Avoid recursive use of objects

Oct 13, 2025 by pinkandtrash.com 130 views

Introduction

Conditional rendering of RigidBody in a scene can sometimes lead to unexpected crashes, especially when using hand tracking and external state management. In this article, we will explore a specific scenario where toggling the rendering of a component with a RigidBody can cause the scene to crash with the error "recursive use of an object detected".

The Problem

The issue arises when a component uses the useHandJoint hook to track the hand joint, sets a value in an external Svelte state based on the presence of the hand joint using an effect, and conditionally renders a RigidBody based on that state value. When the rendering of the component is toggled off and back on again while in XR with the hand joint present, the scene crashes.

Error Message

The error message is quite cryptic, but it points to a recursive use of an object detected, which would lead to unsafe aliasing in Rust. The error is thrown by the webxr-polyfill.js file and is related to the Rapier physics engine.

Minimal Reprex

The issue can be reproduced with a minimal reprex available on GitHub: https://github.com/wmurphyrd/threlte-reprex/tree/hand-physics-crash. The reprex includes a Svelte component that uses the useHandJoint hook and conditionally renders a RigidBody based on the presence of the hand joint.

Component Code

The component code is as follows:

<script lang="ts">
	import { RigidBody } from '@threlte/rapier';
	import { useHandJoint } from '@threlte/xr';
	import { testState } from './testState.svelte';

	const hand = useHandJoint('left', 'index-finger-tip');
	$effect(() => {
		testState.interactionMode = $hand ? 'hand' : 'pointer';
	});
</script>

{#if testState.interactionMode === 'hand'}
	<RigidBody type="kinematicPosition" />
{/if}

Rendering the Component

The component is rendered using the following code:

{#if show}
	<Test />
{/if}

Reproducing the Issue

To reproduce the issue, follow these steps:

Enter XR with either emulator or Quest Browser.
Ensure the right hand is tracking.
Wait up to 4 seconds (I have an interval in Scene.svelte to toggle the component rendering).

Workarounds

Some workarounds that avoid the crash are:

Change the {#if} for the RigidBody to check hand directly instead of the external state.
Set the external state with $effect.pre instead of $effect.

Conclusion

Toggling conditional rendering of RigidBody can crash the scene with the error "recursive use of an object detected". This issue arises when using hand tracking and external state management. By understanding the problem and using workarounds, developers can avoid this crash and ensure a smooth user experience.

Troubleshooting Steps

If you encounter this issue, follow these troubleshooting steps:

Check the component code: Ensure that the component code is correct and does not contain any errors.
Verify the external state: Check that the external state is being updated correctly and is not causing the crash.
Use a debugger: Use a debugger to step through the code and identify the source of the crash.
Try a different approach: Consider using a different approach, such as using a different physics engine or a different way of managing the RigidBody.

Best Practices

To avoid this issue, follow these best practices:

Use a consistent approach: Use a consistent approach to managing the RigidBody and external state.
Avoid recursive use of objects: Avoid using objects recursively, as this can lead to crashes.
Use a debugger: Use a debugger to step through the code and identify potential issues.
Test thoroughly: Test the code thoroughly to ensure that it works correctly in all scenarios.
Toggling Conditional Rendering of RigidBody Can Crash Scene with "Recursive Use of an Object Detected" - Q&A ===========================================================

Introduction

In our previous article, we explored a specific scenario where toggling the rendering of a component with a RigidBody can cause the scene to crash with the error "recursive use of an object detected". In this article, we will answer some frequently asked questions related to this issue.

Q: What is the cause of the "recursive use of an object detected" error?

A: The "recursive use of an object detected" error is caused by the Rapier physics engine detecting a recursive use of an object, which would lead to unsafe aliasing in Rust. This error is thrown when the engine tries to access an object that has already been accessed, causing a recursive loop.

Q: Why does this issue only occur when using hand tracking and external state management?

A: This issue only occurs when using hand tracking and external state management because these features introduce additional complexity to the scene. When the hand joint is tracked and the external state is updated, the RigidBody is created and updated multiple times, leading to the recursive use of an object detected error.

Q: How can I reproduce this issue?

A: To reproduce this issue, follow these steps:

Enter XR with either emulator or Quest Browser.
Ensure the right hand is tracking.
Wait up to 4 seconds (I have an interval in Scene.svelte to toggle the component rendering).

Q: What are some workarounds to avoid this crash?

A: Some workarounds to avoid this crash are:

Change the {#if} for the RigidBody to check hand directly instead of the external state.
Set the external state with $effect.pre instead of $effect.

Q: Can I use a different physics engine to avoid this issue?

A: Yes, you can use a different physics engine to avoid this issue. However, this may require significant changes to your code and may not be feasible in all cases.

Q: How can I troubleshoot this issue?

A: To troubleshoot this issue, follow these steps:

Check the component code: Ensure that the component code is correct and does not contain any errors.
Verify the external state: Check that the external state is being updated correctly and is not causing the crash.
Use a debugger: Use a debugger to step through the code and identify the source of the crash.
Try a different approach: Consider using a different approach, such as using a different physics engine or a different way of managing the RigidBody.

Q: What are some best practices to avoid this issue?

A: To avoid this issue, follow these best practices:

Use a consistent approach: Use a consistent approach to managing the RigidBody and external state.
Avoid recursive use of objects: Avoid using objects recursively, as this can lead to crashes.
Use a debugger: Use a debugger to step through the code and identify potential issues.
Test thoroughly: Test the code thoroughly to ensure that it works correctly in all scenarios.

Additional Resources

For more information on this issue and how to troubleshoot it, refer to the following resources:

Conclusion

The "recursive use of an object detected" error is a complex issue that can be challenging to troubleshoot. By understanding the cause of the issue and following best practices, developers can avoid this crash and ensure a smooth user experience.