Building a Firefox OS App for my favorite Internet radio station

I recently created a Firefox OS app for my favourite radio station — radio paradise. It was a lot of fun making this app, so I thought it would be good to share some notes about how I built it.

The audio tag

It started by implementing the main functionality of the app, playing an ogg stream I got from the Internet radio station, using the HTML5 audio element

<pre lang="xml">
<audio src="" controls preload></audio>

That was easy! At this point our app is completely functional. If you don’t believe me, checkout this jsfiddle. But please continue reading, since there will be a few more sweet features added. In fact, checkout the short video below to see how it will turn out.

Because this content belongs to radio paradise, before implementing the app, I contacted them to ask for their permission to make a Firefox OS app for their radio station; they responded:

Thanks. We’d be happy to have you do that. Our existing web player is html5-based. That might be a place to start. Firefox should have native support for our Ogg Vorbis streams.

I couldn’t have asked for a more encouraging response, and that was enough to set things in motion.

Features of the app

I wanted the app to be very minimal and simple — both in terms of user experience and the code backing it. Here is a list of the features I decided to include:

  • A single, easy to access, button to play and pause the music
  • Artist name, song title and album cover for the current song playing should fill up the interface
  • Setting option to select song quality (for situation when bandwidth is not enough to handle highest quality)
  • Setting option to start app with music playing or paused
  • Continue playing even when the app is sent to the background
  • Keep the screen on when the app is running in the forground

Instead of using the HTML tag, I decided to create the audio element and configure it in JavaScript. Then I hooked up an event listener for a button to play or stop music.

<pre lang="javascript">
var btn = document.getElementById('play-btn');
var state = 'stop';
btn.addEventListener('click', stop_play);

// create an audio element that can be played in the background
var audio = new Audio();
audio.preload = 'auto';
audio.mozAudioChannelType = 'content';

function play() {;
state = 'playing';

function stop() {
state = 'stop';

// toggle between play and stop state
function stop_play() {
(state == 'stop') ? play() : stop();

Accessing current song information

The first challenge I faced was accessing the current song information. Normally we should not need any special privilege to access third party API’s as long as they provide correct header information. However, the link radio paradise provided me for getting the current song information did not allow for cross origin access. Luckily FirefoxOS has a special power reserved for this kind of situation — systemXHR comes to the rescue.

<pre lang="javascript">
function get_current_songinfo() {
var cache_killer = Math.floor(Math.random() * 10000);
var playlist_url =
'' +
var song_info = document.getElementById('song-info-holder');
var crossxhr = new XMLHttpRequest({mozSystem: true});
crossxhr.onload = function() {
var infoArray = crossxhr.responseText.split('|');
song_info.innerHTML = infoArray[1];
next_song = setInterval(get_current_songinfo, infoArray[0]);
crossxhr.onerror = function() {
console.log('Error getting current song info', crossxhr);
nex_song = setInterval(get_current_singinfo, 200000);
};'GET', playlist_url);

This meant that the app would have to be privileged and thus packaged. I normally would try to keep my apps hosted, because that is very natural for a web app and has several benefits including the added bonus of being accessible to search engines. However, in cases such as this we have no other option but to package the app and give it the special privileges it needs.

<pre lang="javascript">
"version": "1.1",
"name": "Radio Paradise",
"launch_path": "/index.html",
"description": "An unofficial app for radio paradise",
"type": "privileged",
"icons": {
"32": "/img/rp_logo32.png",
"60": "/img/rp_logo60.png",
"64": "/img/rp_logo64.png",
"128": "/img/rp_logo128.png"
"developer": {
"name": "Aras Balali Moghaddam",
"url": ""
"permissions": {
"systemXHR": {
"description" : "Access current song info on"
"audio-channel-content": {
"description" : "Play music when app goes into background"
"installs_allowed_from": ["*"],
"default_locale": "en"

Updating song info and album cover

That XHR call to radio paradise proides me with three important pieces of information:

  • Name of the current song playing and it’s artist
  • An image tag containing the album cover
  • Time left to the end of current song in miliseconds

Time left to the end of current song is very nice to have. It means that I can execute the XHR call and update the song information only once for every song. I first tried using the setTimeout function like this:

<pre lang="javascript">
//NOT working example. Can you spot the error?
crossxhr.onload = function() {
var infoArray = crossxhr.responseText.split('|');
song_info.innerHTML = infoArray[1];
setTimeout('get_current_songinfo()', infoArray[0]);

To my surprise, that did not work, and I got a nice error in logcat about a CSP restriction. It turns out that any attempt at dynamically executing code is banned for security reasons. All we have to do in this scenario to avoid the CSP issue is to pass a callable object, instead of a string.

<pre lang="javascript">
// instead of passing a string to setTimout we pass
// a callable object to it
setTimeout(get_current_songinfo, infoArray[0]);

Update: Mindaugas pointed out in the comments below that using innerHTML to parse unknown content in this way, introduces some security risks. Because of these security implications, we should retrieve the remote content as text instead of HTML. One way to do this is to use song_info.textContent which does not interpret the passed content as HTML. Another option, as Frederik Braun pointed out is to use a text node which can not render HTML.

radio paradise mobile web app running on FirefoxOS

With a bit of CSS magic, things started to fall into place pretty quickly

Adding a unique touch

One of the great advantages of developing mobile applications for the web is that you are completely free to design your app in any way you want. There is no enforcement of style or restriction on interaction design innovation. Knowing that, it was hard to hold myself back from trying to explore new ideas and have some fun with the app. I decided to hide the settings behind the main content and then add a feature so user can literally cut open the app in the middle to get to setting. That way they are tucked away, but still can be discovered in an intuitive way. For UI elements in the setting page to toggle options I decided to give Brick a try., with a bit of custom styling added.

radio paradise app settings

User can slide open the cover image to access app settings behind it

Using the swipe gesture

As you saw in the video above, to open and close the cover image I use pan and swipe gestures. To implement that, I took gesture detector from Gaia. It was very easy to integrated the gesture code as a module into my app and hook it up to the cover image.

Organizing the code

For an app this small, we do not have to use modular code. However, since I have recently started to learn about AMD practices, I decided to use a module system. I asked James Burke about implications of using requirejs in an app like this. He suggested I use Alameda instead, since it is geared toward modern browsers.

Saving app settings

I wanted to let users choose stream quality as well as whether they want the app to start playing music as soon as it opens. Both of these options need to be persisted somewhere and retrieved when the app starts. I just needed to save a couple of key/value pairs. I went to #openwebapps irc channel and asked for advice. Fabrice pointed me to a nice piece of code in Gaia (again!) that is used for asynchronous storing of key/value pairs and even whole objects. That was perfect for my use case, so I took it as well. Gaia appears to be a goldmine. Here is the module I created for settings.

<pre lang="javascript">
define(['helper/async_storage'], function(asyncStorage) {
var setting = {
values: {
quality: 'high',
play_on_start: false
get_quality: function() {
return setting.values.quality;
set_quality: function(q) {
setting.values.quality = q;;
get_play_on_start: function() {
return setting.values.play_on_start;
set_play_on_start: function(p) {
setting.values.play_on_start = p;;
save: function() {
asyncStorage.setItem('setting', setting.values);
load: function(callback) {
asyncStorage.getItem('setting', function(values_obj) {
if (values_obj) setting.values = values_obj;
return setting;

Splitting the cover image

Now we get to the really fun part that is splitting the cover image in half. To achieve this effect, I made two identical overlapping canvas element both of which are sized to fit the device width. One canvas clips the image and keeps the left portion of it while the other keeps the right side.

Each canvas clips and renders half of the image

Each canvas clips and renders half of the image

Here is the code for draw function where most of the action is happening. Note that this function runs only once for each song, or when user changes the orientation of the device from portrait to landscape and vice versa.

<pre lang="javascript">
function draw(img_src) {
width = cover.clientWidth;
height = cover.clientHeight;
draw_half(left_canvas, 'left');
draw_half(right_canvas, 'right');
function draw_half(canvas, side) {
canvas.setAttribute('width', width);
canvas.setAttribute('height', height);
var ctx = canvas.getContext('2d');
var img = new Image();
var clip_img = new Image();
// opacity 0.01 is used to make any glitch in clip invisible
ctx.fillStyle = 'rgba(255,255,255,0.01)';

if (side == 'left') {
ctx.moveTo(0, 0);
// add one pixel to ensure there is no gap
var center = (width / 2) + 1;
} else {
ctx.moveTo(width, 0);
var center = (width / 2) - 1;

ctx.lineTo(width / 2, 0);

// Draw a wavy pattern down the center
var step = 40;
var count = parseInt(height / step);
for (var i = 0; i < count; i++) {
ctx.lineTo(center, i * step);

// alternate curve control point 20 pixels, every other time
ctx.quadraticCurveTo((i % 2) ? center - 20 :
center + 20, i * step + step * 0.5, center, (i + 1) * step);
ctx.lineTo(center, height);
if (side == 'left') {
ctx.lineTo(0, height);
ctx.lineTo(0, 0);
} else {
ctx.lineTo(width, height);
ctx.lineTo(width, 0);


img.onload = function() {
var h = width * img.height / img.width;
ctx.drawImage(img, 0, 0, width, h);
img.src = img_src;

Keeping the screen on

The last feature I needed to add was keeping the screen on when the app is running in foreground and that turned out to be very easy to implement as well. We need to request a screen wake lock

<pre lang="javascript">
var lock = window.navigator.requestWakeLock(resourceName);

The screen wake lock is actually pretty smart. It will be automatically released when app is sent to the background, and then will given back to your app when it comes to the foreground. Currently in this app I have not provided an option to release the lock. If in future I get requests to add that option, all I have to do is release the lock that has been obtained before setting the option to false

<pre lang="javascript">

Getting the app

If you have a FirefoxOS device and like great music, you can now install this app on your device. Search for “radio paradise” in the marketplace, or install it directly from this link. You can also checkout the full source code from github. Feel free to fork and modify the app as you wish, to create your own Internet Radio apps! I would love it if you report issues, ask for features or send pull requests.


I am more and more impressed by how quickly we can create very functional and unique mobile apps using web technologies. If you have not build a mobile web app for Firefox OS yet, you should definitely give it a try. The future of open web apps is very exciting, and Firefox OS provides a great platform to get a taste of that excitement.

Now it is your turn to leave a comment. What is your favourite feature of this app? What things would you have done differently if you developed this app? How could we make this app better (both code and UX)?

About Aras Balali Moghaddam

Aras is an interaction designer and a frontend engineer living in beautiful British Columbia, Canada. He is passionate about the open web and likes to build awesome mobile web apps. You can learn more about him on his blog.

More articles by Aras Balali Moghaddam…

About Robert Nyman [Editor emeritus]

Technical Evangelist & Editor of Mozilla Hacks. Gives talks & blogs about HTML5, JavaScript & the Open Web. Robert is a strong believer in HTML5 and the Open Web and has been working since 1999 with Front End development for the web - in Sweden and in New York City. He regularly also blogs at and loves to travel and meet people.

More articles by Robert Nyman [Editor emeritus]…


  1. SanThosh K

    Awesome…. :) :) Animation is Simply Superb..

    October 24th, 2013 at 23:06

    1. Aras Balali Moghaddam

      Thanks for your comment SanThosh, glad you like it :)

      October 25th, 2013 at 11:43

  2. Ivan Dejanovic

    I really like how your app looks in the video. Even after going through the whole article I am still amazed how cool this app looks given the amount of code needed to build it.

    I also really like your two canvas idea. First time I looked at the video I was really intrigued how you made this splitting effect look so good and natural. If you don’t mind I will examine closely how you implemented this image splitting. I might be able to use something similar in some of my on work.

    October 25th, 2013 at 01:19

    1. Aras Balali Moghaddam

      Absolutely! that is the whole idea Ivan. I would be very happy to see similar implementation/variations of this idea. Perhaps after we implement several variations of it, we can look into turning it into a reusable component — if it proves to be useful. Keep me posted.

      October 25th, 2013 at 11:42

  3. Mindaugas J.

    Using innerHTML with unknown content from some site via simple http protocol does not look good. You are vulnerable to access elevation of malicious code.

    October 25th, 2013 at 09:44

    1. Aras Balali Moghaddam

      Thanks for your comment Mindaugas. That is a valid concern, and I agree that people should be very careful when using this technique. However, In this case I have trusted in the third party radioparadise which my app depends on entirely anyway. In a way you can say that it is their app not mine, and I think it would be to their best interest if they do not suddenly change the content of that page — which their other apps depend on as well. Also note that because of the CSP restrictions there are no dynamic evaluation of any code.

      October 25th, 2013 at 11:52

      1. Mindaugas J.

        The point is, even if you trust the site now, it may get compromised or spoofed by man-in-the-middle. Arbitrary code will have the same access as your app.

        October 28th, 2013 at 09:21

        1. Aras Balali Moghaddam

          I see. Can you make recommendations on how to make this code more secure in case of MITM attack? Is there a way to strip away scripts that may be injected into the HTML content perhaps? I would be happy to update the post if concrete recommendations to improve security are provided.

          October 28th, 2013 at 11:42

          1. Frederik Braun

            I recommend replacing the innerHTML assignment with a function (e.g. updateText) and this function then updates a text node (document.createTextNode) which is set as a child element of your tag. Text nodes can not render HTML, so no injection ;)

            October 29th, 2013 at 12:46

          2. Aras Balali Moghaddam

            Thank you both for your comments regarding this security issue. I have updated the post to address this.

            October 30th, 2013 at 14:53

  4. nadrimajstor

    How would you aproach additonal features such as:
    1. Opening radio station’s about web page in FxOS main browser,
    2. Restricting download when device’s wireless connection is down?

    October 25th, 2013 at 15:39

    1. Aras Balali Moghaddam

      Great questions!
      1) This is actually already the case when you user clicks on the radio paradise link in the settings page, I just didn’t show it in the demo. All you need to do in order to open a link from your app in the default browser, is to set target=”_blank”.
      2) I have been wondering about that too and asked this question a while ago:!searchin/$20api/

      I believe once the relevant bug is fixed ( network API will become available and we could add that very useful feature as you suggested.

      October 25th, 2013 at 18:05

  5. nadrimajstor

    You didn’t get to the bottom of the Brick’s require/alameda issue #83? ;(

    October 26th, 2013 at 12:02

    1. Aras Balali Moghaddam

      Nadrimajstor, unfortunately that compatibility issue has not been resolved yet, but I am sure they are working on it. For others curious about what this issue is, here is the link:

      October 26th, 2013 at 13:28

  6. Kevin

    I unfortunately don’t have a FFOS phone to test this with, but what happens when you get a disconnect, can you automatically reconnect?

    Also, last time I tried simply playing an .ogg stream in Firefox, it would buffer a whole lot compared to e.g. mplayer – can you control this from js?

    And is there a way to ensure you don’t play from an old buffer? (E.g. I want the stream to stop playing if it’s not “current”, and on reconnect, re-buffer so we never play “old” content.)

    (By the way, this form tries to validate my email and fail badly, had to remove the +)

    October 30th, 2013 at 00:57

    1. Aras Balali Moghaddam

      Thanks for your comments Kevin. I try to answer to the best of my knowledge.
      > can you automatically reconnect?
      Yes. I have already implemented a way to try another stream from a queue of similar quality streams, when there is an error:
      > can we control buffer from js?
      I have not tried this, so I can not answer yet. But it looks like `audioContext` is in nightly: and with that I think we can control the buffer (and lots more). I don’t know if this is yet available in FxOS
      > Is there a way to ensure you don’t play from an old buffer?
      The easiest way I can think of to do this is to reset the stream source every time we pause and play which will throw away the old buffer. I am sure there are other ways to do this as well.

      October 30th, 2013 at 15:43

      1. Kevin

        Thanks for the pointers! AudioContext looks quite powerful and complicated, I guess that’s what I’ll have to look into if audio.addEventListener(‘error’,f) doesn’t cut it for me :)

        October 31st, 2013 at 00:28

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