Saturday, April 28, 2012

HTML5 AppCache API - How to prevent browser caching unsupported resources in manifest file

The method describe below doesn't work in all browsers.  
For now I'm using two different html files. But the best solution is on server side, making a rewrite rule for redirecting the user to the correct manifest file.


I'm writing a web application that uses the AppCache API for offline browsing. But I'm also using the Audio API to play back-ground music and a few audio effects.

For audio support in different browsers I'm delivering each sound/music in two different file formats: OGG and MP3.

The problem is regarding the cache manifest file. If we add all audio files (MP3 and OGG) in the cache manifest file, all browsers will cache all files. Including the unsupported ones. So, we end up with a huge storage requirement. Which is really bad if you are on a 3G connection for example.

So, to prevent browser caching unsupported resources, the best approach I've found was to split the manifest file. This way we can tell browsers that sopport OGG files to cache only OGG files, and do tha same for other formats like MP3.

Ok then. I have two manifest cache files, one with all OGG files listed (ogg.appcache) and the other one with the MP3 files (mp3.appcache).

Now what!? How do I swap from one to other?

Easy. First we should consider the browser supports OGG. If so, we can add the manifest attribute into the html tag with the right manifest file, just like that:


<html manifest="ogg.appcache">

Now, we have to check if the browser supports MP3. If it does, we change the value of the attribute "manifest" from the html tag. We can do that with this javascript:

var audio = document.createElement('audio');
if(audio.canPlayType('audio/mpeg;'))
  document.documentElement.setAttribute("manifest"document.documentElement.getAttribute("manifest").replace('ogg','mp3'));