variable method
Implementation
@override
Future<DVCVariable> variable(String key, dynamic defaultValue) async {
final result = await methodChannel.invokeMethod(
'variable',
{"key": key, "defaultValue": defaultValue}
) ?? {};
final map = Map<String, dynamic>.from(result);
return map.isNotEmpty
? DVCVariable.fromCodec(map)
: DVCVariable.fromDefault(key, defaultValue);
}