flutter-htmltextview-demo.dart

import 'package:flutter/widgets.dart';
import 'package:flutter/material.dart';
import 'dart:convert';

/*
 * TextView with HTML tags support By Kyle Katarn for Dart
 *
 * Original code by Erik Arvidsson, Mozilla Public License
 * http://erik.eae.net/simplehtmlparser/simplehtmlparser.js
 * and ported it on JavaScript by John Resig (ejohn.org)
 */

import 'dart:ui';

class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
        return new MaterialApp(
                title: 'Flutter Demo',
                theme: new ThemeData(
                primarySwatch: Colors.blue,
            ),

            home: new MyHomePage(title: 'HtmlTextView Demo'),
        );
    }
}

// =====================================================================================================================

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

    final String title;

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

// =====================================================================================================================

class _MyHomePageState extends State<MyHomePage> {
    Widget buildListItem(BuildContext context, String item) {
        return new ListItem(
            title: new HtmlTextView(data: item)
        );
    }

    // =================================================================================================================

    @override
    Widget build(BuildContext context) {
        String lorem = '<b>Lorem</b> <i>ipsum</i> <u>dolor</u> <span style="color: #FF0000">sit</span> ' +
                        '<span style="text-decoration: underline; font-weight: bold; font-style: italic">amet</span>, ' +
                        '<a hef="#">consectetur adipiscing elit</a>, sed do eiusmod tempor incididunt ut ' +
                        'labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation';

        List<String> items = <String>[];

        for (var i = 0; i < 100; i++) {
            items.add(lorem);
        }

        Iterable<Widget> listItems = items.map((String item) => buildListItem(context, item));

        return new Scaffold(
            appBar: new AppBar(
                title: new Text(config.title),
            ),

            body: new Scrollbar(
                child: new MaterialList(
                    padding: new EdgeInsets.symmetric(vertical: 4.0),
                    children: listItems
                )
            )
        );
    }
}

// =====================================================================================================================

class HtmlTextView extends StatelessWidget {
    final String data;

    // =================================================================================================================

    HtmlTextView({
        this.data
    });

    // =================================================================================================================

    @override
    Widget build(BuildContext context) {
        HtmlParser parser = new HtmlParser();
        List nodes        = parser.parse(this.data);
        TextSpan span     = this._stackToTextSpan(nodes, context);
        RichText contents = new RichText(text: span);

        return new Container(
            padding: const EdgeInsets.all(16.0),
            child:   contents
        );
    }

    // =================================================================================================================

    TextSpan _stackToTextSpan(List nodes, BuildContext context) {
        List<TextSpan> children = <TextSpan>[];

        for (int i = 0; i < nodes.length; i++) {
            children.add(_textSpan(nodes[i]));
        }

        return new TextSpan(
            text:     '',
            style:    DefaultTextStyle.of(context).style,
            children: children
        );
    }

    // =================================================================================================================

    TextSpan _textSpan(Map node) {
        TextSpan span = new TextSpan(text: node['text'], style: node['style']);

        return span;
    }
}

// =====================================================================================================================

class HtmlParser {
    // Regular Expressions for parsing tags and attributes
    RegExp _startTag;
    RegExp _endTag;
    RegExp _attr;
    RegExp _style;
    RegExp _color;

    final List _emptyTags     = const ['area', 'base', 'basefont', 'br', 'col', 'frame', 'hr', 'img', 'input',
                                           'isindex', 'link', 'meta', 'param', 'embed'];
    final List _blockTags     = const ['address', 'applet', 'blockquote', 'button', 'center', 'dd', 'del', 'dir',
                                           'div', 'dl', 'dt', 'fieldset', 'form', 'frameset', 'hr', 'iframe', 'ins',
                                           'isindex', 'li', 'map', 'menu', 'noframes', 'noscript', 'object', 'ol',
                                           'p', 'pre', 'script', 'table', 'tbody', 'td', 'tfoot', 'th', 'thead',
                                           'tr', 'ul'];
    final List _inlineTags    = const ['a', 'abbr', 'acronym', 'applet', 'b', 'basefont', 'bdo', 'big', 'br', 'button',
                                           'cite', 'code', 'del', 'dfn', 'em', 'font', 'i', 'iframe', 'img', 'input',
                                           'ins', 'kbd', 'label', 'map', 'object', 'q', 's', 'samp', 'script',
                                           'select', 'small', 'span', 'strike', 'strong', 'sub', 'sup', 'textarea',
                                           'tt', 'u', 'var'];
    final List _closeSelfTags = const ['colgroup', 'dd', 'dt', 'li', 'options', 'p', 'td', 'tfoot', 'th', 'thead', 'tr'];
    final List _fillAttrs     = const ['checked', 'compact', 'declare', 'defer', 'disabled', 'ismap', 'multiple',
                                           'nohref', 'noresize', 'noshade', 'nowrap', 'readonly', 'selected'];
    final List _specialTags   = const ['script', 'style'];

    List _stack  = [];
    List _result = [];

    Map<String, dynamic> _tag;

    // =================================================================================================================

    HtmlParser() {
        this._startTag = new RegExp(r'^<([-A-Za-z0-9_]+)((?:\s+\w+(?:\s*=\s*(?:(?:"[^"]*")' + "|(?:'[^']*')|[^>\s]+))?)*)\s*(\/?)>");
        this._endTag   = new RegExp("^<\/([-A-Za-z0-9_]+)[^>]*>");
        this._attr     = new RegExp(r'([-A-Za-z0-9_]+)(?:\s*=\s*(?:(?:"((?:\\.|[^"])*)")' + r"|(?:'((?:\\.|[^'])*)')|([^>\s]+)))?");
        this._style    = new RegExp(r'([a-zA-Z\-]+)\s*:\s*([^;]*)');
        this._color    = new RegExp(r'^#([a-fA-F0-9]{6})$');
    }

    // =================================================================================================================

    List parse(String html) {
        String last = html;
        Match match;
        int index;
        bool chars;

        while (html.length > 0) {
            chars = true;

            // Make sure we're not in a script or style element
            if (this._getStackLastItem() == null || !this._specialTags.contains(this._getStackLastItem())) {
                // Comment
                if (html.indexOf('<!--') == 0) {
                    index = html.indexOf('-->');

                    if (index >= 0) {
                        html = html.substring(index + 3);
                        chars = false;
                    }
                }
                // End tag
                else if (html.indexOf('</') == 0) {
                    match = this._endTag.firstMatch(html);

                    if (match != null) {
                        String tag = match[0];

                        html  = html.substring(tag.length);
                        chars = false;

                        this._parseEndTag(tag);
                    }
                }
                // Start tag
                else if (html.indexOf('<') == 0) {
                    match = this._startTag.firstMatch(html);

                    if (match != null) {
                        String tag = match[0];

                        html  = html.substring(tag.length);
                        chars = false;

                        this._parseStartTag(tag, match[1], match[2], match.start);
                    }
                }

                if (chars) {
                    index = html.indexOf('<');

                    String text = (index < 0) ? html : html.substring(0, index);

                    html = (index < 0) ? '' : html.substring(index);

                    this._appendNode(text);
                }
            }
            else {
                RegExp re = new RegExp(r'(.*)<\/' + this._getStackLastItem() + r'[^>]*>');

                html = html.replaceAllMapped(re, (Match match) {
                    String text = match[0]
                               ..replaceAll(new RegExp('<!--(.*?)-->'), '\$1')
                               ..replaceAll(new RegExp('<!\[CDATA\[(.*?)]]>'), '\$1');

                    this._appendNode(text);

                    return '';
                });

                this._parseEndTag(this._getStackLastItem());
            }

            if (html == last) {
                throw 'Parse Error: ' + html;
            }

            last = html;
        }

        // Cleanup any remaining tags
        this._parseEndTag();

        List result = this._result;

        // Cleanup internal variables
        this._stack  = [];
        this._result = [];

        return result;
    }

    // =================================================================================================================

    void _parseStartTag(String tag, String tagName, String rest, int unary) {
        tagName = tagName.toLowerCase();

        if (this._blockTags.contains(tagName)) {
            while (this._getStackLastItem() != null && this._inlineTags.contains(this._getStackLastItem())) {
                this._parseEndTag(this._getStackLastItem());
            }
        }

        if (this._closeSelfTags.contains(tagName) && this._getStackLastItem() == tagName) {
            this._parseEndTag(tagName);
        }

        if (this._emptyTags.contains(tagName)) {
            unary = 1;
        }

        if (unary == 0) {
            this._stack.add(tagName);
        }

        Map attrs = {};

        Iterable<Match> matches = this._attr.allMatches(rest);

        if (matches != null) {
            for (Match match in matches) {
                String attribute = match[1];
                String value;

                if (match[2] != null) {
                    value = match[2];
                }
                else if (match[3] != null) {
                    value = match[3];
                }
                else if (match[4] != null) {
                    value = match[4];
                }
                else if (this._fillAttrs.contains(attribute) != null) {
                    value = attribute;
                }

                attrs[attribute] = value;
            }
        }

        this._appendTag(tagName, attrs);
    }

    // =================================================================================================================

    void _parseEndTag([String tagName]) {
        int pos;

        // If no tag name is provided, clean shop
        if (tagName == null) {
            pos = 0;
        }
        // Find the closest opened tag of the same type
        else {
            for (pos = this._stack.length - 1; pos >= 0; pos--) {
                if (this._stack[pos] == tagName) {
                    break;
                }
            }
        }

        if (pos >= 0) {
            // Remove the open elements from the stack
            this._stack.removeRange(pos, this._stack.length);
        }
    }

    // =================================================================================================================

    TextStyle _parseStyle(String tag, Map attrs) {
        Iterable<Match> matches;
        String style = attrs['style'];
        String param;
        String value;

        Color color                   = new Color(0xFF000000);
        FontWeight fontWeight         = FontWeight.normal;
        FontStyle fontStyle           = FontStyle.normal;
        TextDecoration textDecoration = TextDecoration.none;

        switch (tag) {
            case 'a':
                color = new Color(int.parse('0xFF1965B5'));
                break;

            case 'b':
            case 'strong':
                fontWeight = FontWeight.bold;
                break;

            case 'i':
            case 'em':
                fontStyle = FontStyle.italic;
                break;

             case 'u':
                textDecoration = TextDecoration.underline;
                break;
        }

        if (style != null) {
            matches = this._style.allMatches(style);

            for (Match match in matches) {
                param = match[1].trim();
                value = match[2].trim();

                switch (param) {
                    case 'color':
                        if (this._color.hasMatch(value)) {
                            value = value.replaceAll('#', '').trim();
                            color = new Color(int.parse('0xFF' + value));
                        }

                        break;

                     case 'font-weight':
                        fontWeight = (value == 'bold') ? FontWeight.bold : FontWeight.normal;

                        break;

                     case 'font-style':
                        fontStyle = (value == 'italic') ? FontStyle.italic : FontStyle.normal;

                        break;

                    case 'text-decoration':
                        textDecoration = (value == 'underline') ? TextDecoration.underline : TextDecoration.none;

                        break;
                }
            }
        }

        TextStyle textStyle = new TextStyle(
            color:      color,
            fontWeight: fontWeight,
            fontStyle:  fontStyle,
            decoration: textDecoration
        );

        return textStyle;
    }

    // =================================================================================================================

    void _appendTag(String tag, Map attrs) {
        this._tag = {
            'tag':   tag,
            'attrs': attrs
        };
    }

    // =================================================================================================================

    void _appendNode(String text) {
        if (this._tag == null) {
            this._tag = {
                'tag':   'p',
                'attrs': {}
            };
        }

        this._tag['text']  = text;
        this._tag['style'] = this._parseStyle(this._tag['tag'], this._tag['attrs']);
        this._tag['href']  = (this._tag['attrs']['href'] != null) ? this._tag['attrs']['href'] : '';

        this._tag.remove('attrs');

        this._result.add(this._tag);

        this._tag = null;
    }

    // =================================================================================================================

    String _getStackLastItem() {
        if (this._stack.length <= 0) {
            return null;
        }

        return this._stack[this._stack.length - 1];
    }
}

// =====================================================================================================================

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

Could you please guide me?

This is a very old snippet, try this widgets:
https://pub.dev/packages/flutter_html (official Google implementation)
https://pub.dev/packages/flutter_widget_from_html

I tried using the flutter_html but its not rendering as a Text-kind of widget and we don't have any control over the Text properties in there. We cant set the maxLines, etc. Here it is rendered as a RichText Widget, that's why this is a more suitable solution for me.

Unfortunately, I have not been involved in this script for a long time. You can try to remove br from _emptyTags, _inlineTags. Or add it to _blockTags. What exactly to change in the script logic to work as you need - now, unfortunately, I can not say.

@krishnakumarcn you can try this package simple_html_css
@Katarn if you can, I would really appreciate a PR to the above package cos I'm struggling with rendering <ol> <ul> tags.