Bug ARFoundation: ARCameraBackground CommandBuffer freezes Editor

The CommandBuffer in ARCameraBackground freezes the Editor after a reset (leaving and re-entering playmode).

The exact method is

Code (CSharp):

1. internal static void AddBeforeBackgroundRenderHandler()
2. {
3.     commandBuffer.IssuePluginEvent(s_BeforeBackgroundRenderHandlerFuncPtr, 0);
4. }

If this method runs, and you leave and try to re-enter play mode, the editor remains stuck at 'Application.Reload' indefinitely.

 

I stumbled on this issue too in the development of my plugin. I ended up writing a script that will automatically apply this fix:

Code (CSharp):

1. // calling commandBuffer.IssuePluginEvent is crashing Unity Editor 2019.2 and freezing newer versions of Unity
2. #if !UNITY_EDITOR
3.     commandBuffer.IssuePluginEvent(s_BeforeBackgroundRenderHandlerFuncPtr, 0);
4. #endif

 

Here is a script to apply the fix automatically. This is not the perfect piece of code, but you can use it as an example
Please place it inside the Editor folder.

Code (CSharp):

1. using System;
2. using System.IO;
3. using UnityEditor;
4.  
5.  
6. [InitializeOnLoad]
7. public static class ARCameraBackgroundFixer {
8.     static ARCameraBackgroundFixer() {
9.         if (ApplyFixIfNeeded()) {
10.             AssetDatabase.Refresh();
11.         }
12.     }
13.  
14.     static bool ApplyFixIfNeeded() {
15.         var path = "Packages/com.unity.xr.arfoundation/Runtime/AR/ARCameraBackground.cs";
16.         var script = AssetDatabase.LoadAssetAtPath<MonoScript>(path);
17.         var text = script.text;
18.         if (text.Contains("AR_FOUNDATION_EDITOR_REMOTE")) {
19.             return false;
20.         }
21.  
22.         var index = text.IndexOf("commandBuffer.IssuePluginEvent(", StringComparison.Ordinal);
23.         var withFix = text.Insert(index, @"// AR_FOUNDATION_EDITOR_REMOTE: calling commandBuffer.IssuePluginEvent is crashing Unity Editor 2019.2 and freezing newer versions of Unity
24.            " + @"#if !UNITY_EDITOR
25.            ");
26.         withFix = withFix.Insert(withFix.IndexOf(";", index, StringComparison.Ordinal) + 1, @"
27.            " + "#endif");
28.         File.WriteAllText(AssetDatabase.GetAssetPath(script), withFix);
29.         return true;
30.     }
31. }
32.  

 

Thanks @KirillKuzyk
Your plugin is actually what led me to finding the bug.
My own code would just freeze the editor whenever I did return a texture, and not freeze if I didn't.

I'm currently creating a similar plugin to yours (but without the 'Remote'-part), and would like to not have to implement the same 'hacky' fixes you did (especially when looking at future compatibility), which is why I made this thread.

 

Ok, so after looking at ARCameraBackground.cs a bit more, three things stood out to me:



This creates a pointer to a function in the native subsystem for the render-thread to call just before rendering.
As neither mine nor @KirillKuzyk's XRCameraSubsystem exists in unmanaged code when Unity reloads before entering Play Mode the call to the static constructor of ARCameraBackground will attempt to re-marshall the pointer.
This somehow puts the Editor into an infinite loop or await.

Adding a #if !UNITY_EDITOR around the [MonoPInvokeCallback(typeof(Action<int>))]-Attribute allows for our managed XRSubsystems to work just fine, keeping the commandbuffer while gaining a call to subsystem.OnBeforeBackgroundRender in the process.
Example:

Code (CSharp):

1. #if !UNITY_EDITOR
2. [MonoPInvokeCallback(typeof(Action<int>))]
3. #endif
4. static void BeforeBackgroundRenderHandler(int eventId)
5. {
6.     if (s_CameraSubsystem != null)
7.         s_CameraSubsystem.OnBeforeBackgroundRender(eventId);
8. }

Of course this is still a 'hacky' fix. Any 'natively' supported editor-attached XRDevice (as in native to the Unity team) would not run from within the Editor.
A better fix would be for ARFoundation to provide built-in defines for managed vs unmanaged subsystems.
Is there any way to file bugreports specifically to ARFoundation devs?

 

I think this repo is the best place to file bug reports:
https://github.com/Unity-Technologies/arfoundation-samples

 

Hi i just downloaded the new version of ARF remote and I get

AR Foundation Remote. ARCameraBackground calls CommandBuffer.IssuePluginEvent which crashes/freezes some versions of Unity.

Im using Unity 2022.2.1f
and AR foundation 5.0
any idea's how to fix this?

 

Neverminded embeding the project fixed issue.
however when i run it it's just black image on the phone and it's not tracking