Howdy, We encountered a need for the feature described in JIRA-3813 earlier this year and have implemented a solution locally. I'd like to contribute some of this back to the community and have attached a patch which is a bit of a mix of Bruno's design with ours.
There are a few major changes from Bruno's original thoughts. FIrst, we implemented the fallback inside of a Device object. This way the structure of fallbacks is far more flexible to certain devices. This is especially important if projects treat the devices differently (e.g. whether an iPad falls back to a tablet design or an iOS device). Second, we allowed individual components to specify their device. This is important for us as we have many reusable components which use device specific markup in differing ways. Third, we added a hook point in which this component specific device logic can be externalized to a central location (Session). This is important if there's an external rules engine which is determining which devices should be used for which components). Finally, we moved the logic for determining the correct extension to the DefaultMarkupResourceStreamProvider. This aligns it with the logic for finding Style, Variation, and Locale, as well as giving it access to the component (for our above requirements) without requiring an API change. Thanks, -David Phillips - USAA
