how to open a Popup Menu Entry code example

Example: open popupbutton onclick in flutter

_showPopupMenu(){
    showMenu<String>(
      context: context,
      position: RelativeRect.fromLTRB(25.0, 25.0, 0.0, 0.0),      //position where you want to show the menu on screen
      items: [
        PopupMenuItem<String>(
            child: const Text('menu option 1'), value: '1'),
        PopupMenuItem<String>(
            child: const Text('menu option 2'), value: '2'),
        PopupMenuItem<String>(
            child: const Text('menu option 3'), value: '3'),
      ],
      elevation: 8.0,
    )
    .then<void>((String itemSelected) {

      if (itemSelected == null) return;

      if(itemSelected == "1"){
        //code here
      }else if(itemSelected == "2"){
        //code here
      }else{
        //code here
      }

    });
}