What AnimationSystem can advance: a Tween, a Timeline, or an
AnimationMixer.
Structural rather than a union of the three classes, for the same reason
TimelineChild is: the system states its actual requirements — something that
moves by a delta in seconds and can say whether it is done — so a future
player satisfies it without this file importing it, and a test can drive it
with a three-line stub.
What AnimationSystem can advance: a
Tween, aTimeline, or anAnimationMixer.Structural rather than a union of the three classes, for the same reason
TimelineChildis: the system states its actual requirements — something that moves by a delta in seconds and can say whether it is done — so a future player satisfies it without this file importing it, and a test can drive it with a three-line stub.