[native_pdf_view] Provide a onPageChanged callback

[IchordeDionysos]

Is your feature request related to a problem? Please describe.
We would like to show the user that he is on page 1/n,

Describe the solution you'd like
To achieve that I believe that a simple onPageChanged callback could be easily implemented and would give us a lot of flexibility.

Describe alternatives you've considered
A built-in page indicator would also work, but might not fit into the UI of every app.

Additional context

[SergeShkurko]

Show code example

import 'package:flutter/material.dart';
import 'package:native_pdf_view/native_pdf_view.dart';
import 'package:native_pdf_view_example/has_support.dart';

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

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

class _MyAppState extends State<MyApp> {
  // Create page controller
  final _pageController = PageController();
  int _actualPageNumber = 1;

  @override
  void initState() {
    // listen page number change
    _pageController.addListener(() {
      final pageIndex = _pageController.page.round();
      final pageNumber = pageIndex + 1;
      // Set actual page
      if (pageNumber != _actualPageNumber) {
        setState(() {
          _actualPageNumber = pageNumber;
        });
      }
    });
    super.initState();
  }

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

  @override
  Widget build(BuildContext context) => MaterialApp(
        theme: ThemeData(primaryColor: Colors.white),
        home: Scaffold(
          appBar: AppBar(title: Text('PDFView example')),
          body: FutureBuilder<PDFDocument>(
            future: _getDocument(),
            builder: (_, snapshot) {
              if (snapshot.hasData) {
                return Stack(
                  children: <Widget>[
                    PDFView(
                      controller: _pageController, // set page controller
                      document: snapshot.data,
                    ),
                    Align(
                      alignment: Alignment.center,
                      child: Text(
                        '$_actualPageNumber/${snapshot.data.pagesCount}',
                        style: TextStyle(fontSize: 34),
                      ),
                    )
                  ],
                );
              }

              if (snapshot.hasError) {
                return Center(
                  child: Text(
                    'PDF Rendering does not '
                    'support on the system of this version',
                  ),
                );
              }

              return Center(child: CircularProgressIndicator());
            },
          ),
        ),
      );

  Future<PDFDocument> _getDocument() async {
    if (await hasSupport()) {
      return PDFDocument.openAsset('assets/sample.pdf');
    } else {
      throw Exception(
        'PDF Rendering does not '
        'support on the system of this version',
      );
    }
  }
}

@IchordeDionysos will this solve your problem?

[SergeShkurko]

Got your idea, I agree
Added in 2.1.0

[IchordeDionysos]

Yes, the example you provided worked, but is so think far less beautiful and less intuitive/obvious to us :)