Adding buttons to Finder toolbar?

Question

Dropbox has the option of adding a button to the Finder toolbar. It even appears in the 'Customize Toolbar...' window.   I can't find any documentation on how this sort of thing is done.... Does anyone know how to do this, or can anyone point me towards some documentation or sample code?

EDIT ONE:

I guess a start would be finding the location of the existing icons, and any related code.

I noticed that Dropbox has files in Library/DropboxHelperTools/Dropbox_u501 called mach_inject_bundle_stub.bundle and FinderLoadBundle which might be doing the magic. Dropbox is also putting the 'tick' badge on Finder icons. This code might be involved: github.com/rentzsch/mach_star

EDIT TWO: A Dropbox talk by Rian Hunter about the process is here, at around 15:30 http://blip.tv/pycon-us-videos-2009-2010-2011/pycon-2011-how-dropbox-did-it-and-how-python-helped-4896698

Looking in the DropboxBundle file in the Dropbox_u501 shows the icons and some compiled code. The code suggests that Rian Hunter is the author: http://twitter.com/timeserena

Rian has his own version of the Mach Star code on his github - https://github.com/rianhunter/mach_star - tho it seems much older

EDIT THREE

There was a 'Code Injection Workshop' at Stanford a few weeks ago - http://stanfordacm.com/past/ - hosted by Rian. If anyone has notes from this please let me know!

Answer 1

You can use Finder Sync app extension(Starting in OS X v10.10).

A Finder Sync extension can:

• Add, remove, and update badges and labels on items in a monitored folder.
• Display a contextual menu when the user Control-clicks an item inside a monitored folder.
• Add a custom button to the Finder’s toolbar.