Apple Watch embodies the following themes:

• Personal. Because Apple Watch is designed to be worn, its UI is attuned to the wearer’s presence. A raise of the wrist shows the time and new alerts. Digital Touch—particularly with its Heartbeat and Sketch features—enables new types of communication that are incredibly personal. An accelerometer and heart rate sensor provide personalized information about the wearer’s activity from day to day. No other Apple device has ever been so connected to the wearer. It’s important to be mindful of this connection as you design apps for Apple Watch.
• Holistic. Apple Watch was designed to blur the boundaries between physical object and software. The Digital Crown is a finely tuned hardware control that enables nuanced software navigation. The Taptic Engine produces subtle, physical feedback associated with alerts and onscreen interaction. And Force Touch—a physical gesture interpreted by hardware—reveals a new dimension of contextual software controls. Even the physical border of the Retina display has been considered, resulting in edge-to-edge UI design that effectively renders that border invisible. Thoughtful app design should contribute to this experience of hardware and software feeling indistinguishable.
• Lightweight. Apps on Apple Watch are designed for quick, lightweight interactions that make the most of the display size and its position on the wrist. Information is accessible and dismissible quickly and easily, for both privacy and usability. The notification Short Look, for example, is designed to provide a minimal alert, only revealing more information if the wearer remains engaged. And Glances provide information from apps in an easy-to-access, swipe-able interface. Apps designed for Apple Watch should respect the context in which the wearer experiences them: briefly, frequently, and on a small display.

A WatchKit app complements your iOS app; it does not replace it. If you measure interactions with your iOS app in minutes, you can expect interactions with your WatchKit app to be measured in seconds. So interactions need to be brief and interfaces need to be simple.

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https://developer.apple.com/library/prerelease/ios...