WatchKit
The original toolkit for Apple Watch apps and complications.
Overview
WatchKit was how the first Apple Watch apps were built, with interface controllers laid out in storyboards. Today most watch apps use SwiftUI.
WatchKit still provides the app lifecycle, haptics, extended runtime sessions and other watch-specific services that SwiftUI apps rely on.
WKApplication
The app object and delegate that manage your watch app's lifecycle.
Haptics
Play system haptics that users recognize from the rest of watchOS.
Extended runtime sessions
Keep running for mindfulness, therapy, alarms or workouts.
Background refresh
Schedule tasks to update data and complications in the background.
History
How it grew up.
Introduced
The WatchKit SDK lets developers build for Apple Watch.
Native apps
watchOS 2 runs app code directly on the watch.
Independent apps
watchOS 6 brings the App Store to the watch.
SwiftUI lifecycle
watchOS 7 apps can be written entirely in SwiftUI.