Support conditional update on panel-class

[rohdef]

I have a set of options that I group in the accordion, I'd like to do something like this:

thing is this works on load, but does not update when the isValid setting is changed :(

[Foxandxss]

There is no support for that at this moment. panel-class is set once.

Not hard to do, but every watch is a performance hit on the app and we need to consider if that feature is worthwhile or not for that.

[wesleycho]

We could support an optional @ binding. I think I would be fine with that, seems reasonable - the workaround is to do something like class="{{foo ? 'panel-danger'}}", which potentially adds likely two watchers anyway. (1 interpolation * 2, 2 for the original element and for the element replacing it)

[rohdef]

@Foxandxss that makes sense, I guess for the majority of the uses cases you're quite right. The suggestion by @wesleychy seemss quite reasonable to me :)