The Animatable interface was created specifically for the Animation
class. It turns out that it might be fairly useful to others - such as
ClutterAnimator and ClutterState.
The newly-added API in this cycle for querying and accessing custom
properties should not require that we pass a ClutterAnimation to the
implementations: the Animatable itself should be enough.
This is necessary to allow language bindings to wrap
clutter_actor_animate() correctly and do type validation and
demarshalling between native values and GValues; an Animation instance
is not available until the animate() call returns, and validation must
be performed before that happens.
There is nothing we can do about the animate_property() virtual
function - but in that case we might want to be able to access the
animation from an Animatable implementation to get the Interval for
the property, just like ClutterActor does in order to animate
ClutterActorMeta objects.
The Animatable interface implementation will always have the computed
value applied, whilst the non-Animatable objects go through the
interval validation first to avoid incurring in assertions and
warnings.
The Animatable::animate_property() should also be able to validate the
property it's supposed to interpolate, and eventually discard it. This
requires adding a return value to the virtual function (and its wrapper
function).
The Animation code will then apply the computed value only if the
animate_property() returns TRUE -- unifying the code path with the
non-Animatable objects.
The ClutterAnimatable interface is meant to be used by GObject
classes to override the value computation for an animatable
property within the boundaries of an interval.
It is composed of a single virtual function, animate_property();
its implementation will receive the ClutterAnimation used to
animate the object; the property name; the initial and final
interval values; and the progress factor as retrieved by the
Alpha object bound to the Animation instance.
