Re: [css3-fonts][cssom] Load events for webfonts?

On Thu, May 24, 2012 at 6:26 PM, Tab Atkins Jr. <jackalmage@gmail.com>wrote:

> I couldn't find anything about this from a cursory search of the list.
>
> Has any thought been given to exposing when fonts in @font-face are
> loaded?  (Where "loaded" means "one of the sources is either
> successfully downloaded and parsed, or is successfully found on the
> local system, or all the sources fail".)
>
> Our Docs team needs to know this information so it can do accurate
> measurements of things, which are based on the metrics of the font
> being used.  If a fallback font is being used because the main one
> hasn't loaded yet, we'll do an initial sizing sweep, then watch for
> the load using silly indirect means so we can redo all the sizing when
> it loads and we swap out the fonts.
>
> Right now, our technique is to render two spans positioned off-screen,
> one with the fallback font and one with the @font-face font, and poll
> the sizes on a timer until the second is a different size than the
> first.  This is, of course, very silly.  It's also not reliable - if
> the fallback font happens to produce a span the same width as the
> loaded font, you'll never notice the load.  This may or may not be
> okay for the overall sizing (depends on how close the metrics actually
> are, and thus how much of a coincidence the same-size thing was), but
> it's definitely bad that it means we have a timer running forever (or
> until we hit our own timeout).
>
> Other fairly advanced apps probably have similar needs.  Providing a
> load event we can listen to instead of polling would be much less
> fragile, more reliable, and cheaper for the user.
>
> Preferably, it would be fired at the @font-face rule in the CSSOM, so
> you don't have to go to any particular effort to figure out which font
> it was.  If you do something more general like firing it at document,
> you have to provide all the descriptors so you can distinguish
> separate rules.
>
> Thoughts?
>
> ~TJ
>
>
+1