Listening for "owner change" events for the clipboard in gtk2hs. How to set up?

Using Gtk2Hs, I want to set up a callback to listen for "Owner change" events for the clipboard.

I can find out if this is supported by the X server by querying:

display <- fmap fromJust displayGetDefault
canBeNotified <- displayRequestSelectionNotification display selectionPrimary

It should be straight forward to do like this in C:

GtkClipboard* clipboard = gtk_clipboard_get(GDK_SELECTION_PRIMARY);
g_signal_connect(clipboard, "owner-change",
G_CALLBACK(handle_owner_change), NULL);

I see that callbacks in Gtk2Hs are set up like this:

<widget> `on` <event> $ <action>

Problems

I cannot find any OwnerChangeEvent. Graphics.UI.Gtk.Gdk.EventM does only provide functions of type EventM to query the owner change events.
on from System.Glib.Signals ~~needs an associated widget~~ takes any object and an event associated to that object (on :: object -> Signal object callback -> callback -> IO (ConnectId object). If I have understood X correctly, it should not be necessary to have a window to listen for this event.

Either I have misunderstood how to wrap this together, or I have reached upon a yet not implemented part of gtk in Gtk2Hs.

I therefore looked at the definition of keyPressEvent which looks like this:

keyPressEvent :: WidgetClass self => Signal self (EventM EKey Bool)
keyPressEvent = Signal (eventM "key_press_event" [KeyPressMask])

So, I tested by simply adding a definition for ownerChangeEvent in gtk like keyPressEvent, but subsituted "key_press_event" for "owner_change_event".

That did not work as I got a runtime error about not finding the event. Naturally.

Any ideas on how to properly listen for "owner change" events in gtk if possible?

Solution

You're right that this is not implemented at the moment. Adding it shouldn't be too involved, and might be a fun project if you're looking to get into gtk2hs development. You'll want to add a new constructor to the Event type, together with a descriptive type alias like type EventOwnerChange = Event. You will need to extend marshalEvent to handle this new kind of event, and there may be an unmarshalEvent hanging around though I can't remember.

Once that's all done, it should be pretty easy to connect to the appropriate signal using the connect_* family of functions (which are for gtk2hs internal use only and therefore are not exposed in the API). From the documentation for the owner-change event, you will probably need to use something like Signal (connect_OBJECT__NONE "owner-change").

It may also be necessary to add to the EventMask type, though I'm not confident -- test it out and see.