I have a concept for an extremely simple Mac application, but I have no idea how to make it. I'd like it to be a program that runs invisibly in the background and detects when a key is pressed and released. When the key is pressed, it should lock a file, when the key is released, it should unlock the file. That's all. The issue is, I've never made an application that runs completely without a UI. I don't even want the menubar to change at all. Also, not particularly important as I'm content with just running the application on my own computer and those of a few friends that want it, but do you think Apple would allow such a program on the App Store? It's not a virus or male ware or anything of that nature.