Ubuntu – the correct way to use g_signal_connect() in C++ for dynamic unity quicklists

application-developmentcdbusgtkqt

I want to make my application use dynamic unity quicklists. For building my application I am using C++ and the QtCreator IDE. When a menu action is triggered I want to be able to have access to a non-static function of my MainWindow class so as to be able to update the Graphical User Interface which can be accessed from inside 'normal' MainWindow's functions.

So, I am building up my quicklist like this (mainwindow.cpp):

void MainWindow::enable_unity_quicklist(){
    Unity_Menu = dbusmenu_menuitem_new();

    dbusmenu_menuitem_property_set_bool (Unity_Menu, DBUSMENU_MENUITEM_PROP_VISIBLE, FALSE);

    Unity_Stop = dbusmenu_menuitem_new();
    dbusmenu_menuitem_property_set(Unity_Stop, DBUSMENU_MENUITEM_PROP_LABEL, "Stop");

    dbusmenu_menuitem_child_append (Unity_Menu, Unity_Stop);

    g_signal_connect (Unity_Stop, DBUSMENU_MENUITEM_SIGNAL_ITEM_ACTIVATED, G_CALLBACK(&fake_callback), (gpointer)this);

    if(!unity_entry)
        unity_entry = unity_launcher_entry_get_for_desktop_id("myapp.desktop");

    unity_launcher_entry_set_quicklist(unity_entry, Unity_Menu);

    dbusmenu_menuitem_property_set_bool(Unity_Menu, DBUSMENU_MENUITEM_PROP_VISIBLE, true);
    dbusmenu_menuitem_property_set_bool(Unity_Stop, DBUSMENU_MENUITEM_PROP_VISIBLE, true);
}

void MainWindow::fake_callback(gpointer data){
    MainWindow* m = (MainWindow*)data;
    m->on_stopButton_clicked();
}

void MainWindow::on_stopButton_clicked(){
   //stopping the process...
}

mainwindow.h:

private slots:
   void enable_unity_quicklist();
   void on_stopButton_clicked();
public slots:
   static void fake_callback(gpointer data);

This suggestion was taken from http://old.nabble.com/Using-g_signal_connect-in-class-td18461823.html

The program crashes immediately after I choose the 'Stop' action from the Unity Quicklist. Debugging the program shows that I am not able to access anything MainWindow related inside the on_stopButton_clicked() without crashing. For example, it crashes when doing this check (which is the first 2 lines of code inside this function):

if (!ui->stopButton->isEnabled())
        return;

I have also tested lots of other things that I found at the internet, but nothing of them worked. One interesting solution would be to use gtkmm (http://developer.gnome.org/gtkmm-tutorial/stable/sec-connecting-signal-handlers.html.en) but I am not used at all working on GTK applications (I work solely in Qt) and I don't know if this even suits to my occasion.

Best Answer

Edit: okay, after a fair amount of investigation (which involved running valgrind), I have finally managed to figure out what's going on here. Your callback's signature isn't correct.

The signature for the static method should be:

void MainWindow::fake_callback(DbusmenuMenuitem *, guint, gpointer data)
{
    //...
}

Notice the addition of DbusmenuMenuitem * and guint. The first is a pointer to the DBus menu item emitting the signal and the second is a timestamp. Both must be present in the signature even if you don't use them.

g_signal_connect is invoked at runtime and has no way of knowing what parameters your function accepts and blindly just pushes the parameters on the stack assuming that the signatures match. (This differs from what you are probably used to in Qt where the MOC (Meta Object Compiler) generates enough information to know when you are trying to connect a signal to a slot with an incompatible signature.)

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