how to open popup in flutter code example

Example 1: 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
      }

    });
}

Example 2: popup flutter

import 'package:flutter/material.dart';

void main() {
  runApp(new MaterialApp(home: new MyApp()));
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  final _formKey = GlobalKey<FormState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Flutter"),
      ),
      body: Center(
        child: RaisedButton(
          onPressed: () {
            showDialog(
                context: context,
                builder: (BuildContext context) {
                  return AlertDialog(
                    content: Stack(
                      overflow: Overflow.visible,
                      children: <Widget>[
                        Positioned(
                          right: -40.0,
                          top: -40.0,
                          child: InkResponse(
                            onTap: () {
                              Navigator.of(context).pop();
                            },
                            child: CircleAvatar(
                              child: Icon(Icons.close),
                              backgroundColor: Colors.red,
                            ),
                          ),
                        ),
                        Form(
                          key: _formKey,
                          child: Column(
                            mainAxisSize: MainAxisSize.min,
                            children: <Widget>[
                              Padding(
                                padding: EdgeInsets.all(8.0),
                                child: TextFormField(),
                              ),
                              Padding(
                                padding: EdgeInsets.all(8.0),
                                child: TextFormField(),
                              ),
                              Padding(
                                padding: const EdgeInsets.all(8.0),
                                child: RaisedButton(
                                  child: Text("Submitß"),
                                  onPressed: () {
                                    if (_formKey.currentState.validate()) {
                                      _formKey.currentState.save();
                                    }
                                  },
                                ),
                              )
                            ],
                          ),
                        ),
                      ],
                    ),
                  );
                });
          },
          child: Text("Open Popup"),
        ),
      ),
    );
  }
}