The Waterwheel Swift SDK provides classes to natively connect iOS, macOS, tvOS, and watchOS applications to Drupal 7 and 8. It is formerly known as the Drupal iOS SDK.
It used to support Drupal 6, and technically still does but at your own risk.
I wanted to build an SDK that would allow users to focus on the functionality of their application and not how they talk to Drupal. Ive given various talks on the SDK, and made my first commit to the project in 2010 and have been working on it ever since.
Originally an objective-c project, it has seen been converted to Swift, and currently tracks Drupal 8.