compute method
Implementation
@override
T compute(TimelineAnimation<T> timeline, int index, double t) {
var value = this.value;
if (value == null) {
assert(
index > 0, 'Relative still keyframe must have a previous keyframe');
value = timeline.keyframes[index - 1].compute(timeline, index - 1, 1.0);
}
return value as T;
}