Fix build failing when defining signals with Gdk.ScrollEvent

[ryonakano]

Followup of #295

I suspect this is a bug of Vala. It tries to call gdk_value_get_event() which does not exist and fails to build with the following error message:

../src/Widgets/DisplayWidget.vala: In function ‘g_cclosure_user_marshal_VOID__GDK_EVENT’:
../src/Widgets/DisplayWidget.vala:18:19: error: implicit declaration of function ‘gdk_value_get_event’; did you mean ‘g_value_get_uint’? [-Wimplicit-function-declaration]
   18 | public class Sound.DisplayWidget : Gtk.Box {
      |                   ^~~~~~~~~~~~~~~~~~~
      |                   g_value_get_uint
../src/Widgets/DisplayWidget.vala:18:19: error: passing argument 2 of ‘callback’ makes pointer from integer without a cast [-Wint-conversion]
   18 | public class Sound.DisplayWidget : Gtk.Box {
      |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~
      |                   |
      |                   int

This commit resolves the above build error by wrapping Gdk.ScrollEvent with GLib.Value.

[ryonakano]

@danirabbit Would you test if this workaround actually works as expected? I'm not familiar with how to use GTK4 version of Wingpanel and its indicators.

[danirabbit]

Hm I'm not getting scroll events with this branch. But that could be some issue elsewhere. Will keep investigating. Thanks for this fix though!

[ryonakano]

I suspect this is a bug of Vala.

Opened https://gitlab.gnome.org/GNOME/vala/-/work_items/1671

[ryonakano]

@danirabbit

Hm I'm not getting scroll events with this branch.

The scrolling now works as expected at least on my side. Would you check if it works for you too?

Screencast.From.2026-07-28.22-08-16.mp4

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Leaving what I investigated for future reference.

I saw the following warnings in the journal when things didn't work in 493b7b7:

Jul 28 21:54:18 elementary-8-daily io.elementary.w[12494]: g_value_set_boxed: assertion 'G_VALUE_HOLDS_BOXED (value)' failed
Jul 28 21:54:18 elementary-8-daily io.elementary.w[12494]: g_value_get_boxed: assertion 'G_VALUE_HOLDS_BOXED (value)' failed
Jul 28 21:54:18 elementary-8-daily io.elementary.w[12494]: sound_indicator_handle_scroll_event: assertion 'e != NULL' failed

I susupect Gdk.Event can't be used as a boxed type because it's not defined with G_DEFINE_BOXED_TYPE as in https://docs.gtk.org/gobject/gvalue.html.

---

Then I found solution when I was looking at the code of GDK and GLib as a reference.

GdkEvent is derived from GTypeInstance in C:
https://github.com/GNOME/gtk/blob/7d6dc9151d04c094952b8defb0b2839e06825558/gdk/gdkeventsprivate.h#L50-L52

and I found a test class TestFundamentalObject that is also derived from GTypeInstance in a test code of glib:
https://github.com/GNOME/glib/blob/90b0a7e9def3f12a44e5ea8fdecbd912b3ca5189/gobject/tests/signals.c#L180-L189

It uses g_value_set_instance() for a setter function and g_value_peek_pointer() for a getter function to handle TestFundamentalObject in signals:
https://github.com/GNOME/glib/blob/90b0a7e9def3f12a44e5ea8fdecbd912b3ca5189/gobject/tests/signals.c#L1111-L1119

Using these methods in 6dabd76 resolves our issue!

[danirabbit]

Works as expected! Thanks for researching this ❤️