AVCaptureSession+Rx.swift

import Foundation
import AVFoundation
import RxSwift
import RxCocoa

extension AVCaptureSession {
  var rx_runnning: AnyObserver<Bool> {
    return RxBindingObserver(element: self) { element, value in
      if value {
        element.startRunning()
      } else {
        element.stopRunning()
      }
    }.asObserver()
  }
}

RxBindingObserver.swift

import Foundation
import RxSwift

class RxBindingObserver<Element: AnyObject, E> : ObserverType {

  private weak var _element: Element?
  private let _binding: (Element, E) -> Void
  
  init(element: Element, binding: (Element, E) -> Void) {
    _element = element
    _binding = binding
  }
    
  func on(event: Event<E>) {
    switch event {
    case .Next(let value):
      if let element = _element {
        _binding(element, value)
      }
    default:
      break
    }
  }
}