forward method
Starts animating forward (toward upperBound).
Returns a Cmd that schedules the first animation frame tick.
The hosting State must return this Cmd from handleInit or
handleUpdate.
If from is provided the controller jumps to that value before
animating. The optional curve defaults to Curves.linear.
Implementation
Cmd forward({double? from, Curve curve = Curves.linear}) {
if (from != null) _value = from.clamp(lowerBound, upperBound);
_targetValue = upperBound;
_startValue = _value;
_curve = curve;
_status = AnimationStatus.forward;
_startTime = null;
_repeating = false;
_notifyStatusListeners();
return _scheduleTick();
}