NSPrincipalClass

NSPrincipalClass is a key in a macOS application's Info.plist that designates the class to instantiate as the app’s principal NSApplication object when the application launches. If the key is not present, the system uses the default NSApplication class.

At startup, the application’s main entry point (such as NSApplicationMain) reads the Info.plist and creates an instance of the class named by NSPrincipalClass to serve as the shared application object. This principal class can be a subclass of NSApplication, and it provides a place to customize the application’s event loop and global behavior by overriding NSApplication methods.

To use NSPrincipalClass, you create a subclass of NSApplication in your project and specify its class name as the value of the NSPrincipalClass key in Info.plist. In Swift, the value may need to include the module name (for example, MyModule.MyApplication) depending on your project setup. In Objective-C, the plain class name is typically sufficient. The subclass can override methods such as -sendEvent:, -finishLaunching:, or other NSApplication lifecycle methods to implement app-wide behavior. Even with a custom principal class, you can still employ an NSApplicationDelegate to handle most application lifecycle events.

Notes and considerations: the custom principal class must be available at launch time and correctly linked to the target. NSPrincipalClass is specific to macOS Cocoa apps and is not used in iOS, where UIApplicationMain handles application initialization.