Why do I get a linking error after updating to Xcode 12?

Question

I have a Unity project that uses a native Swift + ObjC library that I made using Firebase MLKit. When trying to build for iOS, the Unity project always builds with Xcode 11.3.1, but when updating to any Xcode 12.X version, I get the following errors:

ld: warning: Could not find or use auto-linked library 'swiftCoreMIDI'
ld: warning: Could not find or use auto-linked library 'swiftUniformTypeIdentifiers'
Undefined symbols for architecture armv7:
  "__swift_FORCE_LOAD_$_swiftUniformTypeIdentifiers", referenced from:
      __swift_FORCE_LOAD_$_swiftUniformTypeIdentifiers_$_NativeLibrary_iOS in NativeLibrary_iOS.a(NativeLibrary.o)
      __swift_FORCE_LOAD_$_swiftUniformTypeIdentifiers_$_NativeLibrary_iOS in NativeLibrary_iOS.a(UIImage.o)
     (maybe you meant: __swift_FORCE_LOAD_$_swiftUniformTypeIdentifiers_$_NativeLibrary_iOS)
  "__swift_FORCE_LOAD_$_swiftCoreMIDI", referenced from:
      __swift_FORCE_LOAD_$_swiftCoreMIDI_$_NativeLibrary_iOS in NativeLibrary_iOS.a(NativeLibrary.o)
      __swift_FORCE_LOAD_$_swiftCoreMIDI_$_NativeLibrary_iOS in NativeLibrary_iOS.a(UIImage.o)
     (maybe you meant: __swift_FORCE_LOAD_$_swiftCoreMIDI_$_NativeLibrary_iOS)
ld: symbol(s) not found for architecture armv7
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I am not using either swiftCoreMIDI nor swiftUniformTypeIdentifiers in my code. For the record, I did recompile my native library with Xcode 12 before importing it again in Unity.

Things I have tried:

• Building a Unity project with a Swift library that doesn't use Firebase MLKit: works fine
• Building an Xcode only project that uses Firebase MLKit: works fine
• Building the project with a more recent Unity version: fails
• Integrating the Firebase SDK frameworks directly instead of using CocoaPods: fails
• Adding an empty Swift file + bridging header to my project: fails
• Adding the user defined setting LD_VERIFY_BITCODE = NO in XCode: fails

I'm using:

• Xcode 12.2
• Firebase 6.34
• Swift 5.0
• Unity 2019.3.6f1

Any help would be appreciated, I've been stuck on this for a while!

Answer 1

SOLUTION

I finally found the solution, it was linked to the UnityFramework target in Xcode. I had to remove some architectures that are not concerned by my build: i386 and x86_64 . In addition, I updated the Library Search Paths to include $(SDKROOT)/usr/lib/swift .

To execute those actions automatically, you can add those lines in a PostProcessBuild file in your Unity project:

var projPath = pathToBuiltProject + "/Unity-iPhone.xcodeproj/project.pbxproj";
var proj = new PBXProject();
proj.ReadFromFile(projPath);

// Update the Library Search Paths of the whole Xcode project
proj.AddBuildProperty(proj.ProjectGuid(), "LIBRARY_SEARCH_PATHS", "$(SDKROOT)/usr/lib/swift");

// Get the UnityFramework target and exclude the unwanted architectures
var unityFrameworkGuid = proj.TargetGuidByName("UnityFramework");
proj.SetBuildProperty(unityFrameworkGuid, "EXCLUDED_ARCHS", "i386");
proj.AddBuildProperty(unityFrameworkGuid, "EXCLUDED_ARCHS", "x86_64");