Learn all about AppKit with these posts
AppKit is a graphical user interface toolkit on macOS. It is the original macOS development framework, and provides the basic building blocks for application development.
New! How to keep your macOS app's menu bar item running after quitting the app
Learn how you can interrupt the app's termination and change your app's activation policy to keep the menu bar item running after the app has been quit.
How to setup a menu bar only macOS app using AppKit.