Pebble iOS SDK by Pebble interacts with the API to create applications in wearable watches. Resources on GitHub include CocoaPods installation and Xcode documentation. Integration occurs via Bluetooth. See the examples available at https://github.com/pebble/pebble-sdk-examples
At Google I/O 2016, Google announced the Awareness API that would enable apps to use and react to a user's context. Then, the API was available to a limited pool of integration partners. Google has now opened the API to all developers so all apps can leverage context like location and activity.