不显示小吃店

Question

android studio 3.6

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        debugShowCheckedModeBanner: false,
        theme: new ThemeData(
            primaryColor: new Color(Constants.COLOR_PRIMARY),
            primaryTextTheme: TextTheme(headline6: TextStyle(color: Colors.white))),
        home: new SignInForm());
  }
}


class SignInForm extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    logger.d("createState:");
    return new _SignInFormState();
  }
}

class _SignInFormState extends State {
  final _formKey = GlobalKey<FormState>();
  final _scaffoldKey = GlobalKey<ScaffoldState>();
  String _textVersion = "";
  String _email = null;
  String _password = null;

@override
  Widget build(BuildContext context) {
    logger.d("build:");
    //String _errorMessage = null;
    return Scaffold(
        appBar: new AppBar(
            centerTitle: true,
            title: new Text('Sign in',
                style: TextStyle(fontWeight: FontWeight.bold))),
        body: new Container(
            margin: const EdgeInsets.only(
                left: Constants.DEFAULT_MARGIN,
                right: Constants.DEFAULT_MARGIN),
            child: new Form(
                key: _formKey,
                child: new Column(children: [
                  new TextFormField(
                      decoration: new InputDecoration(hintText: 'Email'),
                      keyboardType: TextInputType.emailAddress,
                      onChanged: (value) {
                        setState(() {
                          _email = value;
                        });
                      }),
                  new TextFormField(
                      decoration: new InputDecoration(hintText: 'Password'),
                      obscureText: true),
                  new Container(
                      margin: const EdgeInsets.only(
                          top: Constants.DEFAULT_MARGIN / 2),
                      height: Constants.MIN_HEIGHT,
                      child: new Row(
                          mainAxisAlignment: MainAxisAlignment.spaceBetween,
                          children: [
                            new Text("Forgot password?",
                                style: TextStyle(
                                    color: new Color(Constants.COLOR_PRIMARY))),
                            new Align(
                                alignment: Alignment.centerRight,
                                child: new RaisedButton(
                                    // Buttons are disabled by default
                                    child: Text('Sign in'.toUpperCase()),
                                    color: new Color(Constants.COLOR_PRIMARY),
                                    textColor: new Color(
                                        Constants.COLOR_PRIMARY_TEXT_COLOR),
                                    onPressed: () {
                                      if (_formKey.currentState.validate()) {
                                        logger.d(
                                            "onPressed: check_email = $_email");
                                        if (_email == null ||
                                            _email.trim().isEmpty) {
                                          logger.d(
                                              "onPressed: show_error_message");
                                          Scaffold.of(context).showSnackBar(
                                              SnackBar(
                                                  content: Text("Аll fields must be filled"),
                                                  backgroundColor: Colors.red));
                                        }
                                      }
                                    }))
                          ])),
                  new Container(
                      margin: const EdgeInsets.all(Constants.DEFAULT_MARGIN),
                      child: new Text('Registration'.toUpperCase(),
                          style: new TextStyle(
                              color: new Color(Constants.COLOR_PRIMARY),
                              fontWeight: FontWeight.bold))),
                  new Container(
                      margin: const EdgeInsets.all(Constants.DEFAULT_MARGIN),
                      child: new Text(_textVersion))
                ]))));
  }

按下按钮并得到错误提示 在这一行：

Scaffold.of(context).showSnackBar(

logcat：

The context used was: SignInForm
  state: _SignInFormState#fe66a
When the exception was thrown, this was the stack: 
#0      Scaffold.of (package:flutter/src/material/scaffold.dart:1456:5)
#1      _SignInFormState.build.<anonymous closure> (package:flutter_sample/signinform.dart:86:52)
#2      _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:779:14)
#3      _InkResponseState.build.<anonymous closure> (package:flutter/src/material/ink_well.dart:862:36)
#4      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
...
Handler: "onTap"
Recognizer: TapGestureRecognizer#74eb7
  debugOwner: GestureDetector
  state: possible
  won arena
  finalPosition: Offset(268.7, 208.0)
  finalLocalPosition: Offset(52.7, 18.0)
  button: 1
  sent tap down

Answer 1

发生此问题是因为当前传递的上下文没有任何匹配的祖先（在本例中为匹配的祖先支架）。

选项1：

解决此问题的最简单方法是在您的MyApp中，替换：

home: new SignInForm()

具有：

home: Scaffold(body: SignInForm())

选项2：

您必须指定脚手架的关键属性：

Scaffold(key: _scaffoldKey, body: // Your remaining code);

并使用以下方法显示小吃店

：

_scaffoldKey.currentState.showSnackBar(
     SnackBar(content: Text("Аll fields must be filled"),
              backgroundColor: Colors.red)
 );

选项3：

您可以使用Builder小部件来解决此问题：

Scaffold(body: Builder(builder: (context) {
    return Container(child:// Your widget);
    // No other change needed
},);

---

我个人更喜欢选项3。

在同一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“上方”，因此在小部件中返回了小部件 树）。

参考of method