here goes the loading itself, nothing special...

   container->addChild(node); //that's the container where I keep all the objects

   //boost::dynamic_pointer_cast<IDrawable>(container->getChild(inputName))->update();
   //If I uncomment this line, it all works: the slot is launched.  
   emit loadingFinished(inputName); // it executes the following function
 }

The last emit is connected to this:

 void GeomBox::updateVisualization(const QString& fileName)
 {
   boost::shared_ptr<INode> node = container_->getChild(fileName);
   boost::shared_ptr<IDrawable> nodeDrawable = boost::dynamic_pointer_cast<IDrawable>(node);
   nodeDrawable->update(); //this is the problem line: update() executes, connect() works, but the slot never runs :(
 }

How come? The node object is the same all the way through, it is valid. Every line in code in launched, QObject::connect doesn't write anything to debug window, the signal tmptmp() is emitted, but the slot drawObject() in one case is never reached? Any ideas?

Upd.: If I do not inherit IIOPlugin from QThread, everything works fine (i.e. load the object in the main thread). I expected the signals/slots to work across the threads...

* have you included this last line in the source for plugins? ( Q_EXPORT_PLUGIN2(xxx,XXX)) — Ronny Brendel
yep, I've done all of that it seems something's wrong with signal/slots between DLLs.. — Mikhail
Did you double check if plugin is loaded? Put some asserts on plugin loading code — Kamil Klimek

Dave Ceddia Dave Ceddia · Accepted Answer · 2011-08-25T16:48:03

Since you are sending a signal across to a different thread, you might need to explicitly tell Qt that the connection should be a queued one:

QObject::connect(this, SIGNAL(tmptmp()), this, SLOT(drawObject()), Qt::QueuedConnection );

By default Qt will use Qt::AutoConnection as that last parameter, and it will choose whether to use a direct connection (if the slot is in the same thread as the emitter) or a queued connection (if the slot is in a different thread). But since your thread is in a separate library, maybe Qt isn't making the right assumption here.

qt signals/slots in a plugin

1 Answers