Multiple Threads, how to create qt widgets properly

Question

I have the follow constellation:

A qt gui thread with MainWindow

Another thread which essentially is a CameraManager...everytime a camera is added/removed the MainWindow will be informed.

It roughly looks like this:

Mainwindow derives from ICameraAddedConsumer

MainWindow implements ConsumeCameraAdded and creates widget inside this function. It subscribes itselv as a consumer to the CameraManager

CameraManager calls ConsumeCameraAdded of all it's consumers (MainWindow) when a new camera is added.

The problem is that CameraManager lives in a different thread and Qt will obviously complain about this since a widget is not created in the same thread as the mainwindow was.

Any suggestions how I can solve this?

I'd expect signals and slots to take care of that inter-thread communication. You don't subscribe to the manager but you connect one of its signals to one of your slots. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Mar 4, 2019 at 9:19
I thought about that, but what if I dont want the CameraManager to be a Q_OBJECT? — Captain GouLash
– Captain GouLash, Commented Mar 4, 2019 at 9:31
Or, you can use Boost signals. It is very useful when you don't want to use QObject in you business logic. — Dimitry Ernot
– Dimitry Ernot, Commented Mar 4, 2019 at 10:07

Maxim Paperno · Accepted Answer · 2019-03-05 04:25:13Z

As per comments, using signals/slots between QObjects in different threads should take care of the issue "automagically."

Barring that, and assuming MainWindow/ICameraAddedConsumer is a QObject, one idea could be to use something like:

QMetaObject::invokeMethod(consumer, "ConsumeCameraAdded", Qt::QueuedConnection, ...)

where consumer is a pointer to the MainWindow/ICameraAddedConsumer instance.

There's QWaitCondition but I'm not sure that makes sense in this case (though it could be adapted I suppose).

Otherwise... don't create the widget in ConsumeCameraAdded() but set some flag there (and return) and then use a QTimer or QObject::timerEvent() to periodically check the flag and create widget if it is set. Unfortunately I'm pretty sure you will not be able to create or start a timer within ConsumeCameraAdded() itself because of threading issues.

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