This problem occasionally happens. My view looks like this simplified:

struct CameraView: View {
    @State var camera: Camera
    
    var body: some View {
        VStack{
            if let uiImage = camera.previewPhoto {
                ImageView(uiImage)
                SomeOtherButton()
            } else {
                camera.preview
                CapturePhotoButton(camera) // has a button that calls camera.capturePhoto()
            }
        }
    }
}

And I have a Camera view model that looks something like this:

@Observable
class Camera {
    @ObservationIgnored private let captureSession: MyCustomCaptureSession
    @ObservationIgnored @MainActor var preview: some View {
        captureSession.preview // Just a wrapper view around AVCaptureVideoPreviewLayer
    }
    @MainActor var previewPhoto: UIImage?

    @MainActor func capturePhoto() async {
        // Take a photo using a captureSession
        print("AT POINT A")
        previewPhoto = capturedPhoto
    }
}

As you can see, I made sure to decorate previewPhoto with @MainActor.

On some rare occasions, when I press the “capturePhoto” button (not show here), I see the line AT POINT A get printed, but the view does not update. This baffles me and is really hard to debug, any ideas?

One thing I noticed is that if I break up the CameraView into top and bottoms halves:

struct CameraView: View {
    @State var camera: Camera

    var body: some View {
        VStack{
            if let uiImage = camera.previewPhoto {
                ImageView(uiImage)
            } else {
                camera.preview
            }

            if let uiImage = camera.previewPhoto {
                SomeOtherButton()
            } else {
                CapturePhotoButton(camera) // has a button that calls camera.capturePhoto()
            }
        }
    }
}

The view will sometimes only update the bottom, not the top, this makes me suspect its some kind of race condition or that it’s related to camera.preview.

Another thing to note is that I’m using iOS 17’s new observation framework.