Changing button text from another thread.

[wsmckenz]

I'm fairly new to Qt, so excuse me I'm missing something obvious.

I have an action button named actionConnect on my mainToolbar that is labeled 'Connect'. I'm trying to change the text to read 'Disconnect' from another thread, when I receive an event from a socket saying the connection was successful. The following crashes the app when called from my other thread (which for reasons beyond my control happens to be a pthread, not a QThread):

	QMetaObject::invokeMethod(ui->actionConnect, "setText", Q_ARG(QString, "Disconnect."));

And if I call it from any other place, on my gui thread it does nothing at.

Is there any reason setText would not be invokable on a QAction?

[SGaist]

Hi and welcome to devnet,

QThread is not a thread but a thread handler around platform specific implementation so on unix pthreads.

Where is that ui variable coming from ?

[wsmckenz]

It's the designer-supplied ui in my main window, looks like it is created magically by compiling the .ui xml file. This is all happening in a method of my MainWindow class.

void MainWindow::connect()
{
    // member variable, my websocket client wrapper which runs on its own thread.
     _client.request_session();

    //	add an open handler using a lambda.
    _client.add_open_handler([this]() {
            
	std::cout << "Open Session.\n";   // <=== this works fine.

    // this does nothing.
	QMetaObject::invokeMethod(ui->actionConnect, "setText", Q_ARG(QString, "Disconnect."));

   // ui->actionConnect->setText("Disconnect");    <== this works, oddly enough, even though I'm on another thread at this point.		
});
}

The more I play around with it, though, I don't think this has anything to do with the threading. I happened to be using invokeMethod because I'm coming from another thread (and that is working fine in a few other cases, where I invoke methods on my other widgets). But no matter where I put the call, invokeMethod on a QAction (as opposed to a QWidget) seems to be ignored. There is also no Debug output, which seem to occur when you try to invoke things that don't exist.

[wsmckenz]

OK, so I lied.

This is produced in the debug window:

QMetaObject::invokeMethod: No such method QAction::setText(QString)

But according to these docs, it does exist, and as I said I can call it directly. My ui->actionConnect parameter seems like it must be valid, because QMetaObject is able to determine that it's type is QAction.

http://doc.qt.io/qt-4.8/qaction.html#text-prop

[Devopia]

The setText() is not a signal or a slot name on the QAction.
The invokeMethod() can't invokes general functions.

[wsmckenz]

Right, that makes sense, and now that I look, the api docs tag all the signals and slots, so it easy to know whether you can invoke a given method.

So to achieve this (changing that button label from a non-ui thread), I'm thinking my options would be:

1. Add a custom slot to my main window, invoke it and have it call setText() on the action.
2. Subclass QAction and add a slot to it to do the dirty work.

Option one seems simpler.

Thanks for a clear explanation.

[VRonin]

If you are already subclassing QThread it might be easier to add a signal to that subclass

// Inside MyThread class, the QThread subclass
signals:
void updateConnectedButton(const QString&);

and when the connection is enstablished call

emit updateConnectedButton(tr("Disconnect"));

now you can use Qt5 connection in your main window to link the two

connect(otherthread, &MyThread::updateConnectedButton, ui->actionConnect, &QAction::setText);