Flutter Widget from HTML (core)

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A Flutter plugin for building Flutter-widget tree from html.

This core package implements html parsing and widget building logic so it's easy to extend and fit your app's use case. It tries to render an optimal tree: use Text instead of RichText as much as possible, merge text spans together, show images in AspectRatio, etc.

If this is your first time here, consider using the flutter_widget_from_html package as a quick starting point.

Usage

To use this plugin, add flutter_widget_from_html_core as a dependency in your pubspec.yaml file.

See the Example app for inspiration.

Example

const kHtml = """<h1>Heading 1</h1>
<h2>Heading 2</h2>
<h3>Heading 3</h3>
<h4>Heading 4</h4>
<h5>Heading 5</h5>
<h6>Heading 6</h6>
<p>A paragraph with <strong>strong</strong> <em>emphasized</em> text.</p>

<p>And of course, cat image:</p>
<figure>
  <img src="https://media.giphy.com/media/6VoDJzfRjJNbG/giphy-downsized.gif" width="250" height="171" />
  <figcaption>Source: <a href="https://gph.is/QFgPA0">https://gph.is/QFgPA0</a></figcaption>
</figure>
""";

class HelloWorldCoreScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) => Scaffold(
        appBar: AppBar(
          title: Text('HelloWorldCoreScreen'),
        ),
        body: HtmlWidget(
          kHtml,
          onTapUrl: (url) => showDialog(
                context: context,
                builder: (_) => AlertDialog(
                      title: Text('onTapUrl'),
                      content: Text(url),
                    ),
              ),
        ),
      );
}

Features

HTML tags

Below tags are the ones that have special meaning / styling, all other tags will be parsed as text.

  • A: underline, blue color, no default onTap action (use flutter_widget_from_html for that)
  • H1/H2/H3/H4/H5/H6
  • IMG with support for asset (asset://), data uri and network image without caching (use flutter_widget_from_html for that)
  • LI/OL/UL with support for:
    • Attributes: type, start, reversed
    • Inline style list-style-type values: lower-alpha, upper-alpha, lower-latin, upper-latin, circle, decimal, disc, lower-roman, upper-roman, square
  • TABLE/CAPTION/THEAD/TBODY/TFOOT/TR/TD/TH with support for:
    • Attribute: <table border="1">
    • Inline style: <table style="border: 1px solid #f00">
  • ABBR, ACRONYM, ADDRESS, ARTICLE, ASIDE, B, BIG, BLOCKQUOTE, BR, CENTER, CITE, CODE, DD, DEL, DFN, DIV, DL, DT, EM, FIGCAPTION, FIGURE, FONT, FOOTER, HEADER, HR, I, IMG, INS, KBD, MAIN, NAV, P, PRE, Q, S, SAMP, SECTION, STRIKE, STRONG, TT, U, VAR

However, these tags and their contents will be ignored:

Inline stylings

  • border-top, border-bottom: overline/underline with support for dashed/dotted/double/solid style
  • color: hex values only (#F00, #0F08, #00FF00 or #00FF0080)
  • font-family
  • font-size: absolute (e.g. xx-large), relative (larger, smaller) and value in em/px
  • font-style: italic/normal
  • font-weight: bold/normal/100..900
  • margin, margin-top, margin-right, margin-bottom, margin-left (values in px only)
  • text-align: center/justify/left/right
  • text-decoration: line-through/none/overline/underline

Extensibility

As previously mentioned, this package focuses on the core parsing-building routine with lots of tests to make sure it works correctly. If the flutter_widget_from_html package does not suite your need or you don't like any of the dependencies included in that package, it's time to extend flutter_widget_from_html_core.

Here is how it works:

  1. HtmlWidget parses input html into dom nodes
  2. Builder loops through each node, looking for NodeMetadata (text size, styling, hyperlink or image source, etc.)
  3. Use the metadata to build widget via WidgetFactory

If you want to, you can change the way metadata is collected (in step 2) and build widget however you like (in step 3) by extending the WidgetFactory and give it to HtmlWidget. The example below replace smilie inline image with an emoji:

const kHtml = """
<p>Hello <img class="smilie smilie-1" alt=":)" src="http://domain.com/sprites.png" />!</p>
<p>How are you <img class="smilie smilie-2" alt=":P" src="http://domain.com/sprites.png" />?
""";

const kSmilies = {':)': '🙂'};

class SmilieScreen extends StatelessWidget {
  final smilieOp = BuildOp(
    onPieces: (meta, pieces) {
      final alt = meta.domElement.attributes['alt'];
      final text = kSmilies.containsKey(alt) ? kSmilies[alt] : alt;
      return pieces..first?.block?.addText(text);
    },
  );

  @override
  Widget build(BuildContext context) => Scaffold(
        appBar: AppBar(
          title: Text('SmilieScreen'),
        ),
        body: HtmlWidget(
          kHtml,
          builderCallback: (meta, e) => e.classes.contains('smilie')
              ? lazySet(null, buildOp: smilieOp)
              : meta,
        ),
      );
}

Libraries

flutter_widget_from_html_core