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# kAudioUnitProperty_RenderContextObserver

The block that the system calls when the rendering context changes.

```
kAudioUnitProperty_RenderContextObserver
```

## Discussion

If your Audio Unit creates auxilliary realtime rendering threads, set the value of this key to the [`AURenderContextObserver`](/documentation/AudioToolbox/AURenderContextObserver) block you want the system to execute. The system executes your block when the rendering context changes.

> Important:
> The block you provide is for system use only. Audio Unit hosts must not attempt to interact with the audio unit through this block.

---

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