public AddEntryDialogViewModel(DateTime selectedDayDate)
        {
            AllActivities    = DataBaseConnector.GetAllActivities();
            _selectedDayDate = selectedDayDate;

            MessengerInstance.Register <string>(this, ProcessMessage);
        }
 private void ProcessMessage(string message)
 {
     if (message == "Update")
     {
         AllActivities = DataBaseConnector.GetAllActivities();
     }
 }