void _updatePosition() {
   _configuration = ScrollConfiguration.of(context);
   _physics = _configuration.getScrollPhysics(context);
   if (widget.physics != null)
     _physics = widget.physics.applyTo(_physics);
   final ScrollController controller = widget.controller;
   final ScrollPosition oldPosition = position;
   if (oldPosition != null) {
     controller?.detach(oldPosition);
     // It's important that we not dispose the old position until after the
     // viewport has had a chance to unregister its listeners from the old
     // position. So, schedule a microtask to do it.
     scheduleMicrotask(oldPosition.dispose);
   }

   _position = controller?.createScrollPosition(_physics, this, oldPosition)
     ?? ScrollPositionWithSingleContext(physics: _physics, context: this, oldPosition: oldPosition);
   assert(position != null);
   controller?.attach(position);
 }