Inheriting interface in Vala - incompatible with base method - c

Question

I'm trying to implement a Gtk.StyleProvider in Vala. The "base class" (in C) looks like:
GtkIconFactory * gtk_style_provider_get_icon_factory ()
GtkStyleProperties * gtk_style_provider_get_style ()
gboolean gtk_style_provider_get_style_property ()
and in VAPI:
[CCode (cheader_filename = "gtk/gtk.h")]
public interface StyleProvider {
public abstract unowned Gtk.IconFactory get_icon_factory (Gtk.WidgetPath path);
public abstract unowned Gtk.StyleProperties get_style (Gtk.WidgetPath path);
public abstract bool get_style_property (Gtk.WidgetPath path, Gtk.StateFlags state, GLib.ParamSpec pspec, GLib.Value value);
}
Where the first two methods should only return NULL according to the documentation for GtkStyleProvider.
Thus, I wrote some Vala like this:
public class DerivedStyleProvider : Gtk.StyleProvider
{
public Gtk.IconFactory? get_icon_factory (Gtk.WidgetPath path)
{
return null;
}
public Gtk.StyleProperties? get_style (Gtk.WidgetPath path)
{
return null;
}
bool get_style_property (Gtk.WidgetPath path,
Gtk.StateFlags state,
GLib.ParamSpec pspec,
out GLib.Value value)
{
return false; //TODO
}
}
I have a problem with the first two methods. If I have them as written here (with a ?), then I get the following error:
error: overriding method `DerivedStyleProvider.get_icon_factory' is incompatible
with base method `Gtk.StyleProvider.get_icon_factory': Base method expected
return type `Gtk.IconFactory', but `Gtk.IconFactory?' was provided.
public Gtk.IconFactory? get_icon_factory (Gtk.WidgetPath path)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The gtk_style_provider_get_style() method is the same.
If I remove the ?, I get the following two errors per method:
error: overriding method `DerivedsStyleProvider.get_icon_factory'
is incompatible with base method `Gtk.StyleProvider.get_icon_factory': Base
method expected return type `Gtk.IconFactory', but `Gtk.IconFactory' was provided.
public Gtk.IconFactory get_icon_factory (Gtk.WidgetPath path)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
src/Preferences.vala:138.3-138.14: warning: `null' incompatible with
return type `Gtk.IconFactory`
return null;
^^^^^^^^^^^
The first error especially is a bit strange to me, as the upshot is "error: expected TYPE, got TYPE"!
Adding unowned to the first two methods still results in similar errors.
How should I implement a Gtk.StyleProvider interface in Vala?

Answer 1

This compiles without errors or warnings on my system (Vala 0.32.1):
public class DerivedStyleProvider : GLib.Object, Gtk.StyleProvider
{
public unowned Gtk.IconFactory get_icon_factory (Gtk.WidgetPath path)
{
// Evil cast to work around buggy declaration in VAPI file
return (Gtk.IconFactory) null;
}
public Gtk.StyleProperties get_style (Gtk.WidgetPath path)
{
// Evil cast to work around buggy declaration in VAPI file
return (Gtk.StyleProperties) null;
}
bool get_style_property (Gtk.WidgetPath path,
Gtk.StateFlags state,
GLib.ParamSpec pspec,
out GLib.Value value)
{
// I just assigned something here to make the compiler happy, you should make sure to use a correct value
value = Value (typeof (string));
return false; //TODO
}
}
I made these changes:
Derive from GLib.Object in addition to the interface.
Use unowned on the first method.
Remove the nullable from the return types.
Cast null into the actual class types. (Which is not pretty, but the problem is with the vapi file.)
Assign a dummy value to the out parameter to make compiling warning free ;)