MessageSendingState.fromJson constructor
a MessageSendingState return type can be :
Implementation
factory MessageSendingState.fromJson(Map<String, dynamic> json)  {
  switch(json["@type"]) {
    case MessageSendingStatePending.CONSTRUCTOR:
      return MessageSendingStatePending.fromJson(json);
    case MessageSendingStateFailed.CONSTRUCTOR:
      return MessageSendingStateFailed.fromJson(json);
    default:
      return const MessageSendingState();
  }
}