getObj<T>  static method 
Converts JSON string source to object.
Implementation
static T? getObj<T>(String source, T f(Map v)) {
  if (source == null || source.isEmpty) return null;
  try {
    Map map = json.decode(source);
    return map == null ? null : f(map);
  } catch (e) {
    print('JsonUtil convert error, Exception:${e.toString()}');
  }
  return null;
}