NSApplicationDelegate

NSApplicationDelegate is a protocol in the AppKit framework that defines the delegate interface for NSApplication objects. An object that conforms to the protocol can receive messages about the application's lifecycle and other high-level events, enabling centralized control of app behavior without subclassing NSApplication. The NSApplicationDelegate protocol is part of the Cocoa application architecture; its methods are optional, so a delegate implements only the events it needs.

Common methods include applicationDidFinishLaunching, invoked when the app has completed its launch; applicationWillTerminate, called just before the app terminates; and applicationShouldTerminate, which asks the delegate to decide whether the app should quit, allowing cleanup or cancellation. There are also callbacks for the app becoming active or resigning active, and for various state transitions during startup and termination.

In practice, the delegate is typically implemented by an AppDelegate class that conforms to NSApplicationDelegate and is assigned to NSApp.delegate. In Swift, the AppDelegate implements the protocol and provides implementations for the selected methods. The delegate pattern allows the application to initialize resources, restore state, save data, and handle termination in a centralized, platform-consistent way.

The concept complements the NSApplication and the broader AppKit lifecycle, and is a common component in macOS applications developed with Cocoa.