toOptionsJSON method
Implementation
@override
void toOptionsJSON(StringBuffer buffer) {
  super.toOptionsJSON(buffer);
  if (color != null) {
    buffer.writeAll(['"color":', jsonEncode(color), ','], '');
  }
  if (size != null) {
    buffer.writeAll(['"size":', size, ','], '');
  }
  if (visible != null) {
    buffer.writeAll(['"visible":', jsonEncode(visible), ','], '');
  }
}