AdaptableTextDemo.dart

import 'package:flutter/material.dart';

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

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

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

class _MyAppState extends State<MyApp> {
  String mediumSizeText = "Lorem ipsum dolor sit amet, consectetur adipiscing elit.Lorem ipsum dolor sit amet, consectetur adipiscing elit. ";
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('ExpandableText'),
        ),
        body: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            Text('Normal Text'),
            Text(mediumSizeText),
            SizedBox(
              height: 20,
            ),
            Text('Container(250*20) Normal Text:'),
            Container(
              width: 250,
              height: 20,
              color: Colors.green,
              child: Text(
                mediumSizeText,
                overflow: TextOverflow.ellipsis,
              ),
            ),
            SizedBox(
              height: 20,
            ),
            Text('Container(250*20) => sizebox => Text:'),
            Container(
              width: 250,
              height: 20,
              color: Colors.green,
              child:  FittedBox(fit:BoxFit.fitWidth,
                  child: Text(mediumSizeText, maxLines: 100,),
              ),
            ),
            Text('Container(250*20) => AdaptableText => Text:'),
            Container(
              width: 250,
              height: 20,
              color: Colors.green,
                child: AdaptableText(mediumSizeText,style: TextStyle()),
            ),
          ],
        ),
      ),
    );
  }
}


class AdaptableText extends StatelessWidget {
  final String text;
  final TextStyle style;
  final TextAlign textAlign;
  final TextDirection textDirection;
  final double minimumFontScale;
  final TextOverflow textOverflow;
  const AdaptableText(this.text,
      {this.style,
        this.textAlign = TextAlign.left,
        this.textDirection = TextDirection.ltr,
        this.minimumFontScale = 0.5,
        this.textOverflow = TextOverflow.ellipsis,
        Key key})
      : super(key: key);

  @override
  Widget build(BuildContext context) {

    TextPainter _painter = TextPainter(
        text: TextSpan(text: this.text, style: this.style),
        textAlign: this.textAlign,
        textScaleFactor: 1,
        maxLines: 100,
        textDirection: this.textDirection);

    return LayoutBuilder(
      builder: (context, constraints) {
        _painter.layout(maxWidth: constraints.maxWidth);
        double textScaleFactor = 1;

        if (_painter.height > constraints.maxHeight) { //
          print('${_painter.size}');
           _painter.textScaleFactor  = minimumFontScale;
           _painter.layout(maxWidth: constraints.maxWidth);
          print('${_painter.size}');

           if (_painter.height > constraints.maxHeight) { //
             //even minimum does not fit render it with minimum size
             print("Using minimum set font");
             textScaleFactor = minimumFontScale;
           } else if (minimumFontScale < 1) {
             //binary search for valid Scale factor
             int h = 100;
             int l =   (minimumFontScale * 100).toInt();
             while (h > l) {
               int mid = (l + (h - l) / 2).toInt();
               double newScale = mid.toDouble()/100.0;
               _painter.textScaleFactor  = newScale;
               _painter.layout(maxWidth: constraints.maxWidth);

               if (_painter.height > constraints.maxHeight) { //
                  h = mid - 1;
               } else {
                  l = mid + 1;
               }
               if  (h <= l) {
                 print('${_painter.size}');
                 textScaleFactor = newScale - 0.01;
                 _painter.textScaleFactor  = newScale;
                 _painter.layout(maxWidth: constraints.maxWidth);
                 break;
               }


             }
           }
        }

        return Text(
          this.text,
          style: this.style,
          textAlign: this.textAlign,
          textDirection: this.textDirection,
          textScaleFactor: textScaleFactor,
          maxLines: 100,
          overflow: textOverflow,
        );
      },
    );
  }
}