手势捕获的异常：处理手势时引发以下 NoSuchMethodError：

Question

I'm a newbie to programming and Flutter.

On my app, I've built a page navigator with a int variable. Upon change in the integer value, I would like the app to route the user to the specific designated page. However, I'm getting an 'anonymous closure' error onPressed.

Would appreciate if someone could explain why I'm getting this error and post a rectified version of this code?

Below is the full error snippet in the debugger.

Reloaded 1 of 529 libraries in 885ms.
flutter: Page #1 Clicked

════════ Exception caught by gesture ═══════════════════════════════════════════
The following NoSuchMethodError was thrown while handling a gesture:
The method 'call' was called on null.
Receiver: null
Tried calling: call()

When the exception was thrown, this was the stack
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1      _MyHomePageState.build.<anonymous closure>
package:navigator_test/main.dart:45
#2      _InkResponseState._handleTap
package:flutter/…/material/ink_well.dart:993
#3      _InkResponseState.build.<anonymous closure>
package:flutter/…/material/ink_well.dart:1111
#4      GestureRecognizer.invokeCallback
package:flutter/…/gestures/recognizer.dart:183
...
Handler: "onTap"
Recognizer: TapGestureRecognizer#5df0c
    debugOwner: GestureDetector
    state: possible
    won arena
    finalPosition: Offset(208.7, 509.7)
    finalLocalPosition: Offset(76.7, 19.7)
    button: 1
    sent tap down
════════════════════════════════════════════════════════════════════════════════

Below is the depiction of the Navigator

import 'package:flutter/material.dart';
import 'package:navigator_test/main.dart';
import 'package:navigator_test/page1.dart';

class Navigator extends StatefulWidget {
  @override
  _NavigatorState createState() => _NavigatorState();
}

class _NavigatorState extends State<Navigator> {
  int nav = 0;

  void page1() {
    setState(() {
      nav = 1;
    });
  }

  @override
  Widget build(BuildContext context) {
    if (nav < 1) {
      return MyHomePage(page1: page1);
    } else if (nav < 2) {
      return Page1();
    } else {
      return null;
    }
  }
}

I'm importing the Function on the Main page.

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final Function page1;
  MyHomePage({this.page1});

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Navigation Test'),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            SizedBox(height: 20),
            FlatButton.icon(
              onPressed: () {
                print('Page #1 Clicked');
                widget.page1();
              },
              icon: Icon(Icons.add_shopping_cart),
              label: Text('Goto Page #1'),
            )
          ],
        ),
      ),
      // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

Answer 1

You're getting this error because you're calling MyHomePage directly in MyApp, without passing the argument page1, so when you press 'Goto Page #1' a null function is called. To solve the problem you can instead just call Navigator in MyApp :

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: Navigator(),
    );
  }
}

But the main way to navigate to a new page without needing to call setState is to use the Navigator class of Flutter, like this (Just remember to remove your Navigator class first, so the IDE will know that you're calling flutter's Navigator class) :

onPressed: () {
  print('Page #1 Clicked');
  //push the new route, but allowing you to go back to the previous one
  Navigator.of(context).push(

    //Default page transition of android
    MaterialPageRoute(
      builder: (context) => Page1(),
    ),
  );
},

Or:

Navigator.of(context).push(
  //Default page transition of IOS
  CupertinoPageRoute(
    builder: (context) => Page1(),
  ),
);

For more info see the docs .