Brent Simmons, author of NetNewsWire, writes:
On Macs we have a long-standing culture of apps working together.
On the iPad (and iPhone) we can sort-of do the same thing. We don’t have AppleScript or Apple events, but we do have the URL scheme thing for inter-application communication. It’s technically possible to do some of these same things.
But we don’t have an easy way to get back to the calling app.
What if the calling app added, as a parameter to the URL, a URL to call when the task is completed?
This way the helper app (NetNewsWire in this case) would know, once the task is complete, how to get the user back to their place in the calling app (Twitterrific in this case).
I was thinking about the same issue, coincidentally, and one idea which occurred to me is to use the equivalent of the http referrer URL (often misspelled as “referer” for historical reasons).
For a standard http request the referrer is the URL of the document containing the clicked link, so you can get back to that document by clicking on your browser’s “back” button. Now, this isn’t really contained in the URL itself – rather, the browser appends this as one of the fields in the http request headers – but the analogy is interesting.
For this to work locally between applications, the referrer URL would need to be set into the URL targeted at the called application. And indeed, there is a -[NSURL setResourceValue:forKey:error:] method that, in theory could add any key/value pair to the URL. (Currently only the “scheme” property seems to be available, though.)
Update: Mike Abdullah points out in the comments that, in practice, this method applies properties to the file a file URL points at, not to the URL itself. Drat.
On the receiving side, the “referrer” value would have to be pulled out from the NSAppleEventDescriptor that the application gets with the ‘GURL’ event.
All this needs filing enhancement requests and Apple would have to do the required twiddling inside their code base – or rather, bases, as this would be useful to have both on the iPhone/iPad and the Mac side. It would take a long time, if it’s done at all.
The alternative would be to developers to, informally, establish a convention for incorporating this into the URL itself. So we’d have something like:
Probably the second part would have to be suitable encoded/escaped, too. Maybe this would be a good topic for discussion at WWDC?