Swizzling demo implementation

SimpleSwizzler.swift

///copy the implementation of NSObject.init and exchange it with some new fake init we are going to create.
///https://nshint.io/blog/2019/04/08/testing-the-camera-on-the-simulator/
///https://pspdfkit.com/blog/2019/swizzling-in-swift/
private struct Swizzler {
  private let klass: AnyClass

  init(_ klass: AnyClass) {
      self.klass = klass
  }

  func injectNSObjectInit(into selector: Selector) {
      guard let original = class_getInstanceMethod(klass, selector),
          let swizzled = class_getInstanceMethod(klass, #selector(NSObject.init)) else {
        return
      }
    
    let types = method_getTypeEncoding(original)
    let newImp = method_getImplementation(swizzled)
    class_replaceMethod(klass, selector, newImp, types)
  }
}

final class FakeMachineReadableCodeObject: AVMetadataMachineReadableCodeObject {
  var code: String?
  var dataType: AVMetadataObject.ObjectType = .qr

  override var stringValue: String? {
    code
  }

  override var type: AVMetadataObject.ObjectType {
    dataType
  }


  @objc dynamic private convenience init(fake: String?) {
    fatalError()
  }

  private class func fake(fake: String?, type: AVMetadataObject.ObjectType = .qr) -> FakeMachineReadableCodeObject {
      let m = FakeMachineReadableCodeObject(fake: fake)
      m.code = fake
      m.dataType = type
      return m
  }

  static func createFake(code: String?, type: AVMetadataObject.ObjectType) -> FakeMachineReadableCodeObject {
    //we need to swizzle the implementation of this class's init with NSObject init because AVMetadataMachineReadableCodeObject
    //is not intializable directly and it gets its values/gets intialize at runtime only
      Swizzler(self).injectNSObjectInit(into: #selector(FakeMachineReadableCodeObject.init(fake:)))
      return fake(fake: code, type: type)
  }
}