Swift Unit and AVFoundation testing: how to check for the presence / absence of a valid capture device?

I am coding an application that uses the camera (s) of the device and this is my first time trying to use TDD seriously.

I am trying to test this feature, specifically:

func startRunning(cameraPosition: CameraPosition) -> CaptureSessionPreview?

      

        
        
        
      

    

where CameraPosition

is the enum (front and back) and CaptureSessionPreview

is the protocol that defines the object / structure on which the video slot is displayed (for example AVCaptureVideoPreviewLayer

).

I would like to write tests to determine what happens when there is or is not a specific camera (front or back).

I am using a mock AVCaptureDevice

that I can initialize with a position and inject to check for the presence or absence of a device, but I also want to check that I can actually start AVCaptureSession

with (otherwise I should still get an error).

I'm trying to start init AVCaptureDeviceInput

with a valid layout (it always returns nil for me). Is this even the best way to test it? Help?