range static method

FormeValidator range(double min, double max, { String errorText = '', })

when valid:

1. value is null
2. value >= min && value <= max

Implementation

static FormeValidator range(double min, double max, {String errorText = ''}) {
  return (f, dynamic v) =>
      (v == null || (v as num >= min && v <= max)) ? null : errorText;
}