Make sure your apps take advantage of all the great new features in OS X Yosemite, which will be available to millions of customers around the world later this fall.

Add powerful new functionality throughout OS X Yosemite with app extensions, explore the innovative new Swift programming language, and take advantage of advances in game technologies, Safari, iCloud, and more. To prepare your apps for the Mac App Store, download and build with the OS X Yosemite GM candidate and Xcode 6.1 GM seed from the Mac Dev Center. With these latest releases, Swift is now final and you can submit your Mac apps written with Swift to the Mac App Store.

https://developer.apple.com/osx/whats-new/