appbar_jumbo_title.dart

import 'package:flutter/material.dart';

class ScaleFactorAppBar extends StatelessWidget {
  const ScaleFactorAppBar({
    this.textScaleFactor = 1,
    this.textDirection = TextDirection.ltr,
    this.centerTitle = false,
  });

  final double textScaleFactor;
  final TextDirection textDirection;
  final bool centerTitle;

  @override
  Widget build(BuildContext context) {
    return Directionality(
      textDirection: textDirection,
      child: MediaQuery(
        data: MediaQuery.of(context).copyWith(textScaleFactor: textScaleFactor),
        child: Builder(
          builder: (BuildContext context) {
            return AppBar(
              leading: BackButtonIcon(),
              centerTitle: centerTitle,
              title: Text('Jumbo'),
              actions: <Widget>[ Icon(Icons.menu) ],
            );
          },
        ),
      ),
    );
  }
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    const Widget spacer = SizedBox(height: 32);
    return Container(
      color: Colors.white,
      child: SafeArea(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            spacer,
            ScaleFactorAppBar(textScaleFactor: 1),
            spacer,
            ScaleFactorAppBar(textScaleFactor: 2),
            spacer,
            ScaleFactorAppBar(textScaleFactor: 3),
            spacer,
            ScaleFactorAppBar(textScaleFactor: 3.5),
            spacer,
            ScaleFactorAppBar(textScaleFactor: 4),
          ],

          /*
          children: <Widget>[
            spacer,
            ScaleFactorAppBar(textScaleFactor: 1, textDirection: TextDirection.rtl),
            spacer,
            ScaleFactorAppBar(textScaleFactor: 2, textDirection: TextDirection.rtl),
            spacer,
            ScaleFactorAppBar(textScaleFactor: 3, textDirection: TextDirection.rtl),
            spacer,
            ScaleFactorAppBar(textScaleFactor: 3.5, textDirection: TextDirection.rtl),
            spacer,
            ScaleFactorAppBar(textScaleFactor: 4, textDirection: TextDirection.rtl),
          ],
          */

          /*
          children: <Widget>[
            spacer,
            ScaleFactorAppBar(textScaleFactor: 1, centerTitle: true),
            spacer,
            ScaleFactorAppBar(textScaleFactor: 2, centerTitle: true),
            spacer,
            ScaleFactorAppBar(textScaleFactor: 3, centerTitle: true),
            spacer,
            ScaleFactorAppBar(textScaleFactor: 3.5, centerTitle: true),
            spacer,
            ScaleFactorAppBar(textScaleFactor: 4, centerTitle: true),
          ],
          */
        ),
      ),
    );
  }
}

void main() {
  runApp(
    MaterialApp(
      //theme: ThemeData(platform: TargetPlatform.iOS),
      home: HomePage(),
    ),
  );
}