satr

  GlobalKey<ScaffoldState> scaffoldKey = new GlobalKey();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: scaffoldKey,
...
      scaffoldKey.currentState.showSnackBar(SnackBar(content: Text("some text")));

satr

var text = "some text";
final snackBar = SnackBar(
  content: Text("Snack: $text"),
  action: SnackBarAction(
    label: 'Undo',
    onPressed: () {
      // Some code to undo the change!
    },
  ),
);

satr

<Window x:Class="WpfApp1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <Style TargetType="ComboBox">
            <Setter Property="Template">