[webkit-dev] ProgressEvents for Images

[Bear Travis]

Hello Webkit,

A group of us at Adobe has been looking into adding support for ProgressEvents
to images.  The overall goal is to simplify image download progress reporting
by supporting roughly the same progress events as XHR and the File API for
image elements.   For example one could connect an image to a progress element
like this:

<img id="image" src="sample.jpg"
    onloadstart="showProgressBar()"
    onprogress="updateProgressBar(event)"
    onloadend="hideProgressBar()"/>

Developers have taken various tacks to enable progress reporting, for example
in some cases XHR can be used to download image files.  Max Vujovic just
published a blog about the practicalities of doing so:
http://blogs.adobe.com/openweb/2012/01/13/html5-image-progress-events/.  We
think it would be preferable to provide support for image progress events
directly.

We're working on a prototype implementation for WebKit and have filed a bug
that explains what we're up to in a little more detail:
https://bugs.webkit.org/show_bug.cgi?id=76102

It's probably worth pointing out that the beforeload event, which is currently
under discussion, addresses a different use case.  Our proposal is intended to
enable applications to give the user feedback about image download progress,
it's not intended to enable security or efficiency by preemptively blocking or
transforming image downloads.

We'd appreciate feedback on this proposal.
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