# AURenderObserver A block called when an audio unit renders audio. ``` typealias AURenderObserver = (AudioUnitRenderActionFlags, UnsafePointer, AUAudioFrameCount, Int) -> Void ``` ## Discussion This block is called by the base class’s AURenderBlock block before and after each render cycle. The observer can distinguish between before and after using the [`unitRenderAction_PreRender`](/documentation/AudioToolbox/AudioUnitRenderActionFlags/unitRenderAction_PreRender) and [`unitRenderAction_PostRender`](/documentation/AudioToolbox/AudioUnitRenderActionFlags/unitRenderAction_PostRender) action flag values. The block takes the following parameters: - actionFlags: The pointer to the action flags. - timestamp: The HAL time at which the input data will be rendered. If there is a sample rate conversion or time compression/expansion downstream, the sample time will not have a defined correlation with the `AudioDevice` sample time. - frameCount: The number of sample frames to render. - outputBusNumber: The index of the output bus to render. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)