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# workoutSessionMirroringStartHandler

A block that the system calls when it starts a mirrored workout session.

```
var workoutSessionMirroringStartHandler: (@Sendable (HKWorkoutSession) -> Void)? { get set }
```

## Discussion

The system calls this block on the companion iPhone when someone starts a mirrored workout on Apple Watch. If your iOS app isn’t active, the system launches it in the background.

```swift
// The HealthKit store calls this closure when Apple Watch starts a remote session.
store.workoutSessionMirroringStartHandler = { mirroredSession in
    // Reset the health data.
    self.data = HealthData()

    // Save a reference to the workout session.
    self.session = mirroredSession
    logger.debug("*** A session started on the companion Apple Watch. ***")
}
```

To ensure that your app can always catch incoming mirrored workout sessions, assign this property as soon as your app launches.

> Important:
> Your app may receive multiple calls to ``doc://com.apple.healthkit/documentation/HealthKit/HKHealthStore/workoutSessionMirroringStartHandler``. If iPhone and Apple Watch lose their connection in the middle of a workout session, Apple Watch automatically tries to reconnect. Each call has its own ``doc://com.apple.healthkit/documentation/HealthKit/HKWorkoutSession`` instance.

The system calls this block from an arbitrary background queue.

---

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