How to show snackBar without Scaffold

If you have access to context, from laszlo's answer, just call

ScaffoldMessenger.of(context).showSnackBar(snackBar);

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If you dont have context (eg: showing network request error), use this snippet based on answer from Miguel Ruvio

We are going to use the property scaffoldMessengerKey which allows us to show snackbars without needing context with just one GlobalKey. Also, no need to add 3rd party packages like Get.

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First, create a globals.dart to store a GlobalKey

//globals.dart

import 'package:flutter/material.dart';

final GlobalKey<ScaffoldMessengerState> snackbarKey =
    GlobalKey<ScaffoldMessengerState>();

Second, add scaffoldMessengerKey property to MaterialApp

//main.dart

import 'globals.dart';

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Test',
      scaffoldMessengerKey: snackbarKey, // <= this
      ...

Finally, refer to this Key anywhere to show a Snackbar

// anywhere

import 'globals.dart';

final SnackBar snackBar = SnackBar(content: Text("your snackbar message"));
snackbarKey.currentState?.showSnackBar(snackBar); 

It can get the current context and show snackbar like this:

void _showToast(BuildContext context) {
  final scaffold = Scaffold.of(context);
  scaffold.showSnackBar(
    SnackBar(
      content: const Text('Updating..'),
    ),
  );
}

this._showToast(context);