不随系统字体大小发生变化

app.dart

import 'package:flutter/material.dart';
import 'scale_text_widget.dart';

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

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      builder: (ctx, w) {
        return MaxScaleTextWidget(
            max: 1.0,
            child: w,
        );
      },
    );
  }
}

main.dart

import 'package:flutter/material.dart';

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

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

class _MyAppState extends State<MyApp> {
  double textScale = 1;

  @override
  Widget build(BuildContext context) {
    return NotificationListener<ChangeTextScaleNotification>(
      onNotification: (n) {
        this.textScale = n.max;
        setState(() {});
        return true;
      },
      child: MaterialApp(
        home: HomePage(),
        builder: (context, widget) {
          final data = MediaQuery.of(context);
          print("textScaleFactor: ${data.textScaleFactor}");
          return MediaQuery(
            data: MediaQuery.of(context).copyWith(
              textScaleFactor: textScale,
            ),
            child: widget,
          );
        },
      ),
    );
  }
}

class HomePage extends StatefulWidget {
  const HomePage({
    Key key,
  }) : super(key: key);

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

class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Plugin example app'),
      ),
      body: Column(
        children: <Widget>[
          TextField(
            style: TextStyle(fontSize: 20),
            decoration: InputDecoration(hintText: "Test"),
          ),
          Slider(
            min: 0.8,
            max: 3,
            onChanged: (double value) {
              (ChangeTextScaleNotification()..max = value).dispatch(context);
              setState(() {});
            },
            value: MediaQuery.of(context).textScaleFactor,
          ),
        ],
      ),
    );
  }
}

class ChangeTextScaleNotification extends Notification {
  double max;
}

scale_text_widget.dart

import 'dart:math' as math;

import 'package:flutter/material.dart';

class NoScaleTextWidget extends StatelessWidget {
  final Widget child;

  const NoScaleTextWidget({
    Key key,
    @required this.child,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaxScaleTextWidget(
      max: 1.0,
      child: child,
    );
  }
}

class MaxScaleTextWidget extends StatelessWidget {
  final double max;
  final Widget child;

  const MaxScaleTextWidget({
    Key key,
    this.max = 1.2,
    @required this.child,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    var data = MediaQuery.of(context);
    var scale = math.min(max, data.textScaleFactor);
    return MediaQuery(
      data: data.copyWith(textScaleFactor: scale),
      child: child,
    );
  }
}

class ScaleTextWidget extends StatelessWidget {
  final double scale;

  final Widget child;

  const ScaleTextWidget({
    Key key,
    @required this.scale,
    @required this.child,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    var data = MediaQuery.of(context);
    var scale = this.scale ?? data.textScaleFactor;
    return MediaQuery(
      data: data.copyWith(textScaleFactor: scale),
      child: child,
    );
  }
}

添加了一个单文件Example: main.dart,
可以动态演示当前的字体变化

使用 https://dartpad.cn/d28208f569b44f39dd572a7e8f455912 可以在线查看效果