I'm brand new to OS X and mac programming. I'm looking to write an app that runs continuously or is woken up every 30 seconds to do some minor task. It would do something like check the weather via a web service and shoot off an email if the temperature drops below freezing. In Windows I would just use a service or put a repeating task in the task scheduler. Is there a standard way to accomplish this in OS X which the mac app store allows? I would like to write something that I can put in the store but Apple seems to have vague restrictions against running background processes.
I've learned a little about launchd but I can't tell if I am allowed to use it. Ideally the process would get run even when no one is logged in, but it probably won't need root access. If someone does happen to be logged in, it would trigger some popup if it sent an email temperature alert out. I would also have some GUI which the user would be able to modify the configuration files that determine the behavior of the background process. I don't anticipate any app store restriction for that piece of the project though.
The MAS guidelines have several prohibitions that would impact you. Go to the guidelines and read section 2. In particularly consider 2.15, 2.23, and 2.27. I don't know any way to get your application running prior to login without at some point requesting admin privileges (even if you don't use the privilege when you run).
I would probably make it a Login Item (System Preferences>Users>Login Items), probably as an LSUIElement app as sudo rm -rf suggests. Definitely if it's your first app, I wouldn't go diving into launchd in any case. It is one of the most infuriating system processes I've ever dealt with.