iPhone: Setting up an app for use as an "In App Purchase"

Question

Regarding In App Purchases, I can find a lot of information on all the technicalities of actually making purchases and interacting with the store (how to retrieve product information, verify receipts, etc), but I can't seem to find information on guidelines or special instructions for preparing the actual "apps" or "components," whatever they're to be considered, which will act as the In App Purchases.

For instance, once a component is downloaded into an app, where does it exist in the overall architecture of the app? How do they combine to join forces? How do they know about one another. If I have a game, and using In App Purchases I allow users to both download new levels, but also download new game play modes that can affect any of the built-in or downloaded levels, how do I prepare all of these assets so that they integrate?

I'm not looking for a tutorial, per se, but would love to know if anyone has had experience with In App Purchases or knows of a useful reference besides Apple's In App Purchase programming guide which only speaks to the specifics of making the actual download transaction.

Answer 1

The things you download aren't really "apps", they're just data files like anything else your app can download.

Sometimes, they're not really that, they're just effective "switches", i.e. all of the functionality and data is there in your code already, but it's just protected by a line of code like

if (user has purchased extra levels)
    add extra items to menu/list

You aren't allowed to download new executable code; I admit I'm not sure how carefully Apple works to prevent you from downloading scripts that control your program's behavior, since it would be very difficult for them to tell what is intrinsic to your original app or not.

In my own programs, I've put the control logic and tables into the main application, and separated out big resource files into a separate ZIP file. When the user buys the add-on pack, they do download that ZIP file of images which keeps the original application size down, and the program just uses those images out of the documents directory instead of the application bundle like it would if they were built in.

I am using the Urban Airship in-app purchase support, which insulates you from running your own server or learning most details of the StoreKit, at the cost of a slice of your revenue.