Monday, 8 November 2010

How to stop a video or audio element downloading

Say you've started playback of an HTML5 audio or video element, and you decide you really want to cancel playback and downloading of the media resource. Stopping playback is easy, just call the pause() method. But the network connection won't be stopped until the media element gets garbage collected, and even if you release all references to the media element, it won't be destroyed until the browser decides to runs its garbage collector. How can you stop the download of the media resource in the meantime? Here's a quick hack to achieve this: just reset the element's src attribute to the empty string. This destroys the element's internal decoder, and stops the network download.

For example:

<audio id="audiocontrols>
  <source src="funky-music.ogg">
  <source src="funky-music.mp3">
<!-- ... Some time later, we decide we should stop the audio element playing and downloading... -->
var audio = document.getElementById("audio");
audio.src = ""; // Stops audio download.
audio.load(); // Initiate a new load, required in Firefox 3.x.

Be aware that this will destroy the media element's decoders, so the element won't be playable anymore, and it will be rendered as an "error cross" if it's in a document. Also in Firefox 3.x you need to call load() after changing the source, whereas in Firefox 4 the load is scheduled to run when you change the src attribute, and the extra load() call is not required.

Changes to HTML5 video/audio load() function in Firefox 4

I've updated the media load() implementation in Firefox 4 to match the current WHATWG media load algorithm specification. There are three main changes that web developers using media elements should be aware of.

Firstly, error reporting has slightly changed. When a media element fails to load from its <source> children, an "error" event is dispatched to every child element which failed to load. Previously in Firefox 3.x you'd receive only one "error" event dispatched to the media element once all of its child <source> elements had failed to load. Now you only receive "error" events in the child <source> elements, and not in the media element itself.

For example, suppose you have the following markup:

<source id="mp4_src"
        type='video/mp4; codecs="avc1.42E01E, mp4a.40.2"'>
<source id="3gp_src"
        type='video/3gpp; codecs="mp4v.20.8, samr"'>
<source id="ogg_src"
        type='video/ogg; codecs="theora, vorbis"'>

Firefox 4 does not support the playback of patent encumbered content, so you'll receive "error" events in the <source> elements with the MP4 and 3GP resources, before the Ogg resource is loaded. Note also that the <source> children are loaded in the order in which they appear in the markup, and if one <source> child successfully loads and is playable, the children after it won't be loaded.

To detect that all child <source> elements have failed to load, check the value of the networkState attribute of the media element; if its value is HTMLMediaElement.NETWORK_NO_SOURCE, you know all child <source> elements have failed to load.

If you add another child <source> element to a media element which is in networkState HTMLMediaElement.NETWORK_NO_SOURCE, it will attempt to load the resource specified by the newly added <source>.

Secondly, when the load begins has changed. When you set the src attribute of a media element, or add a <source> child element to a media element, a load will be scheduled to run asynchronously as soon as the current JavaScript context exits (basically the load starts the next time we return to the browser application's main event loop). So for example, suppose you have some timeout set such as:

  function() {
    var v = document.createElement("video");
    v.src = "video.ogg";
    // Do some other stuff...

When the function runs, the load for the video element won't begin until after the function returns, and control returns to the browser. This is important, because in Firefox 3.x, the load is started (and could even run to completion!) as soon as you set the media element's src attribute.

Lastly, the media element's events now no longer bubble. Previously they bubbled, and this was a bug in Firefox 3.x and in violation of the spec.