Navigator operation requested with a context that

问题:

I'm trying to start a new screen within an onTap but I get the following error:

Navigator operation requested with a context that does not include a Navigator.

The code I am using to navigate is:

onTap: () { Navigator.of(context).pushNamed('/settings'); },

I have set up a route in my app as follows:

routes: <String, WidgetBuilder>{
    '/settings': (BuildContext context) => new SettingsPage(),
},

I've tried to copy the code using the stocks sample application. I've looked at the Navigator and Route documentation and can't figure out how the context can be made to include a Navigator. The context being used in the onTap is referenced from the parameter passed into the build method:

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {

SettingsPage is a class as follows:

class SettingsPage extends Navigator {

Widget buildAppBar(BuildContext context) {
  return new AppBar(
    title: const Text('Settings')
  );
}

@override
Widget build(BuildContext context) {
  return new Scaffold(
    appBar: buildAppBar(context),
  );
 }
}

回答1:

I set up this simple example for routing in a flutter app:

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      home: new MyHomePage(),
      routes: <String, WidgetBuilder>{
        '/settings': (BuildContext context) => new SettingsPage(),
      },
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('TestProject'),
      ),
      body: new Center(
        child: new FlatButton(
          child: const Text('Go to Settings'),
          onPressed: () => Navigator.of(context).pushNamed('/settings')
        )
      )
    );
  }
}

class SettingsPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        appBar: new AppBar(
          title: new Text('SettingsPage'),
        ),
        body: new Center(
            child: new Text('Settings')
        )
    );
  }
}

Note, that the SettingsPage extends StatelessWidget and not Navigator. I'm not able to reproduce your error.

Does this example help you in building your app? Let me know if I can help you with anything else.

回答2:

TLDR: Wrap the widget which needs to access to Navigator into a Builder or extract that sub-tree into a class. And use the new BuildContext to access Navigator.

---

This error is unrelated to the destination. It happens because you used a context that doesn't contain a Navigator instance as parent.

How do I create a Navigator instance then ?

This is usually done by inserting in your widget tree a MaterialApp or WidgetApp. Although you can do it manually by using Navigator directly but less recommended. Then, all children of such widget can access NavigatorState using Navigator.of(context).

Wait, I already have a MaterialApp/WidgetApp !

That's most likely the case. But this error can still happens when you use a context that is a parent of MaterialApp/WidgetApp.

This happens because when you do Navigator.of(context), it will start from the widget associated to the context used. And then go upward in the widget tree until it either find a Navigator or there's no more widget.

In the first case, everything is fine. In the second, it throws a

Navigator operation requested with a context that does not include a Navigator.

So, how do I fix it ?

First, let's reproduce this error :

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Center(
        child: RaisedButton(
          child: Text("Foo"),
          onPressed: () => Navigator.pushNamed(context, "/"),
        ),
      ),
    );
  }
}

This example creates a button that attempts to go to '/' on click but will instead throw an exception.

Notice here that in the

  onPressed: () => Navigator.pushNamed(context, "/"),

we used context passed by to build of MyApp.

The problem is, MyApp is actually a parent of MaterialApp. As it's the widget who instantiate MaterialApp! Therefore MyApp's BuildContext doesn't have a MaterialApp as parent!

To solve this problem, we need to use a different context.

In this situation, the easiest solution is to introduce a new widget as child of MaterialApp. And then use that widget's context to do the Navigator call.

There are a few ways to achieve this. You can extract home into a custom class :

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHome()
    );
  }
}

class MyHome extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
        child: RaisedButton(
          child: Text("Foo"),
          onPressed: () => Navigator.pushNamed(context, "/"),
        ),
      );
  }
}

Or you can use Builder :

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Builder(
        builder: (context) => Center(
              child: RaisedButton(
                child: Text("Foo"),
                onPressed: () => Navigator.pushNamed(context, "/"),
              ),
            ),
      ),
    );
  }
}

回答3:

A complete and tested solution:

import 'dart:async';

import 'package:flutter/material.dart';
import 'package:my-app/view/main-view.dart';

class SplashView extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
        home: Builder(
          builder: (context) => new _SplashContent(),
        ),
        routes: <String, WidgetBuilder>{
          '/main': (BuildContext context) => new MainView()}
    );
  }
}

class _SplashContent extends StatefulWidget{

  @override
  _SplashContentState createState() => new _SplashContentState();
}

class _SplashContentState extends State<_SplashContent>
    with SingleTickerProviderStateMixin {

  var _iconAnimationController;
  var _iconAnimation;

  startTimeout() async {
    var duration = const Duration(seconds: 3);
    return new Timer(duration, handleTimeout);
  }

  void handleTimeout() {
    Navigator.pushReplacementNamed(context, "/main");
  }

  @override
  void initState() {
    super.initState();

    _iconAnimationController = new AnimationController(
        vsync: this, duration: new Duration(milliseconds: 2000));

    _iconAnimation = new CurvedAnimation(
        parent: _iconAnimationController, curve: Curves.easeIn);
    _iconAnimation.addListener(() => this.setState(() {}));

    _iconAnimationController.forward();

    startTimeout();
  }

  @override
  Widget build(BuildContext context) {
    return new Center(
        child: new Image(
          image: new AssetImage("images/logo.png"),
          width: _iconAnimation.value * 100,
          height: _iconAnimation.value * 100,
        )
    );
  }
}