Determine if WindowServer is available?

Question

I'm writing a framework for OS X that may be used by applications with or without a connection to the OS X WindowServer (i.e. both GUI apps and command-line apps run via, e.g. an ssh session). One class in the framework is for tracking files in the user's home folder across netework and mobile home directories (on OS X, users can have their home directory served via AFP from a server--a "network home directory"--or have the same home folder syncrhonized for offline access--a "mobile home directory").

Because I need to track files across filesystems, we've chosen to use relative paths, rather than OS X aliases (or the 10.6 NSURL bookmarks). When a file can't be found, I need to ask the user for input to relocate that file (like how the Alias Manager prompts the user to reconnect a broken alias). If the application has (or can make) a connection to the WindowServer, this is as simple as using an NSOpenPanel. However, when the app can't make a connection to the WindowServer I need to use an alternative method to get the user input.

So, how can I tell which method to use from within the famework code? Is there a way to programatically determine if a WindowServer connection is available (or possible)?

I recognize that an alternative architecture, where the framework client provides a callback mechanism to prompt the user would let the input-gathering strategy be provided by the calling application. I'd like to make things as simple as possible for the calling application, however, so my first choice would be to encapsulate these details in the framework if I can.

Answer 1

There's an environment variable named SECURITYSESSIONID that is set by loginwindow.app and get's passed to the user's applications. The variable is not set if you login via ssh. It serves as kind of a handle to talk to the window server.

Problem: The existence of this variable does not mean that this user currently controls the window manager (think fast user switching).

There's a function called CGSessionCopyCurrentDictionary in the ApplicationServices framework which looks promising:

Return Value: A window server session dictionary, or NULL if the caller is not running within a Quartz GUI session or the window server is disabled. You should release the dictionary when you are finished using it. For information about the key-value pairs in this dictionary, see "Window Server Session Properties."