Setting Table Items Across Multiple Threads

Question

I'm trying to update the values in a table using a separate worker thread, using POSIX threads.

The function the worker thread is executing is trying to do the following:

ui->table->setItem(0,0,new QTableWidgetItem(tr("%1").arg(value)));

However, at runtime I get the following error:

QObject::connect: Cannot queue arguments of type 'QVector<int>'
(Make sure 'QVector<int>' is registered using qRegisterMetaType().)

I'm not quite sure how that qRegisterMetaType works. I'm wondering if anyone has any idea how I can grant the worker thread direct access to the widgets?

Answer 1

Accessing any widget from anything but the main thread is not allowed in Qt. All UI operations need to be done from within the main thread (with a few exceptions, such as painting on a QImage).

In your case, emit a signal in the worker thread when you want to add a new item. In the main thread, have a slot that actually creates the item, i.e. calls ui->table->setItem(0,0,new QTableWidgetItem(tr("%1").arg(value)));. Then simply connect() the signal and the slot together. Qt will realize that sender and receiver live in different threads and use a QueuedConnection automatically. You slot will be called whenever Qt's main thread returns to the event loop.

If your signal has parameters, their type needs to be registered with qRegisterMetaType.