In macOS application development, the NSApplicationDelegate protocol is essential for managing the lifecycle of your app. This protocol allows you to respond to important events in the app's life, such as launching, terminating, and handling notifications. Understanding and implementing NSApplicationDelegate can significantly enhance the functionality and user experience of your macOS applications.

Examples:

1. Creating a Basic macOS App with NSApplicationDelegate

   First, let's create a simple macOS application that uses NSApplicationDelegate.

   import Cocoa
   
   @NSApplicationMain
   class AppDelegate: NSObject, NSApplicationDelegate {
   
      func applicationDidFinishLaunching(_ aNotification: Notification) {
          // Insert code here to initialize your application
          print("Application did finish launching")
      }
   
      func applicationWillTerminate(_ aNotification: Notification) {
          // Insert code here to tear down your application
          print("Application will terminate")
      }
   }

   In this example, the AppDelegate class conforms to the NSApplicationDelegate protocol. The applicationDidFinishLaunching method is called when the application has completed its launch process, and the applicationWillTerminate method is called when the application is about to terminate.

2. Handling Notifications

   You can also use NSApplicationDelegate to handle notifications. For instance, if you want to respond to a notification when the app becomes active:

   import Cocoa
   
   @NSApplicationMain
   class AppDelegate: NSObject, NSApplicationDelegate {
   
      func applicationDidFinishLaunching(_ aNotification: Notification) {
          // Insert code here to initialize your application
          print("Application did finish launching")
      }
   
      func applicationWillTerminate(_ aNotification: Notification) {
          // Insert code here to tear down your application
          print("Application will terminate")
      }
   
      func applicationDidBecomeActive(_ notification: Notification) {
          // Insert code here to handle when the app becomes active
          print("Application did become active")
      }
   }

   Here, the applicationDidBecomeActive method is used to perform actions when the application becomes active.

3. Creating a Custom Window

   Suppose you want to create a custom window when the application launches. You can do this in the applicationDidFinishLaunching method:

   import Cocoa
   
   @NSApplicationMain
   class AppDelegate: NSObject, NSApplicationDelegate {
   
      var window: NSWindow?
   
      func applicationDidFinishLaunching(_ aNotification: Notification) {
          // Create the window
          let screenSize = NSScreen.main?.frame.size ?? CGSize(width: 800, height: 600)
          let windowSize = CGSize(width: 400, height: 300)
          let windowRect = NSRect(x: (screenSize.width - windowSize.width) / 2,
                                  y: (screenSize.height - windowSize.height) / 2,
                                  width: windowSize.width,
                                  height: windowSize.height)
          window = NSWindow(contentRect: windowRect,
                            styleMask: [.titled, .closable, .resizable],
                            backing: .buffered,
                            defer: false)
          window?.title = "My Custom Window"
          window?.makeKeyAndOrderFront(nil)
      }
   
      func applicationWillTerminate(_ aNotification: Notification) {
          // Insert code here to tear down your application
          print("Application will terminate")
      }
   }

   In this example, a custom window is created and displayed when the application finishes launching.