A Widget to show effects of some simple html tags

html_span_widget.dart

import 'package:flutter/material.dart';
import 'package:html/dom.dart' as dom;
import 'package:html/parser.dart';

class HtmlSpanView extends StatefulWidget {
  final String content;

  const HtmlSpanView(this.content);

  @override
  State<StatefulWidget> createState() => _HtmlSpanState();
}

class _HtmlSpanState extends State<HtmlSpanView> {
  bool _didFailToParse = false;
  dom.Node _document;

  @override
  void initState() {
    super.initState();
    _parseContent();
  }

  @override
  void didUpdateWidget(HtmlSpanView oldWidget) {
    _parseContent();
  }

  @override
  Widget build(BuildContext context) {
    if (_didFailToParse) {
      return Text(widget.content);
    }

    return SpanViewBuilder(_document.nodes);
  }

  void _parseContent() {
    try {
      var document = parse(widget.content);
      setState(() {
        _document = document.body;
      });
    } on Exception catch (_) {
      setState(() {
        _didFailToParse = true;
      });
    }
  }
}

class SpanViewBuilder extends StatelessWidget {
  final List<dom.Node> nodes;

  const SpanViewBuilder(this.nodes);

  @override
  Widget build(BuildContext context) {
    if (nodes.isEmpty) {
      return SizedBox.shrink();
    }
    if (nodes.length == 1) {
      return Text.rich(nodeToSpan(nodes.single));
    }
    return Text.rich(
      TextSpan(children: nodesToSpan(nodes)),
    );
  }
}

@visibleForTesting
List<TextSpan> nodesToSpan(List<dom.Node> nodes) {
  return nodes.map((node) => nodeToSpan(node)).toList();
}

@visibleForTesting
TextSpan nodeToSpan(dom.Node node) {
  if (node is dom.Text) {
    return TextSpan(text: node.data);
  }

  if (node is dom.Element) {
    var name = node.localName;
    switch (name) {
      case 'b':
        return BoldSpan(node.nodes);
      case 'font':
        var color;
        try {
          var str = node.attributes['color'];
          if (str == null) {
            throw FormatException();
          }
          var raw = int.parse(str, radix: 16);
          color = Color.fromRGBO(raw >> 16, (raw >> 8) & 0xFF, raw & 0xFF, 1.0);
        } on FormatException {
          color = Colors.black;
        }
        return FontSpan(color, node.nodes);
      default:
        return TextSpan(children: nodesToSpan(node.nodes));
    }
  }

  return TextSpan(text: node.toString());
}

class BoldSpan extends TextSpan {
  BoldSpan(dom.NodeList nodes)
      : super(
          children: nodesToSpan(nodes),
          style: TextStyle(fontWeight: FontWeight.bold),
        );
}

class FontSpan extends TextSpan {
  final Color color;

  FontSpan(this.color, dom.NodeList nodes)
      : super(
          children: nodesToSpan(nodes),
          style: TextStyle(color: color),
        );
}