Dart 如何在Flutter中正确显示Snackbar

我正在试着在点击floatingActionbutton的时候显示Snackbar分.但是当我点击floatingactionbutton的时候，它没有显示任何东西.这是我的代码.我用的是StatefulWidget.我调试并判断了onPressed函数也在执行，但不知何故看不到Snackbar.问题的根本原因是什么？我觉得我正在传递的BuildContext有一些问题.

class MyApp extends StatefulWidget{
  @override
  MyAppState createState() {
    // TODO: implement createState
    return new MyAppState();
  }

}

class MyAppState extends State<MyApp>{
  File _image;
  String _text;

  Future getImage() async {
    var image = await ImagePicker.pickImage(source: ImageSource.camera);
    _image = image;
    final FirebaseVisionImage visionImage = FirebaseVisionImage.fromFile(_image);
    final TextRecognizer textRecognizer = FirebaseVision.instance.textRecognizer();
    final VisionText visionText = await textRecognizer.processImage(visionImage);

    String detectedText = visionText.text;

    setState(() {
      _image = image;
      _text = detectedText;
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: new AppBar(
          title: new Text('Image Picker Example'),
        ),
        body: new Center(
          child: _image == null
              ? new Text('No image selected.')
              : new Image.file(_image),
        ),
        floatingActionButton: new FloatingActionButton(
          onPressed: (){
            showSnackBar(context);
//            getImage();
          },
          tooltip: 'Pick Image',
          child: new Icon(Icons.add_a_photo),
        ),
      ),
    );
  }

  void showSnackBar(BuildContext context) {
    final scaffold = Scaffold.of(context);
    final snackBarContent = SnackBar(
      content: Text("sagar"),
      action: SnackBarAction(
          label: 'UNDO', onPressed: scaffold.hideCurrentSnackBar),
    );
    scaffold.showSnackBar(snackBarContent);
  }

}