Provide domain source argument (contextual filter) handler for views

Well the way I'm using it is for a DA installation for a number of museums, with a parent site for all of them. There is a view that displays 1 event that's promoted to front page, with domain source as the argument. The parent site front page is a panel and includes the same view 4 times, with a different argument for each museum. Each museum publishes events that are available on their own domain and the parent domain, and has the source domain set to that of the individual museum. The result is a page showing 4 events from 4 separate museums, linking to those museum sites.

If you're asking about SimpleTest then that's another story.

What URLs are you generating for this? Don't they have to have a domain_id?

I'm not generating URLs, because my display is a panel pane. I'm passing in a domain_id via the pane configuration. It would be better UX if the user could select a domain from a list in the pane configuration. This would also be ideal in the argument configuration in views ui. I also wonder if machine_name would be better for exportability.

I would expect, in most cases, that people want the argument to inherit from the active domain. But that's not what you're doing, is it?

I'm doing the exact opposite, I want to see one item from each specific subsite. I think most people would not need to use this argument, because the views would already be automatically filtered by domain access. Unless they wanted to show a list of content where the source domain is the current domain, in which case they can set the default argument value to "current domain" (from the domain_views module).