Flutter: show snackbar once after navigation

main.dart

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  void goSecond(){
    Navigator.of(context).push(MaterialPageRoute(builder: (context) => SecondPage()));
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: RaisedButton(
          onPressed: goSecond,
          child: Text("go next"),
        ),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}


class SecondPage extends StatefulWidget{
  @override
  State<StatefulWidget> createState() => SecondPageState();
}

class SecondPageState extends State<SecondPage> {
  ScaffoldState scaffold;

  @override
  void initState(){
    super.initState();

    WidgetsBinding.instance
        .addPostFrameCallback((_) => showSnackBar());
  }

  void showSnackBar(){
    scaffold.showSnackBar(SnackBar(content: Text("snackbar")));
  }

  void goThird(){
    Navigator.of(context).push(MaterialPageRoute(builder: (context) => ThirdPage()));
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Second"),
      ),
      body: Builder(
        builder: (BuildContext buildContext) {
          scaffold = Scaffold.of(buildContext);
          return Center(
              child: FlatButton(onPressed: goThird, child: Text("go next"))
          );
        },
      ),
    );
  }
}


class ThirdPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Third"),
      ),
      body: Center(
          child: Text("third page")
      )
    );
  }
}

Works for me! Thank you!

Thank you !!!