Pasteboard Priority

A weird bug cropped up in MarsEdit, in which a URL copied and pasted from Safari, for example, was pasting into the plain text editor as the text from the link instead of the link itself. Daring Fireball’s star glyph permalinks to entries presented a most dramatic example. Right-clicking a star glyph and copying the link to paste into MarsEdit was supposed to yield:

Image of pasted link shown as expected with full text content of URL

But instead gave:

Image of pasted URL showing the text of the link instead of the URL content

How strange. What could have possibly changed something so fundamental as the manner in which a pasted link is processed by my text editor? I leaned on Mercurial’s “bisect” command which led me to the specific source code commit where the behavior had changed, in my text view’s helper method for building a list of acceptable paste types. My color emphasis is on the changed part:

-	return [[NSArray arrayWithObject:NSFilenamesPboardType] arrayByAddingObjectsFromArray:[NSImage imageFileTypes]];
+	return [[NSArray arrayWithObject:NSFilenamesPboardType] arrayByAddingObjectsFromArray:[NSImage imageTypes]];

That’s it? One little tweak to the construction of a list of image types affects the behavior of pasting a URL copied from Safari? Programming is hard.

I had made the change above because imageFileTypes is deprecated. The deprecation warning specifically says: “use imageTypes instead.” Okay. Functionally, everything related to the image types should work the same. Instead of a list of file extensions from -imageFileTypes, I’m now getting a list of UTIs. I scrutinized the lists a bit to satisfy myself that all of the major image types were present in the new list, and I trust that Apple had covered the bases when they made this migration themselves.

It turns out the change above, the one word diff that causes everything to work either as expected or otherwise, is fine. It’s outside of this method where the real problem lies: in my override of NSTextView’s -readablePasteboardTypes method. In it, I endeavor to combine my own list of pasteable types with NSTextView’s own list. To do this, I create a mutable set to combine -[super readablePasteboardTypes] and my own list, and then return an array of the result. The idea is to avoid listing any items redundantly from the built in list and my own:

- (NSArray*) readablePasteboardTypes
	NSMutableSet* allReadableTypes = [NSMutableSet setWithArray:[super readablePasteboardTypes]];
	[allReadableTypes addObjectsFromArray:[self acceptableDragTypes]];
	return [allReadableTypes allObjects];

Ah, my spidey sense is finally starting to tingle. While it’s not mentioned in the NSTextView reference documentation, the NSTextView.h header file includes comments about this method that are pertinent here:

Returns an array of types that could be read currently in order of preference.  Subclassers should take care to consider the "preferred" part of the semantics of this method.

Ah, so order matters. Of course. And what does the -[NSSet allObjects] say about order?

The order of the objects in the array is undefined.

So all this time, I’ve been playing fast and loose with NSSet, lucking out with the coincidence that types I prefer would show up higher in the list than types I don’t prefer. It turns out that “public.url” is among the types included in NSTextView’s own, built-in readablePasteboardTypes method implementation, but previous to this change, it always showed up lower in resulting list than NSStringPboardType. Thus, when faced with an opportunity to paste a URL from Safari, rich with information including the original text from the link, MarsEdit always favored the plain string representation.

Changing from -[NSImage imageFileTypes] to -[NSImage imageTypes] effectively changed the roll of the dice, causing the resulting array from NSSet, documented as being “undefined” in its order, to place the URL type above the string type in the list. Thus it tries to paste as a rich URL with text linking to a URL, but since my plain text HTML editor doesn’t support rich text, all you see is the star.

The fix will be to arrange that my resulting array from readablePasteboardTypes does impose some predictable prioritization. Probably by taking the code that I have now and, after generating the list of all unique types, carefully moving a few types to the top of the list in the order that I’d prefer.