Missing iOS shims _OnUnityMessage / _OnUnitySceneLoaded in v1.3.1 (Unity 6)

[gitterhubbb]

Is there an existing issue for this?

• I have searched the existing issues

Unity version

Unity 6000.0 LTS

Common issues

• I have checked the common issues

Flutter version

3.32.4

Description

I kept getting some build errors related to OnUnityMessage and OnUnitySceneLoaded.

Im not a very proficient developer but I troubleshot this for about 7 hours using chatGPT with little success and going down some fruitless rabbit holes. Finally, it suggested to add the following to this file "ios/.symlinks/plugins/flutter_embed_unity_2022_3_ios/ios/Classes/Messaging/SendToFlutter.swift" and it worked!! (ive only tried on iOS so far).

code to add:
(insert on line 15)
// These two C-callable shims are required by the Flutter embedder. Unity links
// against them when sending generic messages and scene‑loaded notifications.
@cdecl("OnUnityMessage")
public func onUnityMessage( objPtr: UnsafePointer?,
_ jsonPtr: UnsafePointer?) {
// The plugin only cares about the JSON payload coming from Unity
guard let jsonPtr = jsonPtr else { return }
let json = String(cString: jsonPtr)
SendToFlutter.sendToFlutter(json)
}

@cdecl("OnUnitySceneLoaded")
public func onUnitySceneLoaded( namePtr: UnsafePointer?,
_ buildIndex: Int32,
_ isLoaded: Bool,
_ isValid: Bool) {
// Pack the scene‑loaded event into a JSON blob so Flutter can switch UI, etc.
let sceneName = namePtr != nil ? String(cString: namePtr!) : ""
let payload: [String: Any] = [
"sceneName": sceneName,
"buildIndex": buildIndex,
"isLoaded": isLoaded,
"isValid": isValid
]
if let data = try? JSONSerialization.data(withJSONObject: payload),
let json = String(data: data, encoding: .utf8) {
SendToFlutter.sendToFlutter(json)
}
}

ChatGPT's explanation:
I added the two missing C-callable shims (OnUnityMessage and OnUnitySceneLoaded) to SendToFlutter.swift. They forward Unity messages and scene-loaded events to Flutter through the existing SendToFlutter helper.
Rebuild the iOS target and those undefined-symbol linker errors should disappear.

Minimum reproducible example (MRE)

No response

What platforms are you seeing the problem on?

No response

Devices

iphone13

Anything else?

No response

[jamesncl]

Unfortunately I'm not able to reproduce this using the example project (although I have just encountered a separate issue with the example app on ios, a runtime crash after the Unity splash screen which I'm investigating).

When you say 'build errors' do you mean when exporting the Unity project or when building the app?

Could you please provide any build error messages?

Are you sure you have exported the Unity project as iOS project (not Android)?

[gitterhubbb]

I looked back and found the errors I got. It's when I was using Flutter to build and test on my physical iPhone device. This was definitely with the iOS export as I haven't tried Android yet.

error:

Launching lib/main.dart on iPhone XR (2) (wireless) in debug mode...
Automatically signing iOS for device deployment using specified development team in Xcode project: xxxxxxxxxxxxx
Xcode build done. 38.9s
Failed to build iOS app
Could not build the precompiled application for the device.
Error (Xcode): Undefined symbol: _OnUnityMessage

Error (Xcode): Undefined symbol: _OnUnitySceneLoaded
2

Error launching application on iPhone XR (2) (wireless).

Exited (1).

[jamesncl]

It feels like the Unity project isn't linked properly in Xcode somehow, perhaps you have just made a mistake somewhere in the setup? I suggest trying the following:

• remove the Unity-Iphone project from Xcode
• delete the ios/unityLibrary folder
• run flutter clean
• export the unity project again
• follow the steps again in the readme for linking the Unity project to your Xcode workspace: https://pub.dev/packages/flutter_embed_unity#ios

[jamesncl]

Also, after exporting the unity project again, in the Xcode navigator, click on the Unity-iPhone.xcodeproj, then click on the Unity-iPhone project in the editor area, click on Build Settings, and look for the 'Other linker flags' setting (use the search box to help you find it). there should be the following value set:

If this is missing, you may be using an old version of the Unity export scripts (you used to have to add this manually each time you export the project, the newer version does this automatically). If that's the case, grab the latest build scripts as descibed in the readme (https://pub.dev/packages/flutter_embed_unity#import-unity-package) and import the assets into your Unity project. Then try the export again.

[gitterhubbb]

ahhhh I think this is it. When checking the image above, I didnt see anything in may Other Linker Flags. That reminded me that when I imposed the Unity Export scripts, I first imported the 2022.3 one before using the 6000 one. Maybe that's what messed this up. I manually added those elements and not it archives fine. Thanks for the guidance.

Thesis my first time creating a unity project, so sorry for the rookie mistakes.

[gitterhubbb]

I notice that your Unity-iphone project is a child of the runner project. I could have sworn that in the instruction (or maybe chatGPT) told me to keep it outside of Runner...

[jamesncl]

Just a warning, if you have to manually add the 'other linker flags' argument (because the export scripts aren't adding it) you will have to re-do this step after every time you export your unity project, because it will get overwritten.

It's odd, because the latest scripts for both the 2023 and the 6000 versions of unity should both do this automatically. So not sure how that got messed up, unless you used an older version perhaps? If you export the project again with the latest scripts, let me know if the other linker flags is still blank.

I notice that your Unity-iphone project is a child of the runner project. I could have sworn that in the instruction (or maybe chatGPT) told me to keep it outside of Runner...

You're right, I realised after writing that part of the readme that it doesn't matter if the project is a child of Runner or outside, and I have neglected to update the readme. I'll update that in the next version. Thanks!

[gitterhubbb]

You are correct, I did notice I have to keep manually adding that on each export. I even tried deleting and reimporting the Unity Export Package v6000 into my unity project, but still need to each time...

Before I found your package, I was trying to use the flutter_unity_widget. But that hasn't been updated in 17 months. I used ChatGPT to try an troubleshoot it for over a week straight. Lot of frankenstiening code (like my original post above) and probably hardcoded and modified things I shouldn't have (what happens when you a first time dev). ChatGPT helps new devs like me get started, but often times it tries too hard to fix the code in things that often just need a simpler fix. I had to learn a lot of things the hard way.

A separate bug is not being able to multitouch on IOS, but I read that that is more of an issue with flutter and unity. Im sure youre already tracking that. As a workaround I implemented my joystick in unity, then the jump button as a flutter overlay.

[gitterhubbb]

I wonder if this is an issue also, should I not have the second line in my pubspec.yaml file?

because that what the readme says to do: