Swift Plugin Code (unity_kit/ios/Classes/)
├── SwiftUnityKitPlugin.swift      FlutterPlugin entry point, view factory registration
├── UnityKitViewFactory.swift      FlutterPlatformViewFactory
├── UnityKitViewController.swift   FlutterPlatformView + MethodChannel handler
├── UnityKitView.swift             UIView container for the Unity view
├── UnityPlayerManager.swift       Singleton Unity player lifecycle
├── FlutterBridgeRegistry.swift    Routes Unity messages to Flutter controllers
└── UnityEventListener.swift       Protocol for Unity event callbacks

Native Bridge (compiled into UnityFramework)
└── UnityKitNativeBridge.mm        C symbols for IL2CPP DllImport

Scripting Backend = IL2CPP        (Required)
Target SDK        = Device SDK    (Simulator not supported)
Architecture      = ARM64         (Required)

# Build UnityFramework from the Unity export
xcodebuild \
  -project /path/to/Builds/ios/Unity-iPhone.xcodeproj \
  -scheme UnityFramework \
  -configuration Release \
  -sdk iphoneos \
  -arch arm64 \
  ONLY_ACTIVE_ARCH=NO \
  BUILD_DIR=/path/to/Builds/ios/build

# Result: build/Release-iphoneos/UnityFramework.framework

cd unity_kit/ios
ln -s /path/to/Unity/Builds/ios/build/Release-iphoneos/UnityFramework.framework .

platform :ios, '13.0'

post_install do |installer|
  installer.pods_project.targets.each do |target|
    target.build_configurations.each do |config|
      config.build_settings['ENABLE_BITCODE'] = 'NO'
    end
  end
end

Pod::Spec.new do |s|
  s.name             = 'unity_kit'
  s.platform         = :ios, '13.0'
  s.source_files     = 'Classes/**/*'

  # Vendored framework support (when symlinked or copied)
  unity_framework_path = File.join(__dir__, 'UnityFramework.framework')
  if File.exist?(unity_framework_path) || File.symlink?(unity_framework_path)
    s.ios.vendored_frameworks = 'UnityFramework.framework'
    s.preserve_paths = 'UnityFramework.framework'
  end

  s.pod_target_xcconfig = {
    'DEFINES_MODULE' => 'YES',
    'EXCLUDED_ARCHS[sdk=iphonesimulator*]' => 'i386',
    'FRAMEWORK_SEARCH_PATHS' =>
      '$(inherited) "${PODS_TARGET_SRCROOT}" "${PODS_CONFIGURATION_BUILD_DIR}"',
    'OTHER_LDFLAGS' => '$(inherited) -ObjC',
  }
end

// unity_kit/unity/Assets/Plugins/iOS/UnityKitNativeBridge.mm
#import <Foundation/Foundation.h>
#import <objc/runtime.h>

static const NSUInteger kMaxMessageSize = 1048576; // 1 MB

extern "C" {

void SendMessageToFlutter(const char* message) {
    if (message == NULL) return;
    size_t length = strnlen(message, kMaxMessageSize + 1);
    if (length > kMaxMessageSize) return; // size guard before UTF-8 read

    NSString *msg = [NSString stringWithUTF8String:message];

    // Unity calls from the render thread; MethodChannel needs the main thread
    dispatch_async(dispatch_get_main_queue(), ^{
        Class cls = NSClassFromString(@"FlutterBridgeRegistry");
        if (cls == nil) return; // discovered at runtime, not linked
        SEL sel = NSSelectorFromString(@"sendMessageToFlutter:");
        if ([cls respondsToSelector:sel]) {
            [cls performSelector:sel withObject:msg];
        }
    });
}

} // extern "C"

// DOES NOT WORK — symbol lands in unity_kit.framework, not UnityFramework
@_cdecl("SendMessageToFlutter")
func sendMessageToFlutterC(_ message: UnsafePointer<CChar>) { ... }
// IL2CPP linker looks inside UnityFramework → "Undefined symbol" at link time

*** Terminating app due to uncaught exception 'NSInvalidArgumentException'
*** reason: '-[MTLTextureDescriptorInternal validateWithDevice:]:
    MTLTextureDescriptor has width of zero.'

private func waitForNonZeroFrame(attempt: Int = 0) {
    guard !isDisposed else { return }

    if !containerView.bounds.isEmpty {
        autoInitialize()
    } else if attempt < 20 {
        // Poll every 50ms for up to 1 second
        DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(50)) {
            [weak self] in
            self?.waitForNonZeroFrame(attempt: attempt + 1)
        }
    } else {
        autoInitialize() // timeout — proceed anyway, logs the attempt
    }
}

private func waitForUnityView(attempt: Int, maxAttempts: Int) {
    guard !isDisposed else { return }

    // Bounds guard — never attach into a zero-size container
    guard !containerView.bounds.isEmpty else {
        if attempt < maxAttempts {
            let delayMs = 100 * (attempt + 1) // linear backoff
            // retry after delayMs, up to 30 attempts...
        }
        return
    }

    if let unityView = UnityPlayerManager.shared.getView() {
        containerView.attachUnityView(unityView)
        sendEvent(name: "onUnityCreated", data: nil)
    }
}

UnityView(
  config: const UnityConfig(
    sceneName: 'AvatarScene',
    transparentBackground: true, // iOS only; default is false
  ),
)

1. Select your scene's main camera.
2. Clear Flags → Solid Color.
3. Background → RGBA (0, 0, 0, 0)   (alpha must be 0).

'EXCLUDED_ARCHS[sdk=iphonesimulator*]' => 'i386'