# prepareWithDevice A callback function for doing initial setup work. ``` var prepareWithDevice: ((any MTLDevice) -> Void)? ``` ## Discussion If you assign a function to [`prepareWithDevice`](/documentation/RealityKit/ARView/RenderCallbacks-swift.struct/prepareWithDevice), RealityKit calls that function once, after it does its setup work, but before it renders the next frame. If you register a [`prepareWithDevice`](/documentation/RealityKit/ARView/RenderCallbacks-swift.struct/prepareWithDevice) callback during app startup, RealityKit calls it before it renders the first frame. A callback function looks like this: ```swift func myPostProcessSetupCallback(device: MTLDevice) { // Handle setup work here. } ``` To register the function so RealityKit calls it, assign it to the [`prepareWithDevice`](/documentation/RealityKit/ARView/RenderCallbacks-swift.struct/prepareWithDevice) property of the view’s [`renderCallbacks`](/documentation/RealityKit/ARView/renderCallbacks-swift.property) property. ```swift arView.renderCallbacks.prepareWithDevice = myPostProcessSetupCallback ``` To remove the callback, assign `nil`. ```swift arView.renderCallbacks.prepareWithDevice = nil ``` If you assign a new closure or function to the property after setting it to `nil`, RealityKit calls the new closure before it renders the next frame. In that case, avoid doing long-running tasks in the callback function to avoid rendering hitches. --- 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)