Passing a GObjectClass into a function (with GooCanvas2)

[Zakariyya Mughal <zaki mughal gmail com>]

Hello folks,

I am trying to use a function that takes a GObjectClass as an argument
and I'm not sure how to get that using Glib::Object::Introspection.

In particular, I am trying to use the class method
`goo_canvas_item_class_list_child_properties()`
<https://developer.gnome.org/goocanvas2/stable/GooCanvasItem.html#goo-canvas-item-class-list-child-properties>.

It doesn't work if I pass it an instance:

    0:main
    $ GooCanvas2::CanvasItem::class_list_child_properties( GooCanvas2::CanvasTable->new )
    (process:23169): GooCanvas-CRITICAL **: goo_canvas_item_class_list_child_properties: assertion 
'G_IS_OBJECT_CLASS (iclass)' failed
    []

Or if I pass it the type name of the instance class or the derived GObjectClass

    0:main
    $ GooCanvas2::CanvasItem::class_list_child_properties( 'GooCanvas2::CanvasTable' )
    Runtime error: need a hash ref to convert to struct of type ObjectClass at (eval 438) line 5.
    0:main
    $ GooCanvas2::CanvasItem::class_list_child_properties( 'GooCanvas2::CanvasTableClass' )
    Runtime error: need a hash ref to convert to struct of type ObjectClass at (eval 440) line 5.

I noticed that the Python GIR bindings are able to work by doing

    In [2]: from gi.repository import GooCanvas; GooCanvas.CanvasItem.class_list_child_properties( 
GooCanvas.CanvasTable )
    Out[2]:
    [<GParamDouble 'left-padding'>,
     <GParamDouble 'right-padding'>,
     <GParamDouble 'top-padding'>,
     <GParamDouble 'bottom-padding'>,
     <GParamDouble 'x-align'>,
     <GParamDouble 'y-align'>,
     <GParamUInt 'row'>,
     <GParamUInt 'column'>,
     <GParamUInt 'rows'>,
     <GParamUInt 'columns'>,
     <GParamBoolean 'x-expand'>,
     <GParamBoolean 'x-fill'>,
     <GParamBoolean 'x-shrink'>,
     <GParamBoolean 'y-expand'>,
     <GParamBoolean 'y-fill'>,
     <GParamBoolean 'y-shrink'>]


I noticed that Gtk3.pm has overrides for a similar class method in
`Gtk3::ContainerClass::list_child_properties` 
<https://developer.gnome.org/gtk3/stable/GtkContainer.html#gtk-container-class-list-child-properties>,
<https://metacpan.org/source/XAOC/Gtk3-0.032/lib/Gtk3.pm#L993>,
but that works fine with the name of the container class.

    0:main
    $ 0 + @{ Gtk3::ContainerClass::list_child_properties( 'Gtk3::Table' ) }
    8

In case it helps, I noticed that that GIR XML differs in that
`gtk_container_class_list_child_properties` specifies a <method> but
`goo_canvas_item_class_list_child_properties` specifies a <function>.

Regards,
- Zaki Mughal