vtt.js

Implementation of the WebVTT spec in JavaScript. Can be used in NodeJS, on the browser, and many other places. Mozilla uses this implementation for parsing and processing WebVTT files in Firefox/Gecko.

Table of Contents

• Spec Compliance
• API
  • WebVTT.Parser
    • parse(data)
    • flush()
    • onregion(region)
    • oncue(cue)
    • onflush()
    • onparsingerror(error)
  • WebVTT.convertCueToDOMTree(window, cuetext)
  • WebVTT.processCues(window, cues, overlay)
  • ParsingError
  • VTTCue
    • Extended API
      • toJSON()
      • VTTCue.fromJSON(json)
      • VTTCue.create(options)
  • VTTRegion
    • Extended API
      • VTTRegion.fromJSON(json)
      • VTTRegion.create(options)
• Browser
  • Building Yourself
  • Bower
  • Usage
• Node
  • vtt.js
  • node-vtt
• Developing vtt.js
  • Tests
    • Writing Tests
  • Grunt Run Task

Spec Compliance

• Parsing (Completed)
  • File (Completed)
  • Region (Completed)
  • Cue Timings and Settings (Completed)
  • WebVTT Cue Text (Completed)
  • Cue DOM Construction (Completed)
• Rendering (In Progress)
  • Processing Model (In Progress) No VTTRegion or vertical text support
    • Apply WebVTT Cue Settings (In Progress)
      • Steps 1 - 11 (Completed)
      • Step 12 (In progress)
  • Applying CSS Properties (Completed)
  • CSS Extensions (Won't Implement)
• WebVTT API Shim (Completed)
  • VTTCue (Completed) Shims the TextTrackCue interface as well
  • VTTRegion (Completed)

Notes

Our processing model portion of the specification makes use of a custom property, hasBeenReset. It allows us to detect when a VTTCue is dirty, i.e. one of its properties that affects display has changed and so we need to recompute its display state. This allows us to reuse a cue's display state if it has already been computed and nothing has changed to effect its position.

API

WebVTT.Parser

The parser has a simple API:

var parser = new WebVTT.Parser(window, stringDecoder);
parser.onregion = function(region) {};
parser.oncue = function(cue) {};
parser.onflush = function() {};
parser.onparsingerror = function(e) {};
parser.parse(moreData);
parser.parse(moreData);
parser.flush();

The Parser constructor is passed a window object with which it will create new VTTCues and VTTRegions as well as an optional StringDecoder object which it will use to decode the data that the parse() function receives. For ease of use, a StringDecoder is provided via WebVTT.StringDecoder(). If a custom StringDecoder object is passed in it must support the API specified by the #whatwg string encoding spec.

parse(data)

Hands data in some format to the parser for parsing. The passed data format is expected to be decodable by the StringDecoder object that it has. The parser decodes the data and reassembles partial data (streaming), even across line breaks.

var parser = new WebVTT.Parser(window, WebVTT.StringDecoder());
parser.parse("WEBVTT\n\n");
parser.parse("00:32.500 --> 00:33.500 align:start size:50%\n");
parser.parse("<v.loud Mary>That's awesome!");
parser.flush();

flush()

Indicates that no more data is expected and will force the parser to parse any unparsed data that it may have. Will also trigger onflush.

onregion(region)

Callback that is invoked for every region that is correctly parsed. Returns a VTTRegion object.

parser.onregion = function(region) {
  console.log(region);
};

oncue(cue)

Callback that is invoked for every cue that is fully parsed. In case of streaming parsing oncue is delayed until the cue has been completely received. Returns a VTTCue object.

parser.oncue = function(cue) {
  console.log(cue);
};

onflush()

Is invoked in response to flush() and after the content was parsed completely.

parser.onflush = function() {
  console.log("Flushed");
};

onparsingerror(error)

Is invoked when a parsing error has occured. This means that some part of the WebVTT file markup is badly formed. See ParsingError for more information.

parser.onparsingerror = function(e) {
  console.log(e);
};

WebVTT.convertCueToDOMTree(window, cuetext)

Parses the cue text handed to it into a tree of DOM nodes that mirrors the internal WebVTT node structure of the cue text. It uses the window object handed to it to construct new HTMLElements and returns a tree of DOM nodes attached to a top level div.

var div = WebVTT.convertCueToDOMTree(window, cuetext);

WebVTT.processCues(window, cues, overlay)

Converts the cuetext of the cues passed to it to DOM trees—by calling convertCueToDOMTree—and then runs the processing model steps of the WebVTT specification on the divs. The processing model applies the necessary CSS styles to the cue divs to prepare them for display on the web page. During this process the cue divs get added to a block level element (overlay). The overlay should be a part of the live DOM as the algorithm will use the computed styles (only of the divs to do overlap avoidance.

var divs = WebVTT.processCues(window, cues, overlay);

ParsingError

A custom JS error object that is reported through the onparsingerror callback. It has a name, code, and message property, along with all the regular properties that come with a JavaScript error object.

{
  "name": "ParsingError",
  "code": "SomeCode",
  "message": "SomeMessage"
}

There are two error codes that can be reported back currently:

• 0 BadSignature
• 1 BadTimeStamp

Note: Exceptions other then ParsingError will be thrown and not reported.

VTTCue

A DOM shim for the VTTCue. See the spec for more information. Our VTTCue shim also includes properties of its abstract base class TextTrackCue.

var cue = new window.VTTCue(0, 1, "I'm a cue.");

Note: Since this polfyill doesn't implement the track specification directly the onenter and onexit events will do nothing and do not exist on this shim.

Extended API

There is also an extended version of this shim that gives a few convenience methods for converting back and forth between JSON and VTTCues. If you'd like to use these methods then us vttcue-extended.js instead of vttcue.js. This isn't normally built into the vtt.js distributable so you will have to build a custom distribution instead of using bower.

toJSON()

Converts a cue to JSON.

var json = cue.toJSON();

VTTCue.fromJSON(json)

Create and initialize a VTTCue from JSON.

var cue = VTTCue.fromJSON(json);

VTTCue.create(options)

Initializes a VTTCue from an options object where the properties in the option objects are the same as the properties on the VTTCue.

var cue = VTTCue.create(options);

VTTRegion

A DOM shim for the VTTRegion. See the spec for more information.

var region = new window.VTTRegion(0, 1, "I'm a region.");
cue.region = region;

Extended API

There is also an extended version of this shim that gives a few convenience methods for converting back and forth between JSON and VTTRegions. If you'd like to use these methods then us vttregion-extended.js instead of vttregion.js. This isn't normally built into the vtt.js distributable so you will have to build a custom distribution instead of using bower.

VTTRegion.fromJSON(json)

Creates and initializes a VTTRegion from JSON.

var region = VTTRegion.fromJSON(json);

VTTRegion.create(options)

Creates a VTTRegion from an options object where the properties on the options object are the same as the properties on the VTTRegion.

var region = VTTRegion.create(options);

Browser

In order to use the vtt.js in a browser, you need to get the built distribution of vtt.js. The distribution contains polyfills for TextDecoder, VTTCue, and VTTRegion since not all browsers currently support them.

Building Yourself

Building a browser-ready version of the library is done using grunt (if you haven't installed grunt globally, you can run it from ./node_modules/.bin/grunt after running npm install):

$ grunt build
$ Running "uglify:dist" (uglify) task
$ File "dist/vtt.min.js" created.

$ Running "concat:dist" (concat) task
$ File "dist/vtt.js" created.

$ Done, without errors.

Your newly built vtt.js now lives in dist/vtt.min.js, or alternatively, dist/vtt.js for an unminified version.

Bower

You can also get the a prebuilt distribution from Bower. Either run the shell command:

$ bower install vtt.js

Or include vtt.js as a dependency in your bower.json file. vtt.js should now live in bower_components/vtt.js/vtt.min.js. There is also an unminified version included with bower at bower_components/vtt.js/vtt.js.

Usage

To use vtt.js you can just include the script on an HTML page like so:

<html>
<head>
  <meta charset="utf-8">
  <title>vtt.js in the browser</title>
  <script src="bower_components/vtt.js/vtt.min.js"></script>
</head>
<body>
  <script>
    var vtt = "WEBVTT\n\nID\n00:00.000 --> 00:02.000\nText",
        parser = new WebVTT.Parser(window, WebVTT.StringDecoder()),
        cues = [],
        regions = [];
    parser.oncue = function(cue) {
      cues.push(cue);
    };
    parser.onregion = function(region) {
      regions.push(region);
    }
    parser.parse(vtt);
    parser.flush();

    var div = WebVTT.convertCueToDOMTree(window, cues[0].text);
    var divs = WebVTT.processCues(window, cues, document.getElementById("overlay"));
  </script>
  <div id="overlay" style="position: relative; width: 300px; height: 150px"></div>
</body>
</html>

Node

You have a couple of options if you'd like to run the library from Node.

vtt.js

vtt.js is on npm. Just do:

npm install vtt.js

Require it and use it:

var vtt = require("vtt.js"),
    WebVTT = vtt.WebVTT,
    VTTCue = vtt.VTTCue,
    VTTRegion = vtt.VTTRegion;

var parser = new WebVTT.Parser(window);
parser.parse();
// etc

var elements = WebVTT.processCues(window, cues, overlay);
var element = WebVTT.convertCueToDOMTree(window, cuetext);

var cue = new VTTCue(0, 1, "I'm a cue.");
var region = new VTTRegion();

See the API for more information on how to use it.

Note: If you use this method you will have to provide your own window object or a shim of one with the necessary functionality for either the parsing or processing portion of the spec. The only shims that are provided to you are VTTCue and VTTRegion which you can attach to your global that is passed into the various functions.

node-vtt

Use node-vtt. Node-vtt runs vtt.js on a PhantomJS page from Node so it has access to a full DOM and CSS layout engine which means you can run any part of the library you want. See the node-vtt repo for more information.

Developing vtt.js

A few things to note:

• When bumping the version remember to use the grunt release task as this will bump package.json + bower.json and build the dist files for vtt.js in one go.
• The Grunt Run Task tool is handy for running the library without having to run the whole test suite or set of tests.

Tests

Tests are written and run using Mocha on node.js.

To run all the tests, do the following:

$ npm test

If you want to run individual tests, you can install the Mocha command-line tool globally, and then run tests per-directory:

$ npm install -g mocha
$ cd tests/some/sub/dir
$ mocha --reporter spec --timeout 200000

See the usage docs for further usage info.

Writing Tests

Tests are done by comparing live parsed output to a last-known-good JSON file. The JSON files can be easily generated using vtt.js, so you don't need to write these by hand (see details below about Grunt Run Task).

TestRunner

There's a prebuilt API in place for testing different parts of vtt.js. Simply require the TestRunner module in the lib directory and start writing tests using mocha. See an example of a test file here with its first test's WebVTT file here and its corresponding parsing JSON file and processing JSON file. You can also check out the tests directory for more examples on how to write tests.

jsonEqual(vttFile, jsonRefFile, message, onDone)

First parses the WebVTT file as UTF8 and compares it to the reference JSON file and then parses the WebVTT file as a string and compares it to the reference JSON file.

jsonEqualStreaming(vttFile, jsonRefFile, message, onDone)

Simulates parsing the file while streaming by splitting the WebVTT file into chunks. Will simulate parsing like this n times for a single WebVTT file where n is the length in unicode characters of the file, so use this only on small files or else you will get a timeout failure on your test.

jsonEqualParsing(vttFile, jsonRefFile, message, onDone)

Runs jsonEqual and jsonEqualStreaming in one go.

jsonEqualProcModel(vttFile, jsonRefFile, message, onDone)

Runs the processing model over the VTTCues and VTTRegions that are returned from parsing the WebVTT file.

jsonEqualAll(vttFile, jsonRefFile, message, onDone)

Runs jsonEqualParsing and jsonEqualProcModel. Note that jsonRefFile should contain JSON that is generated from parsing. The processing model test will compare its results to a JSON file located at [vttFile]-proc.json. Therefore, if you have a WebVTT file named basic.vtt the JSON reference file for the processing model tests will be basic-proc.json.

jsonEqualAllNoStream(vttFile, jsonRefFile, message, onDone)

Runs jsonEqual and jsonEqualProcModel use this if you want to do parsing and processing tests, but do not want to simulate streaming because you have too big of a WebVTT file.

Grunt Run Task

You can automatically generate a JSON file for a given .vtt file using the run Grunt task.

To get parsed JSON output from some WebVTT file do:

$ grunt run:my-vtt-file.vtt
$ grunt run:my-vtt-file.vtt > my-json-file.json

To get processed output from the WebVTT file do:

$ grunt run:my-vtt-file.vtt:p
$ grunt run:my-vtt-file.vtt:p > my-json-file.json

By passing the c flag you can automatically copy the output into a JSON file with the same name as the WebVTT file:

$ grunt run:my-vtt-file.vtt:c
$ grunt run:my-vtt-file.vtt:pc

The parsed JSON output now lives in my-vtt-file.json and the processing JSON output lives in my-vtt-file-proc.json.

You can also run it over a directory copying the output of parsing or processing each WebVTT file to a corresponding JSON file like so:

$ grunt run:my-vtt-file-directory
$ grunt run:my-vtt-file-directory:p

This is useful when you've modified how vtt.js works and each JSON file needs a slight change.

The run task utilizes a script called cue2json, but does a few other things for you before each run like building a development build for cue2json to use. It's also a bit easier to type in the CL options for the task. If you want to know more about cue2json you can run it directly like so:

$ ./bin/cue2json.js 
$ Generate JSON test files from a reference VTT file.
$ Usage: node ./bin/cue2json.js [options]
$ 
$ Options:
$   -v, --vtt      Path to VTT file.                                                                                     
$   -d, --dir      Path to test directory. Will recursively find all JSON files with matching VTT files and rewrite them.
$   -c, --copy     Copies the VTT file to a JSON file with the same name.                                                
$   -p, --process  Generate a JSON file of the output returned from the processing model. 

Notes:

• cue2json utilizes the last development build done. This is why the Grunt run task is good as you don't have to remember to build it yourself. If you don't build it yourself then you could potentially get incorrect results from it.
• Since cue2json uses the actual parser to generate these JSON files there is the possibility that the generated JSON will contain bugs. Therefore, always check the generated JSON files to check that the parser actually parsed according to spec.