all Gdk.Paintable interface members must be implemented although not mandatory
as per documentation in
The list of functions that can be implemented for the #GdkPaintable interface. Note that apart from the first function, no function is mandatory to implement, though it is a good idea to implement GdkPaintable:get_current_image() for non-static paintables and GdkPaintable:get_flags() if the image is not dynamic as the default implementation returns no flags and that will make the implementation likely quite slow.
in vala it complaints if I don't implement all interface methods! Could you please give me an insight in this problem.