Flutter : Exception on using Navigator. of(context) 用于 RaisedButton 的 onPressed 事件

Question

我遇到了这个异常

Exception has occurred. NoSuchMethodError (NoSuchMethodError: The method 'ancestorStateOfType' was called on null. Receiver: null Tried calling: ancestorStateOfType(Instance of 'TypeMatcher'))

关于使用我为 onPressed 函数传递的自定义方法 (navigateToLogin)，对于 RaisedButton，但是，当我直接在 onPressed = (){ ...Navigation code..} 上编写相同的导航时，它工作正常。是因为它没有得到上下文吗？

import 'package:credit/src/pages/credit/home.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:flutter/gestures.dart';
import 'dart:convert';

class LandingPage extends StatelessWidget {
  BuildContext context;

  @override
  Widget build(BuildContext context) {
    //return HomePage();  // Un coment this method for Homepage
    String loginTxt = "Login";

    // Set login value on click of Login button
    setLoginValue() async {
      SharedPreferences loginCheck = await SharedPreferences.getInstance();
      await loginCheck.setString("userstatus", "loggedin");
      print("Set login value");
      print(loginCheck.getString("userstatus"));
    }

    return WillPopScope(
        onWillPop: () {
          SystemNavigator.pop();
          return Future.value(false);
        },
        child: SafeArea(
            child: MaterialApp(
                debugShowCheckedModeBanner: false,
                home: Scaffold(
                    appBar: PreferredSize(
                      preferredSize: Size.fromHeight(35),
                      child: AppBar(
                        backgroundColor: Colors.lightBlueAccent,
                        title: Text("Select Language"),
                        automaticallyImplyLeading: false,
                        primary: false,
                      ),
                    ),
                    bottomNavigationBar: BottomAppBar(
                      child: RaisedButton(
                        onPressed: navigateToLogin(),
                        child: Row(
                          mainAxisAlignment: MainAxisAlignment.center,
                          children: <Widget>[
                            Container(
                              child: new Text("Continue",
                                  style: TextStyle(fontSize: 20)),
                            ),
                            Container(
                              child: new Icon(Icons.navigate_next),
                            )
                          ],
                        ),
                        padding: EdgeInsets.fromLTRB(0, 20, 0, 20),
                        color: Colors.lightBlueAccent,
                        splashColor: Colors.orangeAccent,
                      ),
                    ),
                    body: Column(
                      children: <Widget>[],
                    )))));
  }

  navigateToLogin() {
    Navigator.of(context)
        .push(MaterialPageRoute<Null>(builder: (BuildContext context) {
      return HomePage();
    }));
  }
}

Answer 1

是的，如果你想让它工作，你必须将 context 作为参数传递:

navigateToLogin(BuildContext context) {
    Navigator.of(context)
        .push(MaterialPageRoute<Null>(builder: (BuildContext context) {
      return HomePage();
    }));
  }

当你调用它时:

onPressed: () => navigateToLogin(context);