Flutter - Flutter - Auto Scaling the Text Size

main_using_fontsize.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(),
    );
  }
}

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

class _MyHomePageState extends State<MyHomePage> {
  double _prevScale;
  double _scale;

  @override
  void initState() {
    super.initState();
    _prevScale = _scale = 1.0;
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onScaleUpdate: (ScaleUpdateDetails details) {
        setState(() {
          _scale = (_prevScale * (details.scale));
        });
      },
      onScaleEnd: (ScaleEndDetails details) {
        setState(() {
          _prevScale = _scale;
        });
      },
      child: Scaffold(
        appBar: AppBar(
          title: Text("Home"),
        ),
        body: Center(
          child: new Text("Howdy", 
            style: new TextStyle(fontSize: 16.0 * _scale),
          ),
        ),
      ),
    );
  }
}

main_using_scale_factor.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(),
    );
  }
}

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

class _MyHomePageState extends State<MyHomePage> {
  double _prevScale;
  double _scale;

  @override
  void initState() {
    super.initState();
    _prevScale = _scale = 1.0;
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onScaleUpdate: (ScaleUpdateDetails details) {
        setState(() {
          _scale = (_prevScale * (details.scale));
        });
      },
      onScaleEnd: (ScaleEndDetails details) {
        setState(() {
          _prevScale = _scale;
        });
      },
      child: Scaffold(
        appBar: AppBar(
          title: Text("Home"),
        ),
        body: Center(
          child: new SizedBox(
            height: 20.0 * _scale,
            child: new FittedBox(
              child: new Text("Howdy"),
            ),
          ),
        ),
      ),
    );
  }
}

main_using_transform.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(),
    );
  }
}

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

class _MyHomePageState extends State<MyHomePage> {
  double _prevScale;
  double _scale;

  @override
  void initState() {
    super.initState();
    _prevScale = _scale = 1.0;
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onScaleUpdate: (ScaleUpdateDetails details) {
        setState(() {
          _scale = (_prevScale * (details.scale));
        });
      },
      onScaleEnd: (ScaleEndDetails details) {
        setState(() {
          _prevScale = _scale;
        });
      },
      child: Scaffold(
        appBar: AppBar(
          title: Text("Home"),
        ),
        body: Center(
          child: Transform(
            transform: new Matrix4.diagonal3Values(_scale, _scale, _scale),
            alignment: Alignment.center,
            child: new Text("Howdy"),
          ),
        ),
      ),
    );
  }
}

This is a great example. I used FittedBox and it works. Thanks for sharing!