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  "identifier" : "/documentation/ARKit/ARSession/identifier",
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# identifier

A unique identifier of the running session.

```
var identifier: UUID { get }
```

## Discussion

This property might change after you call the run function, but not immediately. Therefore, to get the new value, listen for its change using key-value observation.

```swift
// Use key-value observation to monitor my ARSession's identifier.
var sessionIDObservation: NSKeyValueObservation?
...
sessionIDObservation = observe(
    .arView.session.identifier,
    options: [.old, .new]) { 
        object, change in
        print("SessionID changed to: \(change.newValue!)")
    }
```

---

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