update method
Called once per frame while the component is mounted, enabled, and
loaded. deltaSeconds is the elapsed time since the previous tick.
A traversal visits each component at most once. Removing this component
or an earlier sibling is safe. A component inserted before the current
traversal position starts on the next frame. Reordering component or
child lists during traversal is unsupported.
Implementation
@override
void update(double deltaSeconds) {
if (!paused) system.step(deltaSeconds);
_repack();
}