Implementation of common "Read more" feature to truncate and expand lengthy text on demand

read_more.dart

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

class ReadMore extends StatefulWidget {
  final String text;
  final TextStyle? textStyle;
  final String readMoreText;
  final TextStyle? readMoreTextStyle;
  final int maxLines;
  final int steps;
  // final bool shouldUseSteps;
  const ReadMore({
    Key? key,
    this.readMoreText = 'Read more',
    this.readMoreTextStyle,
    this.maxLines = 4,
    required this.text,
    this.textStyle,
    this.steps = 4,
    // required this.shouldUseSteps,
  }) : super(key: key);

  @override
  State<ReadMore> createState() => _ReadMoreState();
}

class _ReadMoreState extends State<ReadMore> {
  late int _maxLines = widget.maxLines;

  String get _readMoreText => ' ' + widget.readMoreText;

  @override
  Widget build(BuildContext context) {
    return _buildMessage();
  }

  TextSpan _bodyTextSpan() {
    return TextSpan(text: widget.text, style: widget.textStyle);
  }

  TextSpan _readMoreTextSpan() {
    return TextSpan(
      text: _readMoreText,
      style: widget.readMoreTextStyle,
      recognizer: TapGestureRecognizer()
        ..onTap = () {
          setState(() {
            _maxLines += widget.steps;
          });
        },
    );
  }

  RichText _buildMessage() {
    const double maxWidth = 300;
    const double minWidth = 0;

    TextPainter textPainter = TextPainter(
      text: _readMoreTextSpan(),
      textDirection: TextDirection.ltr,
      maxLines: _maxLines,
      ellipsis: '',
    );
    textPainter.layout(minWidth: minWidth, maxWidth: maxWidth);
    final linkSize = textPainter.size;

    textPainter.text = _bodyTextSpan();
    textPainter.layout(minWidth: minWidth, maxWidth: maxWidth);
    final textSize = textPainter.size;

    int? endIndex;
    final pos = textPainter.getPositionForOffset(Offset(
      textSize.width - linkSize.width,
      textSize.height,
    ));
    endIndex = textPainter.getOffsetBefore(pos.offset);
    TextSpan textSpan;
    if (textPainter.didExceedMaxLines) {
      textSpan = TextSpan(
        text: widget.text.substring(0, endIndex),
        children: <TextSpan>[_readMoreTextSpan()],
      );
    } else {
      textSpan = TextSpan(
        text: widget.text,
      );
    }
    return RichText(
      softWrap: true,
      text: textSpan,
    );
  }
}