How do I take a screengrab of a single window using Cocoa or Carbon on OS X?

Question

I need to be able to identify a single window on the user's screen and take a screen capture of it. The screen data is to be stored in memory and not written to disk.

This is already supported through the commandline tool /usr/sbin/screencapture or through the Grab utility (though their functionality is not extensive enough to justify me launching them as a subprocess).

References / Hints

• nm /usr/sbin/screencapture returns private Cocoa interfaces including _CGSGetSharedWindow that appear to do this.
• Third party application Snapzpro does this (but does not provide source code)

Answer 1

Command+Shift+4 to activate the screenshot selection, then tap the space bar to select the whole window.

Answer 2

Mac OS X 10.5 introduced the Quartz Window Services API to do just this.

Answer 3

The first thing that came to mind was GrabFS from MacFuse. The source is here.