@kaliber/use-subtitles

Easily consume your video elements' WebVTT subtitles in React.

Motivation

WebVTT subtitles are a great feature, but they come with some difficulties:

• Styling the subtitles with CSS to achieve a consistent look across browsers can be challenging.
• Third party dependencies hide the underlaying HTMLMediaElement, and thus;
• Interfacing with the subtitles from JavaScript code is not straightforward.

This hook aims to make that easier for you.

Installation

yarn add @kaliber/use-subtitles

Transpilation

When working with @kaliber/build, add @kaliber/use-subtitles to your compileWithBabel array.

Usage

Here's a short example demonstrating the most common use case.

import { useSubtitles } from '@kaliber/use-subtitles'

function Component() {
  const { ref } = useSubtitles({
    language: "en"
  })

  return (
    <video {... { ref }}>
      <source type="audio/mp3" src="./audio.mp3" />
      <track src="./subtitle.vtt" kind="subtitles" srcLang="en" default />
    </video>
  )
}

Look at the /example directory for further examples.

Usage with ReactPlayer

As long as you are able provide the underlying HTMLMediaElement of your dependency, the library should work.

Because ReactPlayer only provides the underlaying ref whenever it deems it ready, we need to set it through the onReady method (up until that time, its value will otherwise be null).

import { useSubtitles } from '@kaliber/use-subtitles'
import ReactPlayer from 'react-player'

const config = {
  file: { tracks: [{
    kind: "subtitles",
    src: "/subtitle.vtt",
    srcLang: "en",
    default: true
  }]}
}

function Component() {
  const reactPlayerRef = React.useRef(null)
  const { current, ref } = useSubtitles({
    language: "en"
  })

  return (
    <ReactPlayer
      ref={reactPlayerRef}
      onReady={x => { ref.current = x.getInternalPlayer() }}
      url="./audio.mp3"
      {... { config }}
    />
  )
}

Parameters

The hook accepts the following parameters:

Key	Type	Example	Description
language	string	nl	Expects a language code that matches the track language. Bear in mind this has to be a ISO 639-1 string. For example in case of Japan, jp does not work in all browsers, where ja does, which is the valid ISO code.

Return values

The useSubtitles hook returns the following values:

Key	Initial values	Description
subtitles	[]	An array of all subtitles available for the specified language.
metadata	[]	An array of all metadata available for the specified language.
active	See below.	An object representing the currently active subtitle and metadata, with metadata containing the properties startTime, endTime, and text, and subtitle also containing voice1.
ref	{ current: null }	A reference that should be attached to the player element ref attribute.

The structure of the active object:

{
  subtitles: {
    [key]: null
  },
  metadata: {
    [key]: null
  }
}

WebVTT

A WebVTT file typically looks like this:

WEBVTT

00:00:00.000 --> 00:00:20.000
<v Fred>Hi, my name is Fred

00:00:02.500 --> 00:00:22.500
<v Bill>Hi, I’m Bill

00:00:05.000 --> 00:00:25.000
<v Fred>Would you like to get a coffee?

00:00:07.500 --> 00:00:27.500
<v Bill>Sure! I’ve only had one today.

00:00:10.000 --> 00:00:30.000
<v Fred>This is my fourth!

00:00:12.500 --> 00:00:32.500
<v Fred>OK, let’s go.

Metadata

This hook also has the option to extract metadata; additional data to support the subtitles, but are not directly shown to the user. This accepts JSON (which it parses and returns), or a regular string.

Example of a WebVTT metadata file (with JSON):

WEBVTT

00:00:00.000 --> 00:00:01.000
{ "text": "Living on borrowed time", "color": "red" }

00:00:01.000 --> 00:00:02.400
{ "text": "the clock ticks faster.", "color": "red" }

Look at the Karaoke example for an implementation.

Extracted properties

use-subtitles extracts some of the available data from the cues. Here's an overview of what it extracts, and how that is returned.

Kind	Description
00:00:00.000 --> 00:00:20.000	Timestamp. Outputted under current.startTime and currrent.endTime, and available for all cues in the subtitles array.
<v Fred>	voice-tag. Outputted by hook under current.voice.
Hi, my name is Fred	The text, available under current.text, and available for all cues in the subtitles array as text.

---

Disclaimer

This library is intended for internal use, we provide no support, use at your own risk. It does not import React, but expects it to be provided, which @kaliber/build can handle for you.

This library is not transpiled.

Footnotes

1. voice represents the current speakers' name. ↩