未定义的名称“上下文”（在 Flutter 导航中）

Question

所以我尝试使用 Navigator.push（不使用 NamedRoutes）从一个屏幕导航到另一个屏幕。但突然我收到了这个未定义名称“上下文”的错误。我想从“loading_screen.dart”导航到“context_screen.dart”。我不明白为什么我会遇到这个错误。

loading_screen.dart

class LoadingScreen extends StatefulWidget {

  String searchTerm;
  LoadingScreen({this.searchTerm});

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


class _LoadingScreenState extends State<LoadingScreen> {

  @override
  void initState() {
    print("On Loading Screen");
    String searchTerm = widget.searchTerm;
    getMeaning(searchTerm);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: SpinKitWave(
          color: kMainBlueColor,
          size: 70.0,
        ),
      ),
    );
  }
}

void getMeaning(String searchTerm) async {
  print(searchTerm);
  // TODO: get the meaning of the search term.
  // TODO: Test without net and see background loading
  var searchContext;
  // var searchContext = await .....
  Navigator.push(context, MaterialPageRoute(builder: (context) {
    return ContextScreen(searchContext: searchContext);
  }));
}

context_screen.dart

class ContextScreen extends StatefulWidget {

  final searchContext;
  ContextScreen({this.searchContext});

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

class _ContextScreenState extends State<ContextScreen> {
  @override
  Widget build(BuildContext context) {
    var searchContext = widget.searchContext;
    return Scaffold(
      body: SafeArea(
        child: Column(
          children: <Widget>[
            Align(
              alignment: Alignment.topLeft,
              child: FlatButton(
                onPressed: () {
                  int count = 0;
                  Navigator.of(context).popUntil((context) => count++ >= 2);
                },
                child: Icon(
                  Icons.arrow_back_ios,
                  color: kMainBlueColor,
                  size: 50.0,
                ),
              ),
            ),
            Center(
              child: Text(
                searchContext
              ),
            )
          ],
        ),
      )
    );
  }
}

我忽略了上面的 import 语句，但我已将它们添加到我的代码中。任何帮助，将不胜感激。提前致谢。

Answer 1

错误是因为您在initState 中调用了一个使用上下文的方法。context 仅在完成initState 后可用。

所以你可以调用didChangeDependencies中的方法，在initState之后立即调用。

了解更多didChangeDependencies：https://api.flutter.dev/flutter/widgets/State/didChangeDependencies.html

了解更多 StatefulWidget 生命周期：https://medium.com/flutter-community/widget-state-buildcontext-inheritedwidget-898d671b7956

Answer 2

您可以在下面复制粘贴运行完整代码
您可以将getMeaning 移动到_LoadingScreenState 并使用addPostFrameCallback

代码sn-p

class _LoadingScreenState extends State<LoadingScreen> {
  void getMeaning(String searchTerm) async {
    print(searchTerm);
    // TODO: get the meaning of the search term.
    // TODO: Test without net and see background loading
    var searchContext = "";
    // var searchContext = await .....
    await Future.delayed(Duration(seconds: 5), () {
    });

    Navigator.push(context, MaterialPageRoute(builder: (context) {
      return ContextScreen(searchContext: searchContext);
    }));
  }

  @override
  void initState() {
    print("On Loading Screen");
    String searchTerm = widget.searchTerm;

    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_) {
      getMeaning(searchTerm);
    });
  }

工作演示

完整代码

import 'package:flutter/material.dart';
import 'package:flutter_spinkit/flutter_spinkit.dart';

class LoadingScreen extends StatefulWidget {
  String searchTerm;
  LoadingScreen({this.searchTerm});

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

class _LoadingScreenState extends State<LoadingScreen> {
  void getMeaning(String searchTerm) async {
    print(searchTerm);
    // TODO: get the meaning of the search term.
    // TODO: Test without net and see background loading
    var searchContext = "";
    // var searchContext = await .....
    await Future.delayed(Duration(seconds: 5), () {
    });

    Navigator.push(context, MaterialPageRoute(builder: (context) {
      return ContextScreen(searchContext: searchContext);
    }));
  }

  @override
  void initState() {
    print("On Loading Screen");
    String searchTerm = widget.searchTerm;

    super.initState();
    WidgetsBinding.instance.addPostFrameCallback((_) {
      getMeaning(searchTerm);
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: SpinKitWave(
          color: Colors.blue,
          size: 70.0,
        ),
      ),
    );
  }
}

class ContextScreen extends StatefulWidget {
  final searchContext;
  ContextScreen({this.searchContext});

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

class _ContextScreenState extends State<ContextScreen> {
  @override
  Widget build(BuildContext context) {
    var searchContext = widget.searchContext;
    return Scaffold(
        body: SafeArea(
      child: Column(
        children: <Widget>[
          Align(
            alignment: Alignment.topLeft,
            child: FlatButton(
              onPressed: () {
                int count = 0;
                Navigator.of(context).popUntil((context) => count++ >= 2);
              },
              child: Icon(
                Icons.arrow_back_ios,
                color: Colors.blue,
                size: 50.0,
              ),
            ),
          ),
          Center(
            child: Text(searchContext),
          )
        ],
      ),
    ));
  }
}

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: LoadingScreen(searchTerm: "test"),
    );
  }
}