Widget _openPopupMenu() => PopupMenuButton<int>(
      itemBuilder: (context) => [
            PopupMenuItem(
              value: 1,
              child: Text("Present time"),
            ),
            PopupMenuItem(
              value: 2,
              child: Text("Budapest 1884"),
            ),
          ],
      icon: Container(
        height: double.infinity,
        width: double.infinity,
        child: Center(
          child: Icon(
            Icons.calendar_today_rounded,
            color: Colors.white,
            size: 24.0,
          ),
        ),
        decoration: new BoxDecoration(
          color: Colors.blue,
          shape: BoxShape.circle,
        ),
      ),
  );