Why doesn't NSPasteboard's -types return the union of the contained NSPasteboardItem's -types?

Question

The documentation for NSPasteboard's -types reads:

Return Value

An array of NSString objects containing the union of the types of data declared for all the pasteboard items on the receiver. The returned types are listed in the order they were declared.

Despite this, I have an NSPasteboard with only one NSPasteboardItem and [pboard types] returns more types than [item types] returns. Can anyone explain this?

Code

Here's some code that evidences the problem:

- (BOOL)performDragOperation:(id <NSDraggingInfo>)sender {
    NSPasteboard *pboard = [sender draggingPasteboard];

    // Prove that there's only one item
    if ([[pboard pasteboardItems] count] > 1)
        return NO;

    for (NSString* type in [pboard types])
        NSLog(@"Pasteboard type: %@", type);

    NSPasteboardItem* item = [[pboard pasteboardItems] objectAtIndex:0];

    for (NSString* type in [item types])
        NSLog(@"Item type: %@", type);

    return NO; // Ignore for example
}

Output

When I drag a link from Safari I get the following output:

Pasteboard type: dyn.ah62d4rv4gu8zs3pcnzme2641rf4guzdmsv0gn64uqm10c6xenv61a3k
Pasteboard type: WebURLsWithTitlesPboardType
Pasteboard type: dyn.ah62d4rv4gu8yc6durvwwaznwmuuha2pxsvw0e55bsmwca7d3sbwu
Pasteboard type: Apple URL pasteboard type
Pasteboard type: public.url
Pasteboard type: CorePasteboardFlavorType 0x75726C20
Pasteboard type: public.url-name
Pasteboard type: CorePasteboardFlavorType 0x75726C6E
Pasteboard type: public.utf8-plain-text
Pasteboard type: NSStringPboardType
Item type: dyn.ah62d4rv4gu8zs3pcnzme2641rf4guzdmsv0gn64uqm10c6xenv61a3k
Item type: dyn.ah62d4rv4gu8yc6durvwwaznwmuuha2pxsvw0e55bsmwca7d3sbwu
Item type: public.url
Item type: public.url-name
Item type: public.utf8-plain-text

Wild Speculation

It looks like [item types] is basically showing the same types as [pboard types], but only the UTI versions. And since [pboard types] seems to be interleaving the UTI types with the corresponding other type (?) of types, it's basically a mapping...

I could probably ignore this issue by simply using the UTI for the data format I want, but I'm looking for WebURLsWithTitlesPboardType (corresponding to dyn.ah62d4rv4gu8zs3pcnzme2641rf4guzdmsv0gn64uqm10c6xenv61a3k), and I'm wary of those dyn.(...) UTIs. Sounds like something that shouldn't be hardcoded.

Is there a reliable way of transforming WebURLsWithTitlesPboardType-style identifiers into UTIs? I don't trust the approach of actually using [pboard types] as a mapping...

Answer 1

You might try using the UTTypeCreatePreferredIdentifierForTag() function to see if that will retrieve the UTI corresponding to that pboard type. You would pass kUTTagClassNSPboardType for the tagClass argument, and the pboard type as the tag (the third argument can be NULL). The docs imply that this will probably return a dynamic UTI, since non is explicitly declared anywhere for for WebURLsWithTitlesPboardType. What's not clear is whether it will reuse the same one it generated for use on the pasteboard, or if it just makes a new one each time.

Answer 2

It doesn't because it has to maintain backwards compatibility with earlier versions of Mac OS X, none of which are multiple-item-aware.

If you're developing for Mac OS X 10.6 or later only, you can completely disregard the -types method and focus on the pasteboard's items directly. If you're targetting Mac OS X 10.5 or earlier, you'll need to either use the legacy methods (including -types) or check for the existence of the newer methods and types before using them (via weak linking and judicious use of NSClassFromString() and -respondsToSelector:.)

Answer 3

I'm convinced that documentation for NSPasteboard's -types is actually faulty. The correct documentation should be something like:

An array of NSString objects containing the union of the types of data declared for all the pasteboard items on the receiver, with the addition of old-style, non-UTI type identifiers.

If you're targeting OS X 10.6+, you should be able to completely disregard NSPasteboard's -types and focus only on each NSPasteboardItem's -types, but this requires working exclusively with UTIs.

To convert a non-UTI type identifier to a UTI you need to use the UTTypeCreatePreferredIdentifierForTag() function; you also need to know what kind of identifier you already have (kUTTagClassFilenameExtension, kUTTagClassMIMEType, kUTTagClassNSPboardType or kUTTagClassOSType). This type is the first argument to the function. The second is the identifier itself (as a CFStringRef). While the documentation suggests that it's OK to pass NULL for the third argument, it seems to be important to actually pass kUTTypeData when generating these dynamic UTIs.

For example, to get the (dynamic) UTI for data with the old-style identifier "WebURLsWithTitlesPboardType":

CFStringRef webURLsWithTitlesUTI = UTTypeCreatePreferredIdentifierForTag(kUTTagClassNSPboardType, CFSTR("WebURLsWithTitlesPboardType"), kUTTypeData);