IAnimatableObject

currentFrame runs from zero to totalFrames minus one: frame zero is start and the final frame is end. When the start and end states are identical, or when the totalFrames equals zero, the animation server is putting this object back into an original (not-animated) state.

Implementers must change the state of the object to match the interpolated animation states, but they should *not* record undo events or trigger new solutions.

An object with a finite number of states answers with how many of them lie between the two, so the animation visits each exactly once. An object of unlimited accuracy — an angle or a number line, say — answers with whatever sampling is natural over that range instead, since it is the one which knows what a sensible step through its own values looks like. Nothing else is in a position to guess.

It depends on the states because the range does: a slider covering a tenth of its domain has a tenth of the values to visit.

Both states are always supplied. A binding with no limits of its own passes the extremes this object reported from GetCurrentState(IObjectState, IObjectState), so there is never a missing state to defend against — only, as ever, the possibility of one belonging to some other object.