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flutter_html

pub package codecov CircleCI MIT License

A Flutter widget for rendering HTML and CSS as Flutter widgets.

Screenshot 1 Screenshot 2 Screenshot 3
A Screenshot of flutter_html Another Screenshot of flutter_html Yet another Screenshot of flutter_html

Table of Contents:

Installing:

Add the following to your pubspec.yaml file:

dependencies:
  flutter_html: ^3.0.0-alpha.2

Currently Supported HTML Tags:

a abbr acronym address article aside audio b bdi bdo big
blockquote body br caption cite code data dd del details dfn
div dl dt em figcaption figure footer font h1 h2 h3
h4 h5 h6 header hr i iframe img ins kbd li
main mark nav noscript ol p pre q rp rt ruby
s samp section small span strike strong sub sup summary svg
table tbody td template tfoot th thead time tr tt u
ul var video math: mrow msup msub mover munder msubsup moverunder
mfrac mlongdiv msqrt mroot mi mn mo

Currently Supported CSS Attributes:

background-color color direction display font-family font-feature-settings font-size
font-style font-weight height letter-spacing line-height list-style-type list-style-position
padding margin text-align text-decoration text-decoration-color text-decoration-style text-decoration-thickness
text-shadow vertical-align white-space width word-spacing

Currently Supported Inline CSS Attributes:

background-color border (including specific directions) color direction display font-family font-feature-settings
font-size font-style font-weight line-height list-style-type list-style-position padding (including specific directions)
margin (including specific directions) text-align text-decoration text-decoration-color text-decoration-style text-shadow

Don't see a tag or attribute you need? File a feature request or contribute to the project!

Why this package?

This package is designed with simplicity in mind. Originally created to allow basic rendering of HTML content into the Flutter widget tree, this project has expanded to include support for basic styling as well! If you need something more robust and customizable, the package also provides a number of optional custom APIs for extremely granular control over widget rendering!

API Reference:

For the full API reference, see here.

For a full example, see here.

Below, you will find brief descriptions of the parameters theHtml widget accepts and some code snippets to help you use this package.

Constructors:

The package currently has two different constructors - Html() and Html.fromDom().

The Html() constructor is for those who would like to directly pass HTML from the source to the package to be rendered.

If you would like to modify or sanitize the HTML before rendering it, then Html.fromDom() is for you - you can convert the HTML string to a Document and use its methods to modify the HTML as you wish. Then, you can directly pass the modified Document to the package. This eliminates the need to parse the modified Document back to a string, pass to Html(), and convert back to a Document, thus cutting down on load times.

Selectable Text

The package also has two constructors for selectable text support - SelectableHtml() and SelectableHtml.fromDom().

The difference between the two is the same as noted above.

Please note: Due to Flutter #38474, selectable text support is significantly watered down compared to the standard non-selectable version of the widget. The changes are as follows:

  1. The list of tags that can be rendered is significantly reduced. Key omissions include no support for images/video/audio, table, and ul/ol.

  2. No support for customRender, customImageRender, onImageError, onImageTap, onMathError, and navigationDelegateForIframe. (Support for customRender may be added in the future).

  3. Styling support is significantly reduced. Only text-related styling works (e.g. bold or italic), while container related styling (e.g. borders or padding/margin) do not work.

Once the above issue is resolved, the aforementioned compromises will go away. Currently the SelectableText.rich() constructor does not support WidgetSpans, resulting in the feature losses above.

Parameters:

Parameters Description
data The HTML data passed to the Html widget. This is required and cannot be null when using Html().
document The DOM document passed to the Html widget. This is required and cannot be null when using Html.fromDom().
onLinkTap A function that defines what the widget should do when a link is tapped. The function exposes the src of the link as a String to use in your implementation.
customRenders A powerful API that allows you to customize everything when rendering a specific HTML tag.
onImageError A function that defines what the widget should do when an image fails to load. The function exposes the exception Object and StackTrace to use in your implementation.
shrinkWrap A bool used while rendering different widgets to specify whether they should be shrink-wrapped or not, like ContainerSpan
onImageTap A function that defines what the widget should do when an image is tapped. The function exposes the src of the image as a String to use in your implementation.
tagsList A list of elements the Html widget should render. The list should contain the tags of the HTML elements you wish to include.
style A powerful API that allows you to customize the style that should be used when rendering a specific HTMl tag.
selectionControls A custom text selection controls that allow you to override default toolbar and build toolbar with custom text selection options. See an example.

Methods:

Methods Description
disposeAll() Disposes all ChewieControllers, ChewieAudioControllers, and VideoPlayerControllers being used by every Html widget. (Note: Html widgets automatically dispose their controllers, this method is only provided in case you need other behavior)

Getters:

  1. Html.tags. This provides a list of all the tags the package renders. The main use case is to assist in excluding elements using tagsList. See an example below.

  2. SelectableHtml.tags. This provides a list of all the tags that can be rendered in selectable mode.

  3. Html.chewieAudioControllers. This provides a list of all ChewieAudioControllers being used by Html widgets.

  4. Html.chewieControllers. This provides a list of all ChewieControllers being used by Html widgets.

  5. Html.videoPlayerControllers. This provides a list of all VideoPlayerControllers being used for video widgets by Html widgets.

  6. Html.audioPlayerControllers. This provides a list of all VideoPlayerControllers being used for audio widgets by Html widgets.

Data:

The HTML data passed to the Html widget as a String. This is required and cannot be null when using Html. Any HTML tags in the String that are not supported by the package will not be rendered.

Example Usage - Data:

Widget html = Html(
  data: """<div>
        <h1>Demo Page</h1>
        <p>This is a fantastic product that you should buy!</p>
        <h3>Features</h3>
        <ul>
          <li>It actually works</li>
          <li>It exists</li>
          <li>It doesn't cost much!</li>
        </ul>
        <!--You can pretty much put any html in here!-->
      </div>""",
);

Document:

The DOM document passed to the Html widget as a Document. This is required and cannot be null when using Html.fromDom(). Any HTML tags in the document that are not supported by the package will not be rendered. Using the Html.fromDom() constructor can be useful when you would like to sanitize the HTML string yourself before passing it to the package.

Example Usage - Document:

import 'package:html/parser.dart' as htmlparser;
import 'package:html/dom.dart' as dom;
...
String htmlData = """<div>
  <h1>Demo Page</h1>
  <p>This is a fantastic product that you should buy!</p>
  <h3>Features</h3>
  <ul>
    <li>It actually works</li>
    <li>It exists</li>
    <li>It doesn't cost much!</li>
  </ul>
  <!--You can pretty much put any html in here!-->
</div>""";
dom.Document document = htmlparser.parse(htmlData);
/// sanitize or query document here
Widget html = Html(
  document: document,
);

onLinkTap:

A function that defines what the widget should do when a link is tapped.

Example Usage - onLinkTap:

Widget html = Html(
  data: """<p>
   Linking to <a href='https://github.com'>websites</a> has never been easier.
  </p>""",
  onLinkTap: (String? url, RenderContext context, Map<String, String> attributes, dom.Element? element) {
    //open URL in webview, or launch URL in browser, or any other logic here
  }
);

Inner links (such as <a href="#top">Back to the top</a> will work out of the box by scrolling the viewport, as long as your Html widget is wrapped in a scroll container such as a SingleChildScrollView.

customRenders:

A powerful API that allows you to customize everything when rendering a specific HTML tag. This means you can change the default behaviour or add support for HTML elements that aren't supported natively. You can also make up your own custom tags in your HTML!

customRender accepts a Map<CustomRenderMatcher, CustomRender>.

CustomRenderMatcher is a function that requires a bool to be returned. It exposes the RenderContext which provides BuildContext and access to the HTML tree.

The CustomRender class has two constructors: CustomRender.widget() and CustomRender.inlineSpan(). Both require a <Widget/InlineSpan> Function(RenderContext, Function()). The Function() argument is a function that will provide you with the element's children when needed.

To use this API, create a matching function and an instance of CustomRender.

Example Usages - customRenders:

Note: If you add any custom tags, you must add these tags to the tagsList parameter, otherwise they will not be rendered. See below for an example.

  1. Simple example - rendering custom HTML tags
Widget html = Html(
  data: """
  <h3>Display bird element and flutter element <bird></bird></h3>
  <flutter></flutter>
  <flutter horizontal></flutter>
  """,
  customRenders: {
      birdMatcher(): CustomRender.inlineSpan(inlineSpan: (context, buildChildren) => TextSpan(text: "🐦")),
      flutterMatcher(): CustomRender.widget(widget: (context, buildChildren) => FlutterLogo(
        style: (context.tree.element!.attributes['horizontal'] != null)
            ? FlutterLogoStyle.horizontal
            : FlutterLogoStyle.markOnly,
        textColor: context.style.color!,
        size: context.style.fontSize!.size! * 5,
      )),
    },
  tagsList: Html.tags..addAll(["bird", "flutter"]),
);

CustomRenderMatcher birdMatcher() => (context) => context.tree.element?.localName == 'bird';

CustomRenderMatcher flutterMatcher() => (context) => context.tree.element?.localName == 'flutter';
  1. Complex example - wrapping the default widget with your own, in this case placing a horizontal scroll around a (potentially too wide) table.

Note: Requires the flutter_html_table package.

View code
Widget html = Html(
  data: """
  <table style="width:100%">
    <caption>Monthly savings</caption>
    <tr> <th>January</th> <th>February</th> <th>March</th> <th>April</th> <th>May</th> <th>June</th> <th>July</th> <th>August</th> <th>September</th> <th>October</th> <th>November</th> <th>December</th> </tr>
    <tr> <td>\$100</td> <td>\$50</td> <td>\$80</td> <td>\$60</td> <td>\$90</td> <td>\$140</td> <td>\$110</td> <td>\$80</td> <td>\$90</td> <td>\$60</td> <td>\$40</td> <td>\$70</td> </tr>
    <tr> <td>\90</td> <td>\$60</td> <td>\$80</td> <td>\$80</td> <td>\$100</td> <td>\$160</td> <td>\$150</td> <td>\$110</td> <td>\$100</td> <td>\$60</td> <td>\$30</td> <td>\$80</td> </tr>
  </table>
  """,
  customRenders: {
    tableMatcher(): CustomRender.widget(widget: (context, child) {
      return SingleChildScrollView(
        scrollDirection: Axis.horizontal,
        // this calls the table CustomRender to render a table as normal (it uses a widget so we know widget is not null)
        child: tableRender.call().widget!.call(context, buildChildren),
      );
    }),
  },
);

CustomRenderMatcher tableMatcher() => (context) => context.tree.element?.localName == "table";
  1. Complex example - rendering an iframe differently based on whether it is an embedded youtube video or some other embedded content.
View code
Widget html = Html(
   data: """
   <h3>Google iframe:</h3>
   <iframe src="https://google.com"></iframe>
   <h3>YouTube iframe:</h3>
   <iframe src="https://www.youtube.com/embed/tgbNymZ7vqY"></iframe>
   """,
   customRenders: {
      iframeYT(): CustomRender.widget(widget: (context, buildChildren) {
        double? width = double.tryParse(context.tree.attributes['width'] ?? "");
        double? height = double.tryParse(context.tree.attributes['height'] ?? "");
        return Container(
          width: width ?? (height ?? 150) * 2,
          height: height ?? (width ?? 300) / 2,
          child: WebView(
            initialUrl: context.tree.attributes['src']!,
            javascriptMode: JavascriptMode.unrestricted,
            navigationDelegate: (NavigationRequest request) async {
              //no need to load any url besides the embedded youtube url when displaying embedded youtube, so prevent url loading
              if (!request.url.contains("youtube.com/embed")) {
                return NavigationDecision.prevent;
              } else {
                return NavigationDecision.navigate;
              }
            },
          ),
        );
      }),
      iframeOther(): CustomRender.widget(widget: (context, buildChildren) {
        double? width = double.tryParse(context.tree.attributes['width'] ?? "");
        double? height = double.tryParse(context.tree.attributes['height'] ?? "");
        return Container(
          width: width ?? (height ?? 150) * 2,
          height: height ?? (width ?? 300) / 2,
          child: WebView(
            initialUrl: context.tree.attributes['src'],
            javascriptMode: JavascriptMode.unrestricted,
            //on other iframe content scrolling might be necessary, so use VerticalDragGestureRecognizer
            gestureRecognizers: [
              Factory(() => VerticalDragGestureRecognizer())
            ].toSet(),
          ),
        );
      }),
      iframeNull(): CustomRender.widget(widget: (context, buildChildren) => Container(height: 0, width: 0)),
   }
 );

CustomRenderMatcher iframeYT() => (context) => context.tree.element?.attributes['src']?.contains("youtube.com/embed") ?? false;

CustomRenderMatcher iframeOther() => (context) => !(context.tree.element?.attributes['src']?.contains("youtube.com/embed")
  ?? context.tree.element?.attributes['src'] == null);

CustomRenderMatcher iframeNull() => (context) => context.tree.element?.attributes['src'] == null;

More example usages and in-depth details available here.

onImageError:

A function that defines what the widget should do when an image fails to load. The function exposes the exception Object and StackTrace to use in your implementation.

Example Usage - onImageError:

Widget html = Html(
  data: """<img alt='Alt Text of an intentionally broken image' src='https://www.google.com/images/branding/googlelogo/2x/googlelogo_color_92x30d'/>""",
  onImageError: (Exception exception, StackTrace stackTrace) {
    FirebaseCrashlytics.instance.recordError(exception, stackTrace);
  },
);

onImageTap:

A function that defines what the widget should do when an image is tapped.

Example Usage - onImageTap:

Widget html = Html(
  data: """<img alt='Google' src='https://www.google.com/images/branding/googlelogo/2x/googlelogo_color_92x30dp.png' />""",
  onImageTap: (String? url, RenderContext context, Map<String, String> attributes, dom.Element? element) {
    //open image in webview, or launch image in browser, or any other logic here
  }
);

tagsList:

A list of elements the Html widget should render. The list should contain the tags of the HTML elements you wish to whitelist.

Example Usage - tagsList - Excluding Tags:

You may have instances where you can choose between two different types of HTML tags to display the same content. In the example below, the <video> and <iframe> elements are going to display the same content.

The tagsList parameter allows you to change which element is rendered. Iframes can be advantageous because they allow parallel loading - Flutter just has to wait for the webview to be initialized before rendering the page, possibly cutting down on load time. Video can be advantageous because it provides a 100% native experience with Flutter widgets, but it may take more time to render the page. You may know that Flutter webview is a little janky in its current state on Android, so using tagsList and a simple condition, you can get the best of both worlds - choose the video widget to render on Android and the iframe webview to render on iOS.

Widget html = Html(
  data: """
  <video controls>
    <source src="https://www.w3schools.com/html/mov_bbb.mp4" />
  </video>
  <iframe src="https://www.w3schools.com/html/mov_bbb.mp4"></iframe>""",
  tagsList: Html.tags..remove(Platform.isAndroid ? "iframe" : "video")
);

Html.tags provides easy access to a list of all the tags the package can render, and you can remove specific tags from this list to blacklist them.

Example Usage - tagsList - Allowing Tags:

You may also have instances where you would only like the package to render a handful of html tags. You can do that like so:

Widget html = Html(
  data: """
    <p>Render this item</p>
    <span>Do not render this item or any other item</span>
    <img src='https://flutter.dev/images/flutter-mono-81x100.png'/>
  """,
  tagsList: ['p']
);

Here, the package will only ever render <p> and ignore all other tags.

style:

A powerful API that allows you to customize the style that should be used when rendering a specific HTMl tag.

style accepts a Map<String, Style>. The Style type is a class that allows you to set all the CSS styling the package currently supports. See here for the full list.

To use this API, set the key as the tag of the HTML element you wish to provide a custom implementation for, and set the value to be a Style with your customizations.

Example Usage - style:

Widget html = Html(
  data: """
    <h1>Table support:</h1>
    <table>
    <colgroup>
    <col width="50%" />
    <col span="2" width="25%" />
    </colgroup>
    <thead>
    <tr><th>One</th><th>Two</th><th>Three</th></tr>
    </thead>
    <tbody>
    <tr>
    <td rowspan='2'>Rowspan\nRowspan\nRowspan\nRowspan\nRowspan\nRowspan\nRowspan\nRowspan\nRowspan\nRowspan</td><td>Data</td><td>Data</td>
    </tr>
    <tr>
    <td colspan="2"><img alt='Google' src='https://www.google.com/images/branding/googlelogo/2x/googlelogo_color_92x30dp.png' /></td>
    </tr>
    </tbody>
    <tfoot>
    <tr><td>fData</td><td>fData</td><td>fData</td></tr>
    </tfoot>
    </table>""",
  style: {
    // tables will have the below background color
    "table": Style(
      backgroundColor: Color.fromARGB(0x50, 0xee, 0xee, 0xee),
    ),
    // some other granular customizations are also possible
    "tr": Style(
      border: Border(bottom: BorderSide(color: Colors.grey)),
    ),
    "th": Style(
      padding: EdgeInsets.all(6),
      backgroundColor: Colors.grey,
    ),
    "td": Style(
      padding: EdgeInsets.all(6),
      alignment: Alignment.topLeft,
    ),
    // text that renders h1 elements will be red
    "h1": Style(color: Colors.red),
  }
);

More examples and in-depth details available here.

Rendering Reference

This section will describe how certain HTML elements are rendered by this package, so you can evaluate how your HTML will be rendered and structure it accordingly.

Image

This package currently has support for base64 images, asset images, and network images.

The package uses the src of the image to determine which of the above types to render. The order is as follows:

  1. If the src is null, render the alt text of the image, if any.
  2. If the src starts with "data:image" and contains "base64," (this indicates the image data is indeed base64), render an Image.memory from the base64 data.
  3. If the src starts with "asset:", render an Image.asset from the path in the src.
  4. Otherwise, just render an Image.network.

If the rendering of any of the above fails, the package will fall back to rendering the alt text of the image, if any.

Currently the package only considers the width, height, src, and alt text while rendering an image.

If you would like to support SVGs in an <img>, you should use the flutter_html_svg package which provides support for base64, asset, and network SVGs.

External Packages

flutter_html_all

This package is simply a convenience package that exports all the other external packages below. You should use this if you plan to activate all the renders that require external dependencies.

flutter_html_audio

This package renders audio elements using the chewie_audio and the video_player plugin.

The package considers the attributes controls, loop, src, autoplay, width, and muted when rendering the audio widget.

Registering the CustomRender:

Widget html = Html(
  customRenders: {
    audioMatcher(): audioRender(),
  }
);

flutter_html_iframe

This package renders iframes using the webview_flutter plugin.

When rendering iframes, the package considers the width, height, and sandbox attributes.

Sandbox controls the JavaScript mode of the webview - a value of null or allow-scripts will set javascriptMode: JavascriptMode.unrestricted, otherwise it will set javascriptMode: JavascriptMode.disabled.

Registering the CustomRender:

Widget html = Html(
  customRenders: {
    iframeMatcher(): iframeRender(),
  }
);

You can set the navigationDelegate of the webview with the navigationDelegate property on iframeRender. This allows you to block or allow the loading of certain URLs.

NavigationDelegate example:

Widget html = Html(
  customRenders: {
    iframeMatcher(): iframeRender(navigationDelegate: (NavigationRequest request) {
      if (request.url.contains("google.com/images")) {
        return NavigationDecision.prevent;
      } else {
        return NavigationDecision.navigate;
      }
    }),
  }
);

flutter_html_math

This package renders MathML elements using the flutter_math_fork plugin.

When rendering MathML, the package takes the MathML data within the <math> tag and tries to parse it to Tex. Then, it will pass the parsed string to flutter_math_fork.

Because this package is parsing MathML to Tex, it may not support some functionalities. The current list of supported tags can be found above, but some of these only have partial support at the moment.

Registering the CustomRender:

Widget html = Html(
  customRenders: {
    mathMatcher(): mathRender(),
  }
);

If the parsing errors, you can use the onMathError property of mathRender to catch the error and potentially fix it on your end.

The function exposes the parsed Tex String, as well as the error and error with type from flutter_math_fork as a String.

You can analyze the error and the parsed string, and finally return a new instance of Math.tex() with the corrected Tex string.

onMathError example:

Widget html = Html(
  customRenders: {
    mathMatcher(): mathRender(onMathError: (tex, exception, exceptionWithType) {
      print(exception);
      //optionally try and correct the Tex string here
      return Text(exception);
    }),
  }
);

If you'd like to see more MathML features, feel free to create a PR or file a feature request!

Tex

If you have a Tex string you'd like to render inside your HTML you can do that using the same flutter_math_fork plugin.

Use a custom tag inside your HTML (an example could be <tex>), and place your raw Tex string inside.

Then, use the customRender parameter to add the widget to render Tex. It could look like this:

Widget htmlWidget = Html(
  data: r"""<tex>i\hbar\frac{\partial}{\partial t}\Psi(\vec x,t) = -\frac{\hbar}{2m}\nabla^2\Psi(\vec x,t)+ V(\vec x)\Psi(\vec x,t)</tex>""",
  customRenders: {
    texMatcher(): CustomRender.widget(widget: (context, buildChildren) => Math.tex(
      context.tree.element?.innerHtml ?? '',
      mathStyle: MathStyle.display,
      textStyle: context.style.generateTextStyle(),
      onErrorFallback: (FlutterMathException e) {
        //optionally try and correct the Tex string here
        return Text(e.message);
      },
    )),
  },
  tagsList: Html.tags..add('tex'),
);

CustomRenderMatcher texMatcher() => (context) => context.tree.element?.localName == 'tex';

flutter_html_svg

This package renders svg elements using the flutter_svg plugin.

When rendering SVGs, the package takes the SVG data within the <svg> tag and passes it to flutter_svg. The width and height attributes are considered while rendering, if given.

The package also exposes a few ways to render SVGs within an <img> tag, specifically base64 SVGs, asset SVGs, and network SVGs.

Registering the CustomRender:

Widget html = Html(
  customRenders: {
    svgTagMatcher(): svgTagRender(),
    svgDataUriMatcher(): svgDataImageRender(),
    svgAssetUriMatcher(): svgAssetImageRender(),
    svgNetworkSourceMatcher(): svgNetworkImageRender(),
  }
);

flutter_html_table

This package renders table elements using the flutter_layout_grid plugin.

When rendering table elements, the package tries to calculate the best fit for each element and size its cell accordingly. Rowspans and colspans are considered in this process, so cells that span across multiple rows and columns are rendered as expected. Heights are determined intrinsically to maintain an optimal aspect ratio for the cell.

Registering the CustomRender:

Widget html = Html(
  customRenders: {
    tableMatcher(): tableRender(),
  }
);

flutter_html_video

This package renders video elements using the chewie and the video_player plugin.

The package considers the attributes controls, loop, src, autoplay, poster, width, height, and muted when rendering the video widget.

Registering the CustomRender:

Widget html = Html(
  customRenders: {
    videoMatcher(): videoRender(),
  }
);

Notes

  1. If you'd like to use this widget inside of a Row(), make sure to set shrinkWrap: true and place your widget inside expanded:
Widget row = Row(
   children: [
        Expanded(
            child: Html(
              shrinkWrap: true,
              //other params
            )
        ),
	    //whatever other widgets
   ]
);

Migration Guides

  • For Version 1.0 - Guide

Contribution Guide

Coming soon!

Meanwhile, PRs are always welcome