By Daniel Eran Dilger

Published: 05:00 PM EST (02:00 PM PST)

UTI definitions

For example, the "public.html" UTI defines itself as conforming to "public.text," allowing any apps that know how to use text to also access HTML formatted text, even if they never anticipated editing HTML. They could also work with a new vendor-specific UTI such as "com.editmax.htmlplus" that defined itself as conforming to "public.html."

In turn, "public.text" conforms to both "public.data" and "public.content," so any process that knows how to work with files or content in general terms can also work specifically with HTML files, or any new specialized forms of HTML files invented in the future.

UTI definitions also include a human readable (and localizable) description of what that file type is; identify the icon that should be used to represent that file; and outline alternative ways to express that file type. For example, the "public.html" UTI associates itself with the legacy "HTML" Type Code and a variety of file extensions that HTML files may use, such as .html, .htm, .shtml, and .shtm. It also associates itself with the MIME type of "text/html," making UTI the uber-type of data typing.

Developers can make up their own new UTIs, defining them in relation to more general, existing UTIs, without needing to formally register anything. Vendor-specific UTIs use 'reverse DNS' naming just like preference files, preventing different companies from inadvertently using the same label. In the world of file extensions, there's nothing unique about "file.doc," as both WordPerfect and Word used the .doc extension. With UTI, Microsoft uses "com.microsoft.word.doc" to express a unique data type for its files.



As a side note, "reverse DNS" is used because DNS itself is backward. Web URLs like "www.apple.com/mac/features.html" go from most specific to least specific (in server name hierarchy), then hit a slash and begin getting more specific (in web server file hierarchy). This is like writing the number 1,234.567 as 4321.567: nutty. So "reverse DNS" is an attempt to fix a big mistake made in web URLs everywhere else that a similar naming hierarchy might be useful. It's too late to fix the web.

Features of UTI

So, rather than just defining a file in terms of the app that created it and its general type, developers can now assign their files a UTI that identifies a very specific creator and type, and then inform the system of how that specific new UTI label relates with other more general file types that other apps (and system features) already know how to interact with. Developers can also still indicate that their unique UTI-tagged files should open with their "creator" app, a feature the end user can override via a Finder preference.

In addition to using UTI to manage what app to use when opening a file in the Finder (via Launch Services) and in determining how to perform copy and paste, drag and drop, and Application Services (via Pasteboard Manager), Mac OS X also uses UTIs within Spotlight importers, Automator Actions, and Quick Look previews, as well as in Navigation Services to narrow down the relevant files presented in open and save dialogs.

For Launch Services, Apple tells developers to define UTIs for the documents their applications create, and include these within their application. A vendor-specific UTI works like an old fashioned Type and Creator, although developers don't have to register their definitions with Apple in advance. So rather than Adobe registering the "8BIM" creator and "8BPS" type for its Photoshop files, the UTI "com.adobe.photoshop-​image" expressively and uniquely identifies these files as entities that open with Photoshop.

Other applications can also open Photoshop documents. Conceptually, other apps could even create versions of Photoshop documents that open by default using their own app simply by adding a new UTI that defines itself as a specialized version of the existing "com.adobe.photoshop-​image" UTI. This allows the system to do everything it already knows how to do with Photoshop files (copy and paste, index for search, use in Automator Actions, Quick Look) to new documents defined by the specialized "Photoshop+" UTI. Type/Creator Codes can't do that.

The Pasteboard's use of UTI was introduced above. It shouldn't be surprising that iPhone 3.0 also uses UTI in its new copy and paste support. If you were wondering why Apple "took so long" to bring that feature to the iPhone, it's because the platform opted for a long term, robust solution rather than a quick and dirty kluge impacted by security issues.

For Spotlight searching, Apple instructs developers to provide UTI definitions for the types of files their importer plugin can index. The system then uses the most specific and specialized importer available to index files matching that UTI.

For Automator Actions, developers specify the UTIs their Action knows how to input data from and what UTIs the Action will spit out. Automator Actions are essentially a modular string of Services that are compiled together to copy data from one file or selection, transform it, and spit out a result.

For Quick Look, plugins specify UTIs for the files they can view. Again, this allows the system to select the most specialized plugin to use in displaying a given file type. In other words, a developer could create a specific plugin for viewing a fancy text UTI (such as XML) with more features (such as color-coded markup tags) than Apple's own basic text Quick Look plugin offers. Quick Look is also an application of UTIs that allows the system itself to open a file for quick viewing without affecting the user's desired default launching app.

The fix for Creator Code junkies

UTIs provide a standardized way for defining data anywhere in the system, one that transcends the Type/Creator Code's limited concept of a single app that creates given file types. Developers who complain that Snow Leopard doesn't support Creator Codes need to brush up on UTI, which has been in place for several years now. It offers all the benefits of Creator Codes while enabling lots of other modern new features.

Users who miss being able to automatically open a file using the app that originally created it can pester their app's developer to get on the ball with UTI. Any application that has been updated since 2005's Tiger, but which does not yet support UTI, has opted not to support an important feature of the Mac platform.

Everyone else, including many of us who didn't ever understand why the system launched files using a specific app rather than the one we had defined for that given file type, can continue using the Finder's Open With menu, drag and drop app launching, or set a permanent per-item default "creator" app for opening a selection of documents by using the Get Info panel.



Daniel Eran Dilger is the author of "Snow Leopard Server (Developer Reference)," a new book from Wiley available now for pre-order at a special price from Amazon.