How can I sync a timer between iPhone and Apple Watch?

To sync a timer between your iPhone and Apple Watch, you can use the push notification API to handle state changes across devices. Here's a step-by-step approach based on the sessions from WWDC:

Use Push Notifications for State Changes:
- When you start or stop the timer on one device (e.g., iPad), you can trigger a reload of the control on other devices (e.g., iPhone) using push notifications. This ensures that the timer state is synchronized across all devices.
- For more details, refer to the session Extend your app’s controls across the system (11:44).
Live Activity Synchronization:
- If you are using live activities, updates are automatically synchronized to the Apple Watch. This means that any updates to your live activity on the iPhone will reflect on the Apple Watch as well.
- For more information, see the session Bring your Live Activity to Apple Watch (07:03).
Configurable Controls:
- You can create configurable controls using WidgetKit, which allows you to manage different timers (e.g., work and personal) and place them in Control Center. This way, you can start and stop timers from either device, and the state will be synchronized.
- For more details, refer to the session Extend your app’s controls across the system (12:40).

By following these steps, you can ensure that your timer is synchronized between your iPhone and Apple Watch, providing a seamless experience across your devices.