For those who don't watch the video to know what components Apple has mentioned in the video to partially/fully rewritten to Swift:

* Many parts of Music app * Many parts of the Dock (rewrote Mission Control, accessibility features, etc) * Console App new features (e.g. logging) * Many OS Agents/Daemons * The new picture-in-picture feature in Sierra * Xcode features like the new documentation feature * New Playground app