Launch User App with GUI from Launch Daemon

I've got a Launch Daemon that runs as root outside of the user context. This Daemon is always running and monitoring a server. On occasion, my Daemon will get a command from the server telling it to display something to the user. Obviously I can't do this from a Launch Daemon since it's not running in a "WindowsServer" environment - so the other option is to spawn a separate process for each user that has access to be able to display a GUI.

One option I have is to make a Launch Agent and have the OS start it up for me. The launch agent could then communicate with the Launch Daemon to determine when something needs to be displayed.

The problem is this though: the server will sometimes provide updates of the actual binary files. So the server would send the Launch Daemon a series of files which it is expected to copy out on top of the existing files. If I have a Launch Agent runni开发者_开发技巧ng, how can I shut it down so that the agent itself can be updated? Likewise, how could I start the Agent back up after the update is done?

Off the cuff, I would say that you should arrange a way for your daemon to request the launch agent to exit on demand, the same way you would arrange for it to launch on demand. Once you know the agent has exited, your daemon can proceed with the update.

Note that your agent probably should not live forever, and it should definitely not be polling the daemon for work. Instead, you should rig it up so that, when the daemon tries to connect to the agent, launchd launches the agent. You could do this by having the daemon connect to a Unix domain socket registered on behalf of the agent by launchd, for example.

You might do better to look at how Google's UpdateEngine handles software updates. I believe it too has a launch daemon perform the app updates. Since this is already working on many computers, its solution to software update problems is a proven one. (Sparkle's solution is thoroughly proven, as well, but it relies on user interaction, which you seem to want to avoid.)