pass value between bottomNavigationBar views

Nathan Bernard 0
Viewed 34

How am I supposed to pass a value in this big mess called Flutter?

30 years old php global $var wasn't good? All these years were to come up with setState, passed in a controller which get redeclared as a key inside a stateful widget that receive the value from a Navigator?

By the way, I tried using Navigator.push but it seems to open a completely new window, the value is there but I'd need it to show in the tab body not in a new window, below is my code:

main.dart

import 'dart:core';
import 'dart:developer';
import 'dart:io';

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:qr_code_scanner/qr_code_scanner.dart';

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

class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter App',
      theme: ThemeData(
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: HomeView(),
    );
  }
}

class HomeView extends StatefulWidget {
  @override
  _HomeViewState createState() => _HomeViewState();
}

class _HomeViewState extends State<HomeView> {
  final tabs = [QRViewExample(), SecondView(res: '')];

  int _currentIndex = 0;

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        toolbarHeight: 40.0,
        elevation: 0,
        centerTitle: true,
        title: Text('Flutter App'),
      ),
      body: tabs[_currentIndex],
      bottomNavigationBar: BottomNavigationBar(
        backgroundColor: Colors.red,
        currentIndex: _currentIndex,
        type: BottomNavigationBarType.fixed,
        selectedItemColor: Colors.white,
        unselectedItemColor: Colors.white.withOpacity(0.5),
        items: [
          BottomNavigationBarItem(
            icon: Icon(Icons.qr_code),
            label: 'Scan',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.list),
            label: 'List',
          ),
        ],
        onTap: (index) {
          setState(() {
            _currentIndex = index;
          });
        },
      ),
    );
  }
}

// SECOND TAB WIDGET (custom)
class SecondView extends StatelessWidget {
  const SecondView({Key? key, required this.res}) : super(key: key);
  final String? res;

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Center(
        child: Text(res!),
      ),
    );
  }
}

// FIRST TAB WIDGET (qrcode)
class QRViewExample extends StatefulWidget {
  const QRViewExample({Key? key}) : super(key: key);

  @override
  State<StatefulWidget> createState() => _QRViewExampleState();
}

class _QRViewExampleState extends State<QRViewExample> {
  Barcode? result;
  QRViewController? controller;
  final GlobalKey qrKey = GlobalKey(debugLabel: 'QR');

  @override
  void reassemble() {
    super.reassemble();
    if (Platform.isAndroid) {
      controller!.pauseCamera();
    }
    controller!.resumeCamera();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Container(
        height: 500,
        child: Padding(
          padding: EdgeInsets.all(8.0),
          child: Column(
            children: <Widget>[
              Expanded(flex: 4, child: _buildQrView(context)),
              Expanded(
                flex: 1,
                child: FittedBox(
                  fit: BoxFit.contain,
                  child: Column(
                    mainAxisSize: MainAxisSize.min,
                    mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                    children: <Widget>[
                      if (result != null)
                        Text(
                            'Barcode Type: ${describeEnum(result!.format)}   Data: ${result!.code}')
                      else
                        const Text('Scan a code'),
                      Row(
                        mainAxisAlignment: MainAxisAlignment.center,
                        crossAxisAlignment: CrossAxisAlignment.center,
                        children: <Widget>[
                          Container(
                            margin: const EdgeInsets.all(8),
                            child: ElevatedButton(
                                onPressed: () async {
                                  await controller?.toggleFlash();
                                  setState(() {});
                                },
                                child: FutureBuilder(
                                  future: controller?.getFlashStatus(),
                                  builder: (context, snapshot) {
                                    return Text('Flash: ${snapshot.data}');
                                  },
                                )),
                          ),
                          Container(
                            margin: const EdgeInsets.all(8),
                            child: ElevatedButton(
                                onPressed: () async {
                                  await controller?.flipCamera();
                                  setState(() {});
                                },
                                child: FutureBuilder(
                                  future: controller?.getCameraInfo(),
                                  builder: (context, snapshot) {
                                    if (snapshot.data != null) {
                                      return Text(
                                          'Camera facing ${describeEnum(snapshot.data!)}');
                                    } else {
                                      return const Text('loading');
                                    }
                                  },
                                )),
                          )
                        ],
                      ),
                      Row(
                        mainAxisAlignment: MainAxisAlignment.center,
                        crossAxisAlignment: CrossAxisAlignment.center,
                        children: <Widget>[
                          Container(
                            margin: const EdgeInsets.all(8),
                            child: ElevatedButton(
                              onPressed: () async {
                                await controller?.pauseCamera();
                              },
                              child: const Text('pause',
                                  style: TextStyle(fontSize: 20)),
                            ),
                          ),
                          Container(
                            margin: const EdgeInsets.all(8),
                            child: ElevatedButton(
                              onPressed: () async {
                                await controller?.resumeCamera();
                              },
                              child: const Text('resume',
                                  style: TextStyle(fontSize: 20)),
                            ),
                          )
                        ],
                      ),
                    ],
                  ),
                ),
              )
            ],
          ),
        ),
      ),
    );
  }

  Widget _buildQrView(BuildContext context) {
    var scanArea = (MediaQuery.of(context).size.width < 400 ||
            MediaQuery.of(context).size.height < 400)
        ? 150.0
        : 300.0;
    return QRView(
      key: qrKey,
      onQRViewCreated: _onQRViewCreated,
      overlay: QrScannerOverlayShape(
          borderColor: Colors.cyanAccent,
          borderRadius: 10,
          borderLength: 30,
          borderWidth: 10,
          cutOutSize: scanArea),
      onPermissionSet: (ctrl, p) => _onPermissionSet(context, ctrl, p),
    );
  }

  void _onQRViewCreated(QRViewController controller) {
    setState(() {
      this.controller = controller;
    });
    controller.scannedDataStream.listen((scanData) {
      controller.pauseCamera();
      setState(() {
        result = scanData;
      });
      Navigator.push(
              context,
              MaterialPageRoute(
                  builder: (context) => SecondView(res: result!.code)))
          .then((value) => controller.resumeCamera());
    });
  }

  void _onPermissionSet(BuildContext context, QRViewController ctrl, bool p) {
    log('${DateTime.now().toIso8601String()}_onPermissionSet $p');
    if (!p) {
      ScaffoldMessenger.of(context).showSnackBar(
        const SnackBar(content: Text('no Permission')),
      );
    }
  }

  @override
  void dispose() {
    controller?.dispose();
    super.dispose();
  }
}
dart flutter
1 Answers

How am I supposed to pass a value in this big mess called Flutter?

With state management tools like InheritedWidget, InheritedModel, Provider, BloC and many more.

30 years old php global $var wasn't good? All these years were to come up with setState, passed in a controller which get redeclared as a key inside a stateful widget that receive the value from a Navigator?

Well, you shouldn't do that and it's not meant to be done like that. We can use several methods to propagate data down the widget tree. Let me explain this with InheritedWidget. But sometimes you want to go for Provider which is a wrapper class for InheritedWidget.

First we create a class named QRListModel which extends InheritedModel:

class QRListModel extends InheritedWidget {
  
  final List<Barcode> qrList = [];  // <- This holds our data
  QRListModel({required super.child});
  
  @override
  bool updateShouldNotify(QRListModel oldWidget) {
    return !listEquals(oldWidget.qrList, qrList);
  }

  static QRListModel of(BuildContext context) {
    final QRListModel? result = context.dependOnInheritedWidgetOfExactType<QRListModel>();
    assert(result != null, 'No QRListModel found in context');
    return result!;
  }
}

updateShouldNotify is a method we have to override to tell Flutter, when we want the widgets to rebuild. We want this to happen when the list changes. The of method is just a handy way to access the QRListModel.

Now wrap a parent widget of both the scan tab view and the list tab view inside QRListModel. We go for HomeView:

class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter App',
      theme: ThemeData(
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: QRListModel(child: HomeView()), // <- here!
    );
  }
}

We can take any parent widget but it should be a class where we don't call setState. Otherwise our QRListModel also gets rebuilt and our list is gone.

Now we can access QRListModel from anywhere inside the subtree. We need it here:

void _onQRViewCreated(QRViewController controller) {
  setState(() {
    this.controller = controller;
    this.controller!.resumeCamera();
  });
  controller.scannedDataStream.listen((scanData) async {
    controller.pauseCamera();
    QRListModel.of(context).qrList.add(scanData); // <- Here we access the list
    await showDialog(
      context: context, 
      builder: (context) => SimpleDialog(
        title: Text("Barcode was added!"),
        children: [
          Text(scanData.code!)
        ],
      )
    );
  });
}

And here we read the list:

class SecondView extends StatelessWidget {
  const SecondView({Key? key, required this.res}) : super(key: key);
  final String? res;

  @override
  Widget build(BuildContext context) {
    return ListView.builder(
      itemCount: QRListModel.of(context).qrList.length,
      itemBuilder: (context, index) {
        return Card(
          child: ListTile(
            title: Text(QRListModel.of(context).qrList[index].code ?? "NO"),
          ),
        );
      }
    );
  }
}

Now both pages have access to the qr list. Please do mind that a InheritedWidget can only have final fields. So if you need mutable fields, you need an additional wrapper class. We don't need it as we don't change the list but only its elements.

By the way: You shouldn't call setState inside initState. You did this here:

class _HomeViewState extends State<HomeView> {
  final tabs = [QRViewExample(), SecondView(res: '')];

  int _currentIndex = 0;

  @override
  void initState() {
    setState(() {});    // <- Don't call setState inside initState!
    super.initState();
  }
TripleNine0
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