TLDR:
-----------------------
If your app has a document type that may use the same extension as another app,
you should override typeForContentsOfURL:error: because otherwise the existence
of that other application on a user's system can break your app.
Problem summary:
-----------------------
My app and Coda both open plain text .sql files. Coda exports a UTI for the
.sql extension. So does my app. However (because Coda was registered with
Launch Services first), my app will be prevented from opening ANY .sql file
because the system and Cocoa think it's a
com.panic.coda.structured-query-language-file file, and my app can't open files
with that UTI.
The same would happen even if my app does not use a UTI but simply declares
that it opens documents with an .sql extension.
This is a critical problem.
Investigatory work:
-----------------------
- [NSDocumentController typeForContentsOfURL:error:] is the first method being
called to determine what class will open a file. The header file (but not the
documentation) has some useful information:
"The default implementation of this method merely returns the URL's
NSURLTypeIdentifierKey resource value.
You can override this to customize type determination for documents
being opened."
What this says, is that Cocoa doesn't look at my Info.plist file to see what
type *my application* would describe the file at the URL as. Instead, it by
default asks the file system "what's the UTI for this URL" and Launch Services
(because Coda registered its sql UTI long before my app) says: "It's a Coda
file."
Cocoa *then* looks in my Info.plist file (essentially), and sees no
conformity/usage of com.panic.coda.structured-query-language-file so it throws
a fit, preventing my app from opening .sql files.
That's very unfortunate and frankly ridiculous because this means that the
existence of another application on your system can break your app.
The solution however is easy.
Solution
-----------------------
By overriding typeForContentsOfURL:error: as the header suggests, we can cure
the problem.
- (NSString *)typeForContentsOfURL:(NSURL *)url error:(NSError **)outError;
{
if ([url.pathExtension.lowercaseString isEqual:@"sql"]) {
return @"my.uti.type";
}
return [super typeForContentsOfURL:url error:outError];
}
It's also important to note that if your document type declares a UTI, you need
to return that UTI from typeForContentsOfURL:error: even though the docs say it
returns "name of the document type" (AKA the CFBundleTypeName). If you return
the type name and not the UTI then the document architecture will get confused
later and things will fail. However, if you don't declare a UTI type for the
document, then return the CFBundleTypeName.
Conclusion
-----------------------
That's several hours of investigatory work. I went down many rabbit holes
during that, but I'm pretty sure that I understand this and have boiled it down
to the root cause and truth. If I'm wrong, please correct me. (I'd like to be
wrong!)
--
Seth Willits
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