Can QtQuick app create and display MessageDialog from C++ static member function?

Tom asso

I want my C++ static member function to simply display an informational message in a dialog. Does QtQuick provide a convenience function to do this? I see how to do this with a C++ signal connected to QML slot, but in my case the C++ is a static member so cannot emit a signal.

Thanks!

GrecKo

@Tom-asso add a static QObject member to emit the signal. Or add a function in your root QML window and invoke it from c++

Tom asso

@GrecKo - thanks!
I might be over-thinking this, but it's still not clear to me how this would work. I.e. I want my static C++ member function to raise an informational message dialog (with just on 'OK' button). Seems like a common use case? Are there examples that you can point to? In particular, what would the QObject::connect() call look like, if the signal is being emitted by a static member object, assuming my QML ApplicationWindow defines function showInfoDialog()?

class Emitter : public QObject {
signal:
  sendMsg(QString msg);
};

class Backend {
   static Emitter emitter;
}

Now how to connect the signal emitted by static member emitter with QML? What are the first two arguments to connect()?

main.cpp:

Backend backend;

QObject::connect(&backend.emitter, SIGNAL(sendMsg(QString msg)),
		 rootObject, SLOT(showInfoDialog(QString msg)));

The above compiles, but connect() fails at runtime:

QObject::connect: No such signal Emitter::showMessage(QString &msg)

This seems odd because the runtime "knows" that showMessage(QString &msg) is associated with the Emitter class, yet somehow doesn't recognize it as a signal. What am I doing wrong???

Thanks!

Bob64

@Tom-asso I think the issue you are having is that Backend does not expose a signal to allow you to connect, right?

You could add a static function to your Backend class which would make the connection. That would then have access to the emitter. To keep it clean, you could pass a std::function as the target, which you initialise with a lambda.

class Backend
{
public:
    static void connectInfoMsg(std::function<void(QString)> onInfoMsg)
    {
        QObject::connect(&emitter, &Emitter::sendMsg, onInfoMsg);
    }
    ...
};

In your main.cpp:

Backend::connectInfoMsg(
    [&](QString msg){ rootObject->showInfoDialog(msg);
}

Despite what you say, I am not sure how common this actually is as a use case. I don't recall ever having done anything like this. I would be more likely to expose an object to QML using a context property (old-fashioned way) or qmlRegisterSingletonInstance (modern way) that would emit signals when messages are available. Then hook up connections to dialogs on the QML side.

I think you are possibly making things awkward for yourself with a design that relies on a static function.

Tom asso

@Bob64 - Thanks, Bob.
Emitter actually emits a signal sendMsg(). The reason that connect() failed is because I need to specify data passed as QVariant (not QMessage) when data is exchanged between C++ and QML. So the following compiles and connects properly:

Emitter.h:

class Emitter : public QObject {
signal:
  sendMsg(QVariant msg);
};

main.cpp:

QObject::connect(&backend.emitter, SIGNAL(sendMsg(QVariant)),
		 rootObject, SLOT(showInfoDialog(QVariant)));

My application includes a lot of legacy 'C' code, and passes Backend static function pointers to that C code. Now that I have it working it seems ok.
Thanks!