Flutter Keyboard emulation, caret position and selection, para Manu.

main.dart

import 'dart:ui';

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Manu Keyboard'),
    );
  }
}

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          crossAxisAlignment: CrossAxisAlignment.center,
          children: <Widget>[
            buildInput(),
            _row([
              _btn("1", () => "1"),
              _btn("2", () => "2"),
              _btn("3", () => "3"),
            ]),
            _row([
              _btn("4", () => "4"),
              _btn("5", () => "5"),
              _btn("6", () => "6"),
            ]),
            _row([
              _btn("7", () => "7"),
              _btn("8", () => "8"),
              _btn("9", () => "9"),
            ]),
            _row([
              _btn(".", () => ","),
              _btn("0", () => "0"),
              _btn("C", () => "C"),
            ]),
          ],
        ),
      ),
    );
  }

  _row(List<Widget> children) {
    return Padding(
      padding: const EdgeInsets.all(8.0),
      child: Row(
        crossAxisAlignment: CrossAxisAlignment.center,
        mainAxisSize: MainAxisSize.min,
        children: children,
      ),
    );
  }

  Widget _btn(String label, Function callback) {
    return MaterialButton(
      padding: EdgeInsets.all(16),
      child: Text(
        label,
        style: TextStyle(color: Colors.white),
      ),
      shape: CircleBorder(),
      color: Colors.blue,
      highlightColor: Colors.black12,
      onPressed: () {
        var res = callback();
        if (res == 'C') {
          _clearText();
        } else {
          _addChar(res);
        }
      },
    );
  }

  void _clearText() {
    _textController.clear();
  }

  TextEditingController _textController;
  FocusNode _focusNode;
  TextSelection _lastSelection;

  @override
  void initState() {
    _textController = TextEditingController();
    _focusNode = FocusNode();
    _focusNode.addListener(() {
      if (!_focusNode.hasFocus && _focusNode.canRequestFocus) {
        _focusNode.requestFocus();
        _lastSelection = _textController.selection;
      } else {
        if (_lastSelection != null) {
          _textController.selection = _lastSelection;
        }
      }
    });
    super.initState();
  }

  Widget buildInput() {
    return SizedBox(
      width: 300,
      child: TextField(
        controller: _textController,
        focusNode: _focusNode,
        decoration: InputDecoration(
          border: OutlineInputBorder(),
        ),
      ),
    );
  }

  void _addChar(String char) {
    if (!_focusNode.hasFocus) {
      _focusNode.requestFocus();
    }
    if (_lastSelection == null) {
      _lastSelection = _textController.selection;
      print('asd $_lastSelection');
    }
    var txt = _textController.text;
    var base = _lastSelection.baseOffset;
    var part1 = base == -1 ? '' : txt.substring(0, _lastSelection.baseOffset);
    var part2 = _lastSelection.extentOffset == -1
        ? ''
        : txt.substring(_lastSelection.extentOffset);

    // evitar problema de cursor cuando no tiene foco el textfield.
    if (base == -1) base = 0;
    var caretPos = base + char.length;
    // _lastSelection.extentOffset == _lastSelection.baseOffset ? CURSOR : SELECCION

    _textController.text = part1 + char + part2;

    _lastSelection =
        _lastSelection.copyWith(extentOffset: caretPos, baseOffset: caretPos);
    _textController.selection = _lastSelection;
  }
}