如何在 Flutter 中正确显示 Snackbar？

Question

我试图在点击floatingActionbutton 时显示Snackbar。但是当我点击floatingactionbutton 时，它什么也没显示。这是我的代码。我正在使用StatefulWidget。我调试并检查了 onPressed 函数是否也在执行，但不知何故 Snackbar 不可见。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

发生这种情况是因为使用的BuildContext 没有Scaffold 祖先，因此无法找到它来呈现SnackBar，因为它取决于Scaffold 来显示它。

根据of method 文档：

当 Scaffold 实际在同一个构建函数中创建时， 构建函数的上下文参数不能用于查找 脚手架（因为它在被返回的小部件“上方”）。在这样的 在这种情况下，可以使用以下带有 Builder 的技术来提供 具有位于脚手架“下方”的 BuildContext 的新范围：

@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: () {
              ScaffoldMessenger.of(context).showSnackBar(SnackBar(
                content: Text('Hello!'),
              ));
            },
          ),
        );
      },
    ),
  );
}

解决方案

将您的 FloatingActionButton 包装在 Builder 小部件中将比使用 @Epizon 答案中已经提到的 GlobalKey 更优雅。

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

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 4

在最新版本中--

完整示例

import 'package:flutter/material.dart';


void test() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Test App',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: testPage(title: 'Flutter Demo Home Page'),
    );
  }
}

class testPage extends StatefulWidget {
  testPage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  _testPageState createState() => _testPageState();
}

class _testPageState extends State<testPage> {

  GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(

        home: SizedBox(
            child: Scaffold(

                key: _scaffoldKey,

                body: Center(
                    child: RaisedButton(
                      shape: RoundedRectangleBorder(
                        borderRadius: BorderRadius.circular(10.0),
                      ),
                      onPressed: () {

                        final snackBar = SnackBar(content: Text('Your Text'));

                        _scaffoldKey.currentState!.showSnackBar(snackBar);
                      },

                      child: Text("Show SB"),
                    )
                )
            )
        )
    );
  }
}


  [1]: https://i.stack.imgur.com/hKQYZ.jpg