my_editor_mwp.dart

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

void main() async {
  return runApp(MaterialApp(
      theme: ThemeData(brightness: Brightness.light, primaryColor: Colors.red),
      home: const MyEditor()
      // debugShowCheckedModeBanner: false, theme: themeData, home: Document()
      ));
}

class MyEditor extends StatefulWidget {
  const MyEditor({Key? key}) : super(key: key);

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

class _MyEditorController extends State<MyEditor>
    implements DeltaTextInputClient {
  late FocusNode _focusNode;
  late TextInputConnection _inputConnection;
  // same constructor as in TexEditingValue
  TextSelection _selection = const TextSelection.collapsed(offset: -1);
  final _currentDoc = ValueNotifier("Initial Value");
  static const TextInputConfiguration mainTextInputConfiguration =
      TextInputConfiguration(
    inputType: TextInputType.multiline,
    readOnly: false,
    obscureText: false,
    autocorrect: false,
    // SmartDashesType?
    // SmartQuotesType?
    enableSuggestions: false,
    actionLabel: null,
    inputAction: TextInputAction.newline,
    keyboardAppearance: Brightness.light,
    textCapitalization: TextCapitalization.none,
    autofillConfiguration: AutofillConfiguration.disabled,
    enableIMEPersonalizedLearning: true,
    enableDeltaModel: true,
  );

  @override
  TextEditingValue get currentTextEditingValue =>
      TextEditingValue(text: _currentDoc.value, selection: _selection);

  @override
  void updateEditingValue(TextEditingValue value) {
    print("Received:  ${value.toString()}");
    _currentDoc.value = value.text;
    _selection = value.selection;
  }

  @override
  void updateEditingValueWithDeltas(List<TextEditingDelta> textEditingDeltas) {
    var editingValue = currentTextEditingValue;
    for (var delta in textEditingDeltas) {
      print("Received delta:  ${delta.toString()}");
      delta.apply(editingValue);
    }
    _currentDoc.value = editingValue.text;
    _selection = editingValue.selection;
  }

  @override
  void initState() {
    super.initState();
    _focusNode = FocusNode();
    _attachToInput();
  }

  void _attachToInput() {
    _inputConnection = TextInput.attach(this, mainTextInputConfiguration);
    _inputConnection.show();
  }

  @override
  Widget build(BuildContext contex) {
    if (!_focusNode.hasFocus) {
      _focusNode.requestFocus();
    }

    return Focus(
        focusNode: _focusNode,
        autofocus: true,
        child: ValueListenableBuilder(
            valueListenable: _currentDoc,
            builder: (BuildContext context, String doc, Widget? child) {
              return Text(doc);
            }));
  }

  // UNIMPLEMENTED METHODS

  @override
  void connectionClosed() => throw UnimplementedError();

  @override
  AutofillScope? get currentAutofillScope => throw UnimplementedError();

  @override
  void performPrivateCommand(String action, Map<String, dynamic> data) {}

  @override
  void showAutocorrectionPromptRect(int start, int end) =>
      throw UnimplementedError();

  @override
  void updateFloatingCursor(RawFloatingCursorPoint point) =>
      throw UnimplementedError();

  @override
  void performAction(TextInputAction action) => throw UnimplementedError();
}