flutter: bottomNavigationBar cannot switch between html pages using WebView

Question

Goal

I use bottomNavigationBar to switch between a few pages. Two of the pages use WebView to show local html files.

Problem

I found that when I switch between these pages, the pages with pure flutter widgets can load perfectly. But the HTML pages will only show the initially loaded page on switching.

For example, If I have two navigator buttons leading to Page1 and Page2, then at runtime, if I tapped Page1 button first, then tapped Page2 button, then the WebView would still show Page1 instead of Page2. This is wrong.

Code

Here is the HTML page code I use

class LocalLoader {
  Future<String> loadLocal(String filename) async {
    return await rootBundle.loadString('assets/doc/$filename');
  }
}
class HtmlPage extends StatelessWidget {
  final String htmlFile;
  HtmlPage({
    @required this.htmlFile
  });
  @override
  Widget build(BuildContext context) {
    return Container(
      child: FutureBuilder<String>(
        future: LocalLoader().loadLocal(htmlFile),
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            return WebView(
              initialUrl: Uri.dataFromString(snapshot.data,
                  mimeType: 'text/html',
                  // CAUTION
                  // - required for non-ascii chars
                  encoding: Encoding.getByName("UTF-8")
              ).toString(),
              javascriptMode: JavascriptMode.unrestricted,
            );
          } else if (snapshot.hasError) {
            return Text("${snapshot.error}");
          } else {
            print('undefined behaviour');
          }
          return CircularProgressIndicator();
        },
      ),);
  }
}

Then with my bottomNavigationBar, I handle the tap event:

class MyFlutterView extends StatefulWidget {
  @override
  _MyFlutterViewState createState() => _MyFlutterViewState();
}
class _MyFlutterViewState extends State<MyFlutterView> {
  final Keys keys = Keys();
  int _iSelectedDrawerItem = 3; // self
  int _iSelectedNavItem = 0;
  static List<Widget> _widgetOptions = <Widget>[
    MyFlutterPlaceholder(title: 'Index 0: MyFlutter'),
    MyPage(htmlFile: 'page1.html'),
    MyPage(htmlFile: 'page2.html'),
  ];

  void _onItemTapped(int index) {
    setState(() {
      _iSelectedNavItem = index;
    });
  }
  @override
  Widget build(BuildContext context) {
    final deviceSize = MediaQuery.of(context).size;
    final appBar = AppBar(
      backgroundColor: WidgetColors.menubar,
      title: Text('MyFlutter'),
    );
    return Scaffold(
      appBar: appBar,
      endDrawer: NavDrawer(
        keys: keys,
        iSelectedDrawerItem: _iSelectedDrawerItem,
      ),
      body: Container(
        decoration: BoxDecoration(
            gradient: WidgetColors.canvas,
        ),
        child: _widgetOptions.elementAt(_iSelectedNavItem),
      ),
      bottomNavigationBar: BottomNavigationBar(
          currentIndex : _iSelectedNavItem,
          type: BottomNavigationBarType.fixed,
          backgroundColor: WidgetColors.menubar,
          fixedColor: WidgetColors.myColor,
          // selectedItemColor: WidgetColors.myColor,
          unselectedItemColor: Colors.white,
          selectedIconTheme: IconThemeData(color: WidgetColors.myColor),
          // unselectedIconTheme: IconThemeData(color: Colors.white),
          items: [
            BottomNavigationBarItem(
              label: 'MyFlutter',
              icon: Icon(Icons.build)
            ),
            BottomNavigationBarItem(
              label: 'Page1-HTML',
              icon: Icon(Icons.help,),
            ),
            BottomNavigationBarItem(
              label: 'Page2-HTML',
              icon: Icon(Icons.info_outline_rounded),
            ),
          ],
          onTap: _onItemTapped),
    );
  }
}

I've also tried StatefulWidgets but the problem persists.

Workaround

The only workaround I have right now is to derive from the HtmlPage class for every single page I have, like this:

class Page1 extends HtmlPage {
  Page1() : super(htmlFile: 'page1.html');
}

class Page2 extends HtmlPage {
  Page2() : super(htmlFile: 'page2.html');
}

After this, the HTML pages will switch and load as expected.

Question

How should I fix this? Should I work on loading the HTML file more explicitly? I thought setState would handle the loading for me automatically and this certainly applies to the pure flutter widget page (MyFlutterPlaceholder class in the code above).

Also, I ensured that the url loading was called every time I switch page through the nav bar.

Is it buildable? There is no 'title' parameter in 'BottomNavigationBar'. When I test a your code by removing not provided code, it works well. — KuKu
– KuKu, Commented Sep 8, 2020 at 7:25
@KuKu Yeah it's buildable. I checked my post and fixed a markdown syntax error. But it seems title is the AppBar property. — kakyo
– kakyo, Commented Sep 8, 2020 at 7:35

chunhunghan · Accepted Answer · 2020-09-09 10:27:06Z

You can copy paste run full code below
I simulate this case with the full code below
Step 1: Use AutomaticKeepAliveClientMixin

class _WebViewKeepAlive extends State<WebViewKeepAlive>
    with AutomaticKeepAliveClientMixin {
    
    @override
  bool get wantKeepAlive => true;
  
  @override
  Widget build(BuildContext context) {
    super.build(context);

Step 2: Do not direct put function in future attribute, future: LocalLoader().loadLocal(htmlFile), Please use below way

Future<String> _future;

 @override
  void initState() {
    _future = _getUrl(widget.url);
    super.initState();
  }
 
return FutureBuilder(
        future: _future,

Step 3: I use PageView in this case

working demo

full code

import 'package:flutter/material.dart';
import 'package:webview_flutter/webview_flutter.dart';
import 'package:http/http.dart' as http;
import 'dart:convert';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyPortalPage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  final String title;

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

class _MyPortalPageState extends State<MyPortalPage> {
  int _currentIndex = 0;
  PageController _pageController = PageController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: SizedBox.expand(
        child: PageView(
          controller: _pageController,
          children: <Widget>[
            Page1(),
            WebViewKeepAlive(url: "https://flutter.dev/"),
            WebViewKeepAlive(url: "https://stackoverflow.com/"),
            Center(child: Text("Settings")),
          ],
          onPageChanged: (int index) {
            print("onPageChanged");
            setState(() {
              _currentIndex = index;
            });
          },
        ),
      ),
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        selectedItemColor: Colors.amber[800],
        unselectedItemColor: Colors.blue,
        onTap: (index) {
          print("onItemSelected");
          setState(() => _currentIndex = index);
          _pageController.jumpToPage(index);
        },
        items: const <BottomNavigationBarItem>[
          BottomNavigationBarItem(
            icon: Icon(Icons.apps),
            label: 'Challenges',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.people),
            label: 'Users',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.message),
            label: 'Messages',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.settings),
            label: 'Settings',
          ),
        ],
      ),
    );
  }
}

class Page1 extends StatefulWidget {
  const Page1({Key key}) : super(key: key);

  @override
  _Page1State createState() => _Page1State();
}

class _Page1State extends State<Page1> with AutomaticKeepAliveClientMixin {
  @override
  Widget build(BuildContext context) {
    super.build(context);
    return ListView.builder(itemBuilder: (context, index) {
      return ListTile(
        title: Text('Lorem Ipsum'),
        subtitle: Text('$index'),
      );
    });
  }

  @override
  bool get wantKeepAlive => true;
}

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

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

class _WebViewKeepAlive extends State<WebViewKeepAlive>
    with AutomaticKeepAliveClientMixin {

  Future<String> _future;

  @override
  bool get wantKeepAlive => true;

  Future<String> _getUrl(String url) async {
    await Future.delayed(Duration(seconds: 1), () {});
    return Future.value(url);
  }

  @override
  void initState() {
    _future = _getUrl(widget.url);
    super.initState();
  }

  @override
  void dispose() {
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    super.build(context);
    return FutureBuilder(
        future: _future,
        builder: (context, AsyncSnapshot<String> snapshot) {
          switch (snapshot.connectionState) {
            case ConnectionState.none:
              return Text('none');
            case ConnectionState.waiting:
              return Center(child: CircularProgressIndicator());
            case ConnectionState.active:
              return Text('');
            case ConnectionState.done:
              if (snapshot.hasError) {
                return Text(
                  '${snapshot.error}',
                  style: TextStyle(color: Colors.red),
                );
              } else {
                return WebView(
                  initialUrl: snapshot.data,
                  javascriptMode: JavascriptMode.unrestricted,
                );
              }
          }
        });
  }
}

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