In the Cards

John Gruber responds to Justin Blanton’s To Pre or Not To Pre:

The entire user interface is built around the concept of cards rather than apps. So while the iPhone’s Mail app already stays running while in the background, what you cannot do on the iPhone but can on the Pre is have multiple email messages “open” at the same time.

There are a few things going on here. The SDK only displays a single 320 x 480 window per application. This is most likely some combination of a design decision (application switching happens through the depth axis) and an engineering decision (it’s easier to manage windows when an app gets only one of them). Developers, of course, are free to implement an interface like the ‘pages’ in Mobile Safari. (Ah-ha! The card metaphor has been on the iPhone since day one.)

So the decision not to implement pages in Mail has nothing to do with whether the iPhone OS supports cards or whether a developer is capable of displaying and switching between multiple open documents in any other way. My guess is that implementing pages in Mail was either considered too trivial or too confusing to implement.

The interesting thing is that views within an iPhone application belong to a window object. Currently, of course, an application can only display one of these, but I suspect it wouldn’t be too hard for the OS to allow multiple windows of an application to behave like cards.