Question

我试图创建一个用于扫描具有“ fast_qr_reader_view：^ 0.1.5”相关性的条形码的类。但是，当我在main.dart上调用此类时，出现此错误“ NoSuchMethodError：对该method []调用了null.Receiver：null。尝试调用：”

我已经尝试在main.dart上编写并且可以工作，但是当我从其他类调用它时不起作用

这是我的代码，向我显示错误main.dart

import 'package:flutter/material.dart';
import 'RegistroTrajetaOCedulaWidget.dart';
import 'package:pay_id/Screens/testScan.dart';
import 'Screens/dashboard.dart';
void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      debugShowCheckedModeBanner: false,
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

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

class _MyHomePageState extends State<MyHomePage> {

  @override
  Widget build(BuildContext context) {
    return Scan();
  }
}

testScan.dart

import 'package:flutter/material.dart';
import 'dart:async';
import 'package:fast_qr_reader_view/fast_qr_reader_view.dart';
import 'package:lamp/lamp.dart';

List<CameraDescription> cameras;

Future<Null> main() async {
  // Fetch the available cameras before initializing the app.
  try {
    cameras = await availableCameras();
  } on QRReaderException catch (e) {
    logError(e.code, e.description);
  }
  runApp(new Scan());
}

void logError(String code, String message) =>
    print('Error: $code\nError Message: $message');

class Scan extends StatefulWidget {
  @override
  _MyAppState createState() => new _MyAppState();
}

class _MyAppState extends State<Scan> with SingleTickerProviderStateMixin {
  QRReaderController controller;
  final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
  AnimationController animationController;

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

    animationController = new AnimationController(
      vsync: this,
      duration: new Duration(seconds: 3),
    );

    animationController.addListener(() {
      this.setState(() {});
    });
    animationController.forward();
    verticalPosition = Tween<double>(begin: 0.0, end: 300.0).animate(
        CurvedAnimation(parent: animationController, curve: Curves.linear))
      ..addStatusListener((state) {
        if (state == AnimationStatus.completed) {
          animationController.reverse();
        } else if (state == AnimationStatus.dismissed) {
          animationController.forward();
        }
      });

    // pick the first available camera
    onNewCameraSelected(cameras[0]);
  }

  Animation<double> verticalPosition;

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      key: _scaffoldKey,
      appBar: new AppBar(
        title: const Text('Prueba escaner'),
      ),
      floatingActionButton: FloatingActionButton(
        child: new Icon(Icons.check),
        onPressed: () {
          Lamp.turnOn();
        },
      ),
      body: Stack(
        children: <Widget>[
          new Container(
            child: new Padding(
              padding: const EdgeInsets.all(0.0),
              child: new Center(
                child: _cameraPreviewWidget(),
              ),
            ),
          ),
          Center(
            child: Stack(
              children: <Widget>[
                SizedBox(
                  height: 300.0,
                  width: 300.0,
                  child: Container(
                    decoration: BoxDecoration(
                        border: Border.all(color: Colors.red, width: 2.0)),
                  ),
                ),
                Positioned(
                  top: verticalPosition.value,
                  child: Container(
                    width: 300.0,
                    height: 2.0,
                    color: Colors.red,
                  ),
                )
              ],
            ),
          ),
        ],
      ),
    );
  }

  /// Display the preview from the camera (or a message if the preview is not available).
  Widget _cameraPreviewWidget() {
    if (controller == null || !controller.value.isInitialized) {
      return const Text(
        'No camera selected',
        style: const TextStyle(
          color: Colors.white,
          fontSize: 24.0,
          fontWeight: FontWeight.w900,
        ),
      );
    } else {
      return new AspectRatio(
        aspectRatio: controller.value.aspectRatio,
        child: new QRReaderPreview(controller),
      );
    }
  }

  void onCodeRead(dynamic value) {
    showInSnackBar(value.toString());
    // ... do something
    // wait 5 seconds then start scanning again.
    new Future.delayed(const Duration(seconds: 5), controller.startScanning);
  }

  void onNewCameraSelected(CameraDescription cameraDescription) async {
    if (controller != null) {
      await controller.dispose();
    }
    controller = new QRReaderController(cameraDescription,
        ResolutionPreset.high, [CodeFormat.qr, CodeFormat.pdf417], onCodeRead);

    // If the controller is updated then update the UI.
    controller.addListener(() {
      if (mounted) setState(() {});
      if (controller.value.hasError) {
        showInSnackBar('Camera error ${controller.value.errorDescription}');
      }
    });

    try {
      await controller.initialize();
    } on QRReaderException catch (e) {
      logError(e.code, e.description);
      showInSnackBar('Error: ${e.code}\n${e.description}');
    }

    if (mounted) {
      setState(() {});
      controller.startScanning();
    }
  }

  void showInSnackBar(String message) {
    _scaffoldKey.currentState
        .showSnackBar(new SnackBar(content: new Text(message)));
  }
}

Answer 1

相机未初始化
您可以直接在testScan.dart中初始化相机

_asyncMethod() async {
    cameras = await availableCameras();
    onNewCameraSelected(cameras[0]);
    print('camera ${cameras.length}');
  }

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

直接在testScan.dart中演示初始化相机

testScan.dart的完整测试演示

import 'package:flutter/material.dart';
import 'dart:async';
import 'package:fast_qr_reader_view/fast_qr_reader_view.dart';
import 'package:lamp/lamp.dart';

List<CameraDescription> cameras;

/*Future<Null> main() async {
  // Fetch the available cameras before initializing the app.
  try {
    cameras = await availableCameras();
  } on QRReaderException catch (e) {
    logError(e.code, e.description);
  }
  runApp(new Scan());
}*/

void logError(String code, String message) =>
    print('Error: $code\nError Message: $message');

class Scan extends StatefulWidget {
  @override
  _ScanState createState() => new _ScanState();
}

class _ScanState extends State<Scan> with SingleTickerProviderStateMixin {
  QRReaderController controller;
  final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
  AnimationController animationController;

  _asyncMethod() async {
    cameras = await availableCameras();
    onNewCameraSelected(cameras[0]);
    print('camera ${cameras.length}');
  }

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

    animationController = new AnimationController(
      vsync: this,
      duration: new Duration(seconds: 3),
    );

    animationController.addListener(() {
      this.setState(() {});
    });
    animationController.forward();
    verticalPosition = Tween<double>(begin: 0.0, end: 300.0).animate(
        CurvedAnimation(parent: animationController, curve: Curves.linear))
      ..addStatusListener((state) {
        if (state == AnimationStatus.completed) {
          animationController.reverse();
        } else if (state == AnimationStatus.dismissed) {
          animationController.forward();
        }
      });

    // pick the first available camera    
    _asyncMethod();
  }

  Animation<double> verticalPosition;

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      key: _scaffoldKey,
      appBar: new AppBar(
        title: const Text('Prueba escaner'),
      ),
      floatingActionButton: FloatingActionButton(
        child: new Icon(Icons.check),
        onPressed: () {
          Lamp.turnOn();
        },
      ),
      body: Stack(
        children: <Widget>[
          new Container(
            child: new Padding(
              padding: const EdgeInsets.all(0.0),
              child: new Center(
                child: _cameraPreviewWidget(),
              ),
            ),
          ),
          Center(
            child: Stack(
              children: <Widget>[
                SizedBox(
                  height: 300.0,
                  width: 300.0,
                  child: Container(
                    decoration: BoxDecoration(
                        border: Border.all(color: Colors.red, width: 2.0)),
                  ),
                ),
                Positioned(
                  top: verticalPosition.value,
                  child: Container(
                    width: 300.0,
                    height: 2.0,
                    color: Colors.red,
                  ),
                )
              ],
            ),
          ),
        ],
      ),
    );
  }

  /// Display the preview from the camera (or a message if the preview is not available).
  Widget _cameraPreviewWidget() {
    if (controller == null || !controller.value.isInitialized) {
      return const Text(
        'No camera selected',
        style: const TextStyle(
          color: Colors.white,
          fontSize: 24.0,
          fontWeight: FontWeight.w900,
        ),
      );
    } else {
      return new AspectRatio(
        aspectRatio: controller.value.aspectRatio,
        child: new QRReaderPreview(controller),
      );
    }
  }

  void onCodeRead(dynamic value) {
    showInSnackBar(value.toString());
    // ... do something
    // wait 5 seconds then start scanning again.
    new Future.delayed(const Duration(seconds: 5), controller.startScanning);
  }

  void onNewCameraSelected(CameraDescription cameraDescription) async {
    if (controller != null) {
      await controller.dispose();
    }
    controller = new QRReaderController(cameraDescription,
        ResolutionPreset.high, [CodeFormat.qr, CodeFormat.pdf417], onCodeRead);

    // If the controller is updated then update the UI.
    controller.addListener(() {
      if (mounted) setState(() {});
      if (controller.value.hasError) {
        showInSnackBar('Camera error ${controller.value.errorDescription}');
      }
    });

    try {
      await controller.initialize();
    } on QRReaderException catch (e) {
      logError(e.code, e.description);
      showInSnackBar('Error: ${e.code}\n${e.description}');
    }

    if (mounted) {
      setState(() {});
      controller.startScanning();
    }
  }

  void showInSnackBar(String message) {
    _scaffoldKey.currentState
        .showSnackBar(new SnackBar(content: new Text(message)));
  }
}

如何修复“ The Method []在null上被调用”

1 个答案: