WatchConnectivity

WatchConnectivity is a software framework provided by Apple that enables communication between an iOS device and its paired Apple Watch. It is part of the WatchKit and UIKit ecosystems and is available on iOS 8.0 and later, as well as watchOS 1.0 and later. The framework is designed to exchange messages, files, and user activity data in a secure, asynchronous manner while respecting power and connectivity constraints.

Core concepts include the WCSession class, which represents a single communication session between the two devices. A session can be activated on both the iPhone and the Watch, after which they can send data using methods such as sendMessage:, sendMessageData:, transferFile:metadata:, and updateApplicationContext:. The framework supports both session states (reachable, userInfo transfer) and provides delegate callbacks to inform the developer when messages are received and when session state changes. Security is handled automatically; all data is encrypted during transport and is tied to the paired devices. The framework also supports background transfers, allowing large files to be sent without keeping the app active.

Typical usage patterns involve lightweight data updates, such as user activity or UI state synchronization, and heavier data, such as media caching or configuration files. Developers often combine WatchConnectivity with other APIs like HealthKit or CoreMotion to build coordinated Apple Watch apps. The design emphasizes resilience: if the counterpart is temporarily unreachable, messages and files are queued for later delivery once a connection is reestablished.