import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

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

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

class _MyAppState extends State<MyApp> {
  final TextEditingController _textEditingController1 = TextEditingController();
  final TextEditingController _textEditingController2 = TextEditingController();

  String _dummyText = '''This is sample Paragraph with some dummy text. Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.''';
  int _numberOfTextPaintersToBeGenerated = 3500;

  String _logText = '';

  @override
  void initState() {
    _textEditingController1.text = _dummyText;
    _textEditingController2.text = _numberOfTextPaintersToBeGenerated.toString();
    super.initState();
  }

  void _calculateTextPainters() {
    _dummyText = _textEditingController1.text;
    _numberOfTextPaintersToBeGenerated = int.tryParse(_textEditingController2.text)??0;

    if (_numberOfTextPaintersToBeGenerated.toString() != _textEditingController2.text) {
      _logText = 'Please enter numeric input only';
      setState(() {});
      return;
    }

    FocusScope.of(context).requestFocus(new FocusNode());

    _logText = 'Calculating $_numberOfTextPaintersToBeGenerated textPainters\n';

    final DateTime dateTime = DateTime.now();

    for (int i = 0; i < _numberOfTextPaintersToBeGenerated; i++) {
      TextPainter textPainter = TextPainter(
        maxLines: null,
        textDirection: TextDirection.ltr,
        textAlign: TextAlign.justify,
        textScaleFactor: 1,
        text: TextSpan(
          text: _dummyText,
          style: TextStyle(
            fontSize: 20,
            fontWeight: FontWeight.normal,
            color: Colors.black,
          ),
        ),
      );
      textPainter.layout(minWidth: 0, maxWidth: 500);
    }

    _logText += 'All textPainters calculated\n';

    final Duration duration = DateTime.now().difference(dateTime);

    _logText += 'Time taken is ${duration.inMilliseconds} ms\n';

    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        appBar: AppBar(title: Text("Demo")),
        body: Container(
          color: Colors.white,
          constraints: BoxConstraints.expand(),
          child: Column(
            children: [
              Expanded(
                flex: 2,
                child: Padding(
                  padding: EdgeInsets.all(4),
                  child: Container(
                    padding: EdgeInsets.all(4),
                    decoration: BoxDecoration(
                      color: Colors.white,
                      border: Border.all(color: Colors.black)
                    ),
                    constraints: BoxConstraints.expand(),
                    child: Scrollbar(
                      child: TextField(
                        controller: _textEditingController1,
                        maxLines: null,
                        style: TextStyle(
                          fontSize: 20
                        ),
                        decoration: InputDecoration(
                          isDense: true,
                          border: InputBorder.none,
                          labelText: 'Dummy text',
                          labelStyle: TextStyle(color: Colors.blue),
                          focusedBorder: InputBorder.none,
                          enabledBorder: InputBorder.none,
                          errorBorder: InputBorder.none,
                          disabledBorder: InputBorder.none,
                          contentPadding: EdgeInsets.all(5),
                        ),
                      ),
                    ),
                  ),
                ),
              ),
              SizedBox(height: 20,),
              Row(
                mainAxisSize: MainAxisSize.min,
                mainAxisAlignment: MainAxisAlignment.center,
                children: [
                  Container(
                    width: 100,
                    height: 34,
                    child: TextField(
                      controller: _textEditingController2,
                      style: TextStyle(
                        fontSize: 20
                      ),
                    ),
                  ),
                  SizedBox(width: 50,),
                  ElevatedButton(
                    onPressed: _calculateTextPainters,
                    child: Text(
                      'Calculate TextPainters'
                    ),
                  ),
                ],
              ),
              SizedBox(height: 20,),
              Expanded(
                flex: 1,
                child: Padding(
                  padding: EdgeInsets.all(4),
                  child: Container(
                    padding: EdgeInsets.all(4),
                    decoration: BoxDecoration(
                      color: Colors.white,
                      border: Border.all(color: Colors.black)
                    ),
                    constraints: BoxConstraints.expand(),
                    child: Text(
                      _logText,
                      softWrap: true,
                      maxLines: null,
                      style: TextStyle(
                        fontSize: 20,
                        height: 1.4,
                      ),
                    ),
                  ),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}