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

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final GlobalKey text = GlobalKey();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Text.rich(TextSpan(
        children: <InlineSpan>[
          TextSpan(text:'a'),
          WidgetSpan(child: Text('b')), // It works if comment out this line.
          TextSpan(text: 'c'),
        ]
      ), key:text,),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          RenderParagraph paragraph = text.currentContext!.findRenderObject()! as RenderParagraph;
          final String string = paragraph.text.toPlainText();
          final Offset offset = paragraph.getOffsetForCaret(TextPosition(offset: string.length, affinity: TextAffinity.upstream), Rect.zero);
          showDialog<void>(
            context: context,
            barrierDismissible: false,
            builder: (BuildContext context) {
              return AlertDialog(
                title: const Text('getOffsetForCaret'),
                content: SingleChildScrollView(
                  child: ListBody(
                    children: <Widget>[
                      Text('string is $string'),
                      Text('string length is ${string.length}'),
                      Text('getOffsetForCaret at location "${string.length}" is $offset'),
                    ],
                  ),
                ),
                actions: <Widget>[
                  TextButton(
                    child: const Text('Approve'),
                    onPressed: () {
                      Navigator.of(context).pop();
                    },
                  ),
                ],
              );
            },
          );
        },
        child: Icon(Icons.ice_skating),
      ),
    );
  }
}