Can't obtain output buffer from iOS camera in Kotlin Multiplatform - kotlin-multiplatform

Question

I've got a Kotlin Multiplatform multi-module project and one module deals with the camera. I've separated Android and iOS sourcesets for this specific module. Using Kotlin native for iOS, I can succesfully add a video input, a video output and show the preview in a layer, but I can't manage to get the sample buffer in the AVCaptureVideoDataOutputSampleBufferDelegateProtocol delegate.
I'm having this error:
Uncaught Kotlin exception: kotlin.native.IncorrectDereferenceException: illegal attempt to access non-shared <object>#8121d008 from other thread
at 0 MyFramework 0x000000010118fd3c kfun:kotlin.Throwable#<init>(kotlin.String?){} + 92
at 1 MyFramework 0x0000000101188cf0 kfun:kotlin.Exception#<init>(kotlin.String?){} + 88
at 2 MyFramework 0x000000010118883c kfun:kotlin.RuntimeException#<init>(kotlin.String?){} + 88
at 3 MyFramework 0x00000001011b7074 kfun:kotlin.native.IncorrectDereferenceException#<init>(kotlin.String){} + 88
at 4 MyFramework 0x00000001011ba8cc ThrowIllegalObjectSharingException + 428
at 5 MyFramework 0x00000001015fe100 TryAddHeapRef + 0
at 6 MyFramework 0x00000001015fddc4 _ZN27BackRefFromAssociatedObject9tryAddRefEv + 108
at 7 MyFramework 0x00000001015ed7f4 _ZN12_GLOBAL__N_113_tryRetainImpEP11objc_objectP13objc_selector + 64
at 8 libobjc.A.dylib 0x00000002202f1f94 objc_loadWeakRetained + 348
at 9 libobjc.A.dylib 0x00000002202f2060 objc_loadWeak + 20
at 10 AVFoundation 0x00000002271f73b8 <redacted> + 32
at 11 libdispatch.dylib 0x00000001049e8de4 _dispatch_client_callout + 16
at 12 libdispatch.dylib 0x00000001049f7fe4 _dispatch_sync_invoke_and_complete + 124
at 13 AVFoundation 0x00000002271f72c8 <redacted> + 164
at 14 AVFoundation 0x00000002271f715c <redacted> + 36
at 15 AVFoundation 0x00000002271d7fa4 <redacted> + 124
at 16 AVFoundation 0x00000002271d7ce8 <redacted> + 100
at 17 CoreMedia 0x00000002246ffdfc <redacted> + 280
at 18 CoreMedia 0x000000022471d4cc <redacted> + 224
at 19 libdispatch.dylib 0x00000001049e8de4 _dispatch_client_callout + 16
at 20 libdispatch.dylib 0x00000001049ec1e0 _dispatch_continuation_pop + 528
at 21 libdispatch.dylib 0x00000001049fecac _dispatch_source_invoke + 1864
at 22 libdispatch.dylib 0x00000001049f0cd8 _dispatch_lane_serial_drain + 288
at 23 libdispatch.dylib 0x00000001049f1bb4 _dispatch_lane_invoke + 516
at 24 libdispatch.dylib 0x00000001049f0cd8 _dispatch_lane_serial_drain + 288
at 25 libdispatch.dylib 0x00000001049f1b7c _dispatch_lane_invoke + 460
at 26 libdispatch.dylib 0x00000001049fbc18 _dispatch_workloop_worker_thread + 1220
at 27 libsystem_pthread.dylib 0x0000000220d260f0 _pthread_wqthread + 312
at 28 libsystem_pthread.dylib 0x0000000220d28d00 start_wqthread + 4
I guess is related to threading problems, maybe when the images are sendded to the delegate, but I'm not sure. I tried to use #ThreadLocal with no success.
This is the code:
class MyClass(val listener: MyListener) {
...
private val sampleDelegate: AVCaptureVideoDataOutputSampleBufferDelegateProtocol = object : NSObject(), AVCaptureVideoDataOutputSampleBufferDelegateProtocol {
override fun captureOutput(output: AVCaptureOutput, didOutputSampleBuffer: CMSampleBufferRef?, fromConnection: AVCaptureConnection) {
//Not called due to previous error
}
}
...
fun addVideoOutput(captureDevicePosition: AVCaptureDevicePosition) {
val videoOutput = AVCaptureVideoDataOutput()
val myqueue = dispatch_queue_create("cameraQueue", null)
videoOutput.setSampleBufferDelegate(sampleDelegate, myqueue)
val format = kCVPixelFormatType_32BGRA
videoOutput.videoSettings = NSDictionary.dictionaryWithObject(
NSNumber.numberWithUnsignedInt(format),
kCVPixelBufferPixelFormatTypeKey as NSCopyingProtocol
)
if (captureSession.canAddOutput(videoOutput)) {
captureSession.addOutput(videoOutput)
}
val conn = videoOutput.connectionWithMediaType(AVMediaTypeVideo)
if (conn!!.isVideoOrientationSupported()) {
conn.videoOrientation = captureDevicePosition
}
videoOutput.alwaysDiscardsLateVideoFrames = true
captureSession.sessionPreset = AVCaptureSessionPresetHigh
}

Answer 1

Using
val myqueue = dispatch_get_main_queue()
solved the problem