Add RefreshIndicator to flutter webview

Question

I use webview_flutter package to create WebView app. Here is my example code:

import 'dart:async';
import 'dart:io';

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:webview_flutter/webview_flutter.dart';

const purple = 0xFF95377E;
void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    SystemChrome.setPreferredOrientations([
      DeviceOrientation.portraitUp,
    ]);
    return MaterialApp(
      title: 'CustomApp',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primaryColor: Color(purple),
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: CustomApp(),
    );
  }
}

class CustomApp extends StatefulWidget {
  @override
  _CustomAppState createState() => _CustomAppState();
}

class _CustomAppState extends State<CustomApp> {
  WebViewController _controller;
  bool isLoading = true;

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

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () {
        print("Clicked!");
        if (_controller != null) {
          print(_controller.currentUrl());
        }
      },
      child: Scaffold(
        backgroundColor: Color(purple),
        body: SafeArea(
          child: Stack(
            children: [
              WebView(
                initialUrl: 'https://example.com',
                javascriptMode: JavascriptMode.unrestricted,
                onWebViewCreated: (WebViewController webViewController) {
                  _controller = webViewController;
                },
                onPageFinished: (finish) {
                  setState(() {
                    isLoading = false;
                  });
                },
                onWebResourceError: (error) {
                  Loading();
                },
              ),
              isLoading ? Loading() : Stack(),
            ],
          ),
        ),
      ),
    );
  }
}

class Loading extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        fit: StackFit.expand,
        children: <Widget>[
          Container(
            decoration: BoxDecoration(color: Color(purple)),
          ),
          Column(
            mainAxisAlignment: MainAxisAlignment.start,
            children: <Widget>[
              Expanded(
                flex: 2,
                child: Container(
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: <Widget>[
                      Image.asset(
                        'assets/images/bg.png',
                        fit: BoxFit.contain,
                        height: 70.0,
                      ),
                    ],
                  ),
                ),
              ),
              Expanded(
                flex: 1,
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: <Widget>[
                    CircularProgressIndicator(
                      valueColor: AlwaysStoppedAnimation<Color>(Colors.white),
                    ),
                    Padding(
                      padding: EdgeInsets.only(top: 20.0),
                    ),
                    Text(
                      "Загружается...",
                      softWrap: true,
                      textAlign: TextAlign.center,
                      style: TextStyle(fontSize: 18.0, color: Colors.white),
                    )
                  ],
                ),
              )
            ],
          )
        ],
      ),
    );
  }
}

I need to add RefreshIndicator to my webview application. I tried like this but it's doesn't work:

import 'dart:async';
import 'dart:io';

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:webview_flutter/webview_flutter.dart';

const purple = 0xFF95377E;
void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    SystemChrome.setPreferredOrientations([
      DeviceOrientation.portraitUp,
    ]);
    return MaterialApp(
      title: 'CustomApp',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primaryColor: Color(purple),
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: CustomApp(),
    );
  }
}

class CustomApp extends StatefulWidget {
  @override
  _CustomAppState createState() => _CustomAppState();
}

class _CustomAppState extends State<CustomApp> {
  WebViewController _controller;
  bool isLoading = true;
  Completer controller;
  StreamController streamController;
  double webViewHeight = 100;

  void initState() {
    super.initState();
    streamController.stream.listen((value) {
      print(value);
      setState(() {
        webViewHeight = value;
      });
    });
    if (Platform.isAndroid) WebView.platform = SurfaceAndroidWebView();
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () {
        print("Clicked!");
        if (_controller != null) {
          print(_controller.currentUrl());
        }
      },
      child: Scaffold(
        backgroundColor: Color(purple),
        body: RefreshIndicator(
          onRefresh: () => controller.future.then((webViewController) {
            webViewController.reload();
          }),
          child: ListView.builder(
            itemCount: 1,
            itemBuilder: (BuildContext context, int index) {
              return ListTile(
                  title: Container(
                height: webViewHeight,
                child: SafeArea(
                  child: Stack(
                    children: [
                      WebView(
                        initialUrl: 'https://example.com',
                        javascriptMode: JavascriptMode.unrestricted,
                        onWebViewCreated:
                            (WebViewController webViewController) {
                          _controller = webViewController;
                          controller.complete(webViewController);
                        },
                        onPageFinished: (finish) {
                          double height;
                          controller.future.then((value) {
                            value
                                .evaluateJavascript(
                                    "document.documentElement.scrollHeight;")
                                .then((String valueHeight) {
                              height = double.parse(valueHeight);
                              streamController.add(height);
                            });
                          });
                          setState(() {
                            isLoading = false;
                          });
                        },
                        onWebResourceError: (error) {
                          Loading();
                        },
                      ),
                      isLoading ? Loading() : Stack(),
                    ],
                  ),
                ),
              ));
            },
          ),
        ),
      ),
    );
  }
}

class Loading extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        fit: StackFit.expand,
        children: <Widget>[
          Container(
            decoration: BoxDecoration(color: Color(purple)),
          ),
          Column(
            mainAxisAlignment: MainAxisAlignment.start,
            children: <Widget>[
              Expanded(
                flex: 2,
                child: Container(
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: <Widget>[
                      Image.asset(
                        'assets/images/bg.png',
                        fit: BoxFit.contain,
                        height: 70.0,
                      ),
                    ],
                  ),
                ),
              ),
              Expanded(
                flex: 1,
                child: Column(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: <Widget>[
                    CircularProgressIndicator(
                      valueColor: AlwaysStoppedAnimation<Color>(Colors.white),
                    ),
                    Padding(
                      padding: EdgeInsets.only(top: 20.0),
                    ),
                    Text(
                      "Загружается...",
                      softWrap: true,
                      textAlign: TextAlign.center,
                      style: TextStyle(fontSize: 18.0, color: Colors.white),
                    )
                  ],
                ),
              )
            ],
          )
        ],
      ),
    );
  }
}

How I can correctly add RefreshIndicator to webview in my case?