Question

我正在尝试点击Snackbar来显示floatingActionbutton。但是，当我单击floatingactionbutton时，它什么也没有显示。这是我的代码。我正在使用StatefulWidget。我调试并检查了onPressed函数是否也正在执行，但是以某种方式看不见Snackbar。造成此问题的根本原因是什么？我觉得我传递的BuildContext有问题。

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);
  }

}

Answer 1

要显示 SnackBar 你可以这样使用：

 ScaffoldMessenger.of(context).showSnackBar(SnackBar(
          content: Text('User Logged In'),
        ));

以前的 SnackBar 是这样使用的：

Scaffold.of(context).showSnackBar(SnackBar(
                content: Text('Rahul Kushwaha!'),
              ));

现在，SnackBar 由ScaffoldMessenger 管理。有关详细信息，请研究此 link

Answer 2

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

}

class MyAppState extends State<MyApp>{

  final GlobalKey<ScaffoldState> _scaffoldkey = new GlobalKey<ScaffoldState>();
  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(
        key: _scaffoldkey,
        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();
//            getImage();
          },
          tooltip: 'Pick Image',
          child: new Icon(Icons.add_a_photo),
        ),
      ),
    );
  }

  void showSnackBar() {
    final snackBarContent = SnackBar(
      content: Text("sagar"),
      action: SnackBarAction(
          label: 'UNDO', onPressed: _scaffoldkey.currentState.hideCurrentSnackBar),
    );
    _scaffoldkey.currentState.showSnackBar(snackBarContent);
  }



}

Answer 3

之所以会这样，是因为所使用的BuildContext没有Scaffold的祖先，因此无法找到它来渲染SnackBar，因为它取决于Scaffold显示它。

根据of method文档：

当在相同的构建函数中实际创建了脚手架时，构建函数的上下文参数不能用于查找脚手架（因为它在返回的小部件“上方”）。在这样的情况下，可以使用以下带有Builder的技术来提供 BuildContext位于“支架”下方的新作用域：
@override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: Text('Demo')
    ),
    body: Builder(
      // Create an inner BuildContext so that the onPressed methods
      // can refer to the Scaffold with Scaffold.of().
      builder: (BuildContext context) {
        return Center(
          child: RaisedButton(
            child: Text('SHOW A SNACKBAR'),
            onPressed: () {
              Scaffold.of(context).showSnackBar(SnackBar(
                content: Text('Hello!'),
              ));
            },
          ),
        );
      },
    ),
  );
}

解决方案

将FloatingActionButton包装在Builder小部件中，将比使用GlobalKey（@ Epizon答案已经提到）更加优雅。

小吃店在Flutter中不可见

3 个答案: