import 'package:flutter/material.dart';
import 'package:webview_flutter/webview_flutter.dart';
import 'dart:async';
import 'dart:io';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Home(),
      debugShowCheckedModeBanner: false,
    );
  }
}

class Home extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<Home> {
  List<Widget> data = [Container()];

  int initPosition = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: CustomTabView(
          initPosition: initPosition,
          itemCount: data.length,
          tabBuilder: (context, index) {
            if (index == 0) {
              return Tab(icon: Icon(Icons.home));
            } else {
              return Tab(text: index.toString());
            }
          },
          pageBuilder: (context, index) {
            if (index == 0) {
              return Center(
                  child: FloatingActionButton(
                      child: Icon(Icons.add),
                      onPressed: () {
                        setState(() {
                          data.add(Page(
                            url: 'https://www.google.com',
                            key: ValueKey('$index'),
                          ));
                        });
                      }));
            } else {
              return Center(child: (data[index]));
            }
          },
          onPositionChange: (index) {
            print('current position: $index');
            initPosition = index;
          },
          onScroll: (position) => print('$position'),
        ),
      ),
    );
  }
}

/// Implementation

class CustomTabView extends StatefulWidget {
  final int? itemCount;
  final IndexedWidgetBuilder? tabBuilder;
  final IndexedWidgetBuilder? pageBuilder;
  final Widget? stub;
  final ValueChanged<int>? onPositionChange;
  final ValueChanged<double>? onScroll;
  final int? initPosition;

  CustomTabView({
    @required this.itemCount,
    @required this.tabBuilder,
    @required this.pageBuilder,
    this.stub,
    this.onPositionChange,
    this.onScroll,
    this.initPosition,
  });

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

class _CustomTabsState extends State<CustomTabView>
    with TickerProviderStateMixin {
  TabController? controller;
  int? _currentCount;
  int? _currentPosition;

  @override
  void initState() {
    _currentPosition = widget.initPosition ?? 0;
    controller = TabController(
      length: widget.itemCount!,
      vsync: this,
      initialIndex: _currentPosition!,
    );
    controller!.addListener(onPositionChange);
    controller!.animation!.addListener(onScroll);
    _currentCount = widget.itemCount;
    super.initState();
  }

  @override
  void didUpdateWidget(CustomTabView oldWidget) {
    print('current position: $_currentPosition');

    if (_currentCount != widget.itemCount) {
      print('_currentCount != widget.itemCount');
      controller!.animation!.removeListener(onScroll);
      controller!.removeListener(onPositionChange);
      controller!.dispose();

      if (widget.initPosition != null) {
        print('widget.initPosition != null');
        _currentPosition = widget.initPosition;
      }

      if (_currentPosition! > widget.itemCount! - 1) {
        print('_currentPosition > widget.itemCount - 1');
        _currentPosition = widget.itemCount! - 1;
        _currentPosition = _currentPosition! < 0 ? 0 : _currentPosition;
        if (widget.onPositionChange is ValueChanged<int>) {
          WidgetsBinding.instance!.addPostFrameCallback((_) {
            if (mounted) {
              widget.onPositionChange!(_currentPosition!);
            }
          });
        }
      }

      _currentCount = widget.itemCount;
      setState(() {
        controller = TabController(
          length: widget.itemCount!,
          vsync: this,
          initialIndex: _currentPosition!,
        );
        controller!.addListener(onPositionChange);
        controller!.animation!.addListener(onScroll);
      });

      controller!.animateTo(widget.itemCount! - 1);
    } else if (widget.initPosition != null) {
      controller!.animateTo(widget.initPosition!);
    }

    super.didUpdateWidget(oldWidget);
  }

  @override
  void dispose() {
    controller!.animation!.removeListener(onScroll);
    controller!.removeListener(onPositionChange);
    controller!.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    if (widget.itemCount! < 1) return widget.stub ?? Container();

    return Column(
      crossAxisAlignment: CrossAxisAlignment.stretch,
      children: <Widget>[
        Expanded(
          child: TabBarView(
            physics: NeverScrollableScrollPhysics(),
            controller: controller,
            children: List.generate(
              widget.itemCount!,
              (index) => widget.pageBuilder!(context, index),
            ),
          ),
        ),
        Container(
          alignment: Alignment.centerLeft,
          child: TabBar(
            isScrollable: true,
            controller: controller,
            labelColor: Theme.of(context).primaryColor,
            unselectedLabelColor: Theme.of(context).hintColor,
            indicator: BoxDecoration(
              border: Border(
                bottom: BorderSide(
                  color: Theme.of(context).primaryColor,
                  width: 2,
                ),
              ),
            ),
            tabs: List.generate(
              widget.itemCount!,
              (index) => widget.tabBuilder!(context, index),
            ),
          ),
        ),
      ],
    );
  }

  onPositionChange() {
    if (!controller!.indexIsChanging) {
      _currentPosition = controller!.index;
      if (widget.onPositionChange is ValueChanged<int>) {
        widget.onPositionChange!(_currentPosition!);
      }
    }
  }

  onScroll() {
    if (widget.onScroll is ValueChanged<double>) {
      widget.onScroll!(controller!.animation!.value);
    }
  }
}

class Page extends StatefulWidget {
  const Page({key, this.url}) : super(key: key);

  final String? url;

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

class _PageState extends State<Page> with AutomaticKeepAliveClientMixin<Page> {
  final Completer<WebViewController> _controller =
      Completer<WebViewController>();

  late final url = widget.url!;

  @override
  bool get wantKeepAlive => true;

  @override
  void initState() {
    super.initState();
    if (Platform.isAndroid) WebView.platform = SurfaceAndroidWebView();
  }

  @override
  Widget build(BuildContext context) {
    super.build(context);

    return Scaffold(
        body: WebView(
      key: widget.key,
      initialUrl: url,
      javascriptMode: JavascriptMode.unrestricted,
      onWebViewCreated: (WebViewController webViewController) {
        debugPrint("WebView is created");

        _controller.complete(webViewController);
      },
      onProgress: (int progress) {
        debugPrint("WebView is loading (progress : $progress%)");
      },
      onPageStarted: (String url) {
        debugPrint('Page started loading: $url');
      },
      onPageFinished: (String url) {
        debugPrint('Page finished loading: $url');

        ScaffoldMessenger.of(context).showSnackBar(SnackBar(
          content: Text('Page finished loading: $url'),
          duration: Duration(seconds: 5),
        ));
      },
    ));
  }
}