double stepWidth

If non-null, force the child's width to be a multiple of this value.

Source 

double get stepWidth => _stepWidth;
void stepWidth=(double newStepWidth)

Source 

set stepWidth(double newStepWidth) {
  if (newStepWidth == _stepWidth)
    return;
  _stepWidth = newStepWidth;
  markNeedsLayout();
}