How connect a signal from C++ to QML function with parameter

Question

I want to connect a signal which is created by a C++ QObject to a function in a QML item. The signal is the "activated" signal from a QSystemTrayIcon and its argument is ActivationReason (an enum value).

Unfortunately it seems I can't connect a signal with this signature to a slot which seems to only be able to receive QVariant.

In the QML file

function trayIconClicked(reason) { ... }

In the C++ code

QObject::connect(trayIcon, SIGNAL(activated(QSystemTrayIcon::ActivationReason)), rootObject, SLOT(trayIconClicked(QVariant)));

And this is what I get

QObject::connect: Incompatible sender/receiver arguments
        QSystemTrayIcon::activated(QSystemTrayIcon::ActivationReason) --> ApplicationWindow_QMLTYPE_11_QML_60::trayIconClicked(QVariant)

If I completely drop the argument on the slot side, the event calls the function. But then I have no idea what ActivationReason triggred it.

A full working version of the code above is here, except that I can't hand in the reason parameter https://github.com/sturmf/qt_samples/tree/master/trayicon

maybe this page help you: http://stackoverflow.com/questions/8834147/c-signal-to-qml-slot-in-qt — Farhad, Apr 29 '16 at 11:53
unfortunately not, I read this answer before. He is lucky that both sides, signal and slot can be changed to QVariant. I have the fixed parameter from the QSystemTrayIcon. I also know about an intermediate slot which could be used for translation. But my code lives in the main function of the C++ code where I don't have a QObject that could work as an intermediate. — Fabian, Apr 29 '16 at 12:10
@Fabian - conversion to `QVariant` is needed for QML-C++ interop, you can register the custom type to as a meta type so it can be wrapped by a variant so it can be passed to the QML side. — dtech, Apr 29 '16 at 12:38
@ddriver Hmm my problem is that ActivationReason is not a custom type and I can't register it in my main.cpp at least I get the compile error please use the Q_DECLARE_METATYPE. And I can't use Q_DECLARE_METATYPE outside of an QObject. — Fabian, Apr 29 '16 at 13:22
Yes you can: Ideally, this macro should be placed below the declaration of the class or struct. If that is not possible, it can be put in a private header file which has to be included every time that type is used in a QVariant. — dtech, Apr 29 '16 at 14:20
@ddriver I really tried, but to no avail, I now added a link to a small sample app, maybe you can show how it would need to be done? — Fabian, Apr 29 '16 at 14:40
OK, so it is just an enum, that's why it can't be registered as a meta type, and it is basically just an int. So you could connect it to signal on the C++ side that re-emits it cast as an integer, and you connect the QML function to that. — dtech, Apr 29 '16 at 15:11
... can't you just implement a trampoline slot performing the wrapping and re-emitting the signal as a QVariant? — peppe, May 02 '16 at 00:11
@peppe See my answer below, this is my idea of a trampoline slot. Is there even a more elegant or shorter version without subclassing? — Fabian, May 10 '16 at 18:50

score 0 · Accepted Answer · answered May 10 '16 at 18:49

I now subclassed the QSystemTrayicon and added a slot which emits another signal without the parameter.

#ifndef TRAYICON_H
#define TRAYICON_H
#include <iostream>
#include <QObject>
#include <QSystemTrayIcon>

class TrayIcon : public QSystemTrayIcon
{
    Q_OBJECT
public:
    explicit TrayIcon(QObject *parent = 0) : QSystemTrayIcon(parent){}

signals:
    void triggered();

public slots:
    void trayIconActivated(QSystemTrayIcon::ActivationReason reason) {
        std::cout << "activated" << std::endl;
        if (reason == QSystemTrayIcon::Trigger) {
            std::cout << "tiggered" << std::endl;
            emit triggered();
        }
    }
};

#endif // TRAYICON_H

score 0 · Answer 2 · answered May 10 '16 at 18:54

0

In Qt5 you can connect to an anonymous function, by the way.

QObject::connect(trayIcon
    , SIGNAL(activated(QSystemTrayIcon::ActivationReason))
    , [=](QSystemTrayIcon::ActivationReason r){
              rootObject->trayIconClicked(r);
          });

answered May 10 '16 at 18:54

bipll

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How to emit an arbitrary signal in rootObject? – Sassan Aug 15 '16 at 02:09
Do you mean how can an arbitrary signal from an object be emitted from inside a method of rootObject? You can connect a rootObject's signal to that signal and the emit the latter. – bipll Aug 15 '16 at 21:55

How connect a signal from C++ to QML function with parameter

2 Answers2