For this reason, it seems to me that a project-based model like this would work well if it became part of iOS itself. The operating system could allow users to create "workspaces" in which multiple apps can then store their respective data. Internally, the workspace would still be sandboxed so that each app retains exclusive access to its files, but it would also contain a "common" area from which apps are free to read each other's information.
For example, an app like Photoshop could use its sandboxed area of a workspace to store a proprietary PSD file that contains the editable version of an image, with its filters, layers, and other Adobe-specific functionality intact and safe from outside interference. At the same time, the app could store a read-only version of the image in a commonly understood format like JPEG or PNG in the common area of the workspace, thus making it easy for other apps to use it without compromising the integrity of the original. If the user then added, for example, a Pages document to the workspace, the image would be readily available, and would update automatically every time the original was modified.
A Finder for the ages
This approach keeps the data together in one place, making it easy to divide it up in a way that makes sense to a human being, rather than a computer. And, although it relies on a traditional file system internally, it doesn't expose any of its complexity to the end user, satisfying Apple's mission of simplicity and usability.
To make things even neater, the workspaces themselves could be managed by a specialized app provided by Apple itself, whose only job is to "own" the workspaces and help the user organize them and sync them to iCloud--a Finder for the post-PC era that deals with a higher level of data than just today's files and directories.
This would make it easier to transplant this approach to OS X, where the limitations of today's sandbox are even more evident, and where 30 years of legacy based on unfettered access to the file system cannot simply be swept under the carpet.
Of course, I have no way of knowing if this is the direction in which Apple is headed (although I certainly hope so). But I'm fairly sure that today's sandboxing is merely a stepping stone to something better, if for no other reason than the current approach forces users to deal with some significant limitations.
Still, going back to the traditional file system seems like a step backward that would undo many of the innovations that sandboxing has introduced; ultimately, it will be a matter of, as a wise man once quoted another wise man as saying, figuring out where the puck is going to be, and skating there.
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