It looks like the trick here is to hook into a global NSNotification called "UIApplicationWillBeginSuspendAnimationNotification", which is assumed to be "app will enter background", and then throw up a fullscreen UIView on top of everything.<p>I think this is an undocumented notification, so it's possible other iOS versions will break this feature.
I'll wager Apple will shut this down or reject apps that do this.<p>If you're going to do this you have to consider the ramifications of how stale the information on the card becomes over time. The example, showing a "Checked in since..." will become stale if someone checks in/out on another device.<p>A real iOS 7 API to update these cards in the background would be a nice addition.
Maybe I'm too pessimistic, but I expected the example to be just the app's logo. Ie: oh, that's my Dropbox app because instead of the Dropbox UI, I see the big Dropbox logo (Nothing against Dropbox or their app). I also wonder if we'll ever see the app switcher card being used as advertisement space.<p>A little less cynically: If you can redesign a screen in the app to remove clutter and highlight the important information for the user for the app switcher, why don't you do that <i>in</i> the app? Sounds like it might be a better UI design. Now, I can think of many reasonable counter examples, but I'd challenge you to spend more time on the app UI where your users spend most of their time.
Interesting stuff, surprised the built-in apps don't do something similar to this.<p>As an aside, is there anything similar to this for android? I've tried a fair bit of googling but can't find anything or the right search term.
I'm guessing Apple will shut this down. The thumbnails are there for the sole reason of you visually identifying the apps as you left them. If applications can change that snapshot at will then it becomes a mere gimmick.