Mock example for AVFoundation code. This will allow to pass in default values and to isolate testing for specific interfaces. In this example, it mocks the AVCaptureSession through the SessionCapturable.

Mock+AVFoundation.swift

import Foundation
import AVFoundation

enum CameraPosition {
	case front
	case back
}

// interface with functions that our app uses for AVCaptureSession
protocol SessionCapturable {
	func canAddInput(_ input: AVCaptureInput) -> Bool
	func addInput(_ input: AVCaptureInput)
	func canAddOutput(_ output: AVCaptureOutput) -> Bool
	func addOutput(_ output: AVCaptureOutput)
	func startRunning()
}

// All the functions in SessionCapturable are in AVCaptureSession.
extension AVCaptureSession: SessionCapturable {}

// to be adopted by a component that managed the camera. Can be used to mock as well. Ideally, it's better to test out the real implementation with mocked values, hence why captureSession is a SessionCapturable type, allowing to have the real AVCaptureSession or anything else that conforms to it.
protocol CameraRepresentable {

	var captureSession: SessionCapturable? { get }

	var currentCameraPosition: CameraPosition? { get }

	var frontCamera: AVCaptureDevice? { get }
	var frontCameraInput: AVCaptureDeviceInput? { get }

	var photoOutput: AVCapturePhotoOutput? { get }

	var rearCamera: AVCaptureDevice? { get }
	var rearCameraInput: AVCaptureDeviceInput? { get }

}

final class MockAVCaptureSession: SessionCapturable {

	let canAddInputMockValue: Bool
	let canAddOutputMockValue: Bool

    var canAddInputWasCalled: Bool  = false
    var addInputWasCalled: Bool     = false
    var canAddOutputWasCalled: Bool = false
    var addOutputWasCalled: Bool    = false
	var startRunningWasCalled: Bool = false

	init(input: Bool, output: Bool) {
		self.canAddInputMockValue = input
		self.canAddOutputMockValue = output
	}

	func canAddInput(_ input: AVCaptureInput) -> Bool {
		canAddInputWasCalled.toggle()
		return canAddInputMockValue
	}

	func addInput(_ input: AVCaptureInput) {
		canAddInputWasCalled.toggle()
	}

	func canAddOutput(_ output: AVCaptureOutput) -> Bool {
		canAddOutputWasCalled.toggle()
		return canAddOutputMockValue
	}

	func addOutput(_ output: AVCaptureOutput) {
		addOutputWasCalled.toggle()
	}

	func startRunning() {
		startRunningWasCalled.toggle()
	}
	
}