How to create trigger a function when key is pressed in gtkmm?

Question

In the keyboard events section in GTKMM4 official docs there are examples on how to create a program that triggers when using only the enter button is pressed I did try to write my own but it works only if i press alt or ctrl and enter

#include <gtkmm.h>
#include <stdio.h>

class App: public Gtk::Window{
    private:
        bool on_window_key_pressed(guint keyval, guint, Gdk::ModifierType state){
            if (keyval == GDK_KEY_Return)
                printf("Hello there\n");
            return false;
        }
    protected:
        Gtk::Box box;
        Gtk::Text text;
    public:
        App(){
                set_child(box);
                box.append(text);
                text.set_text("keyboard events");
                auto controller = Gtk::EventControllerKey::create();
              controller->signal_key_pressed().connect(sigc::mem_fun(*this, &App::on_window_key_pressed), false);
              add_controller(controller);
        }
};

int main(int argc, char * argv[])
{
    auto app = Gtk::Application::create("org.example.github.basic");
    app->make_window_and_run<App>(argc, argv);
    return 0;
}

Answer 1

Your issue is generated by the textbox. That widget is gaining the focus automatically and with events (signals) there are details to keep in mind.

All you have to do to fix that example is adding this line:

...
box.append(text);

text.set_text("keyboard events");
text.set_can_focus(false); // <== this one

auto controller = Gtk::EventControllerKey::create();
....

But then you should deal with when getting the focus. This sample would have worked much better with using a Gtk::Label instead (less headaches).

I will past a link to the documentation just because it is a formal answer. However it is kind of a mystery that should be explained in the documentation. I shouldn't share the link just that, it is not fair. It says how signals are passed around when they're not completely done handling it, but it does not say anything about how a widget that get the focus automatically will override that behavior. I'm kind of surprised because I was looking for a behavior like this so the less I can say is thank you.