CreateOfferSDPAndSendIt.swift

func offer(completion: @escaping (_ sdp: RTCSessionDescription) -> Void) {
    let constrains = RTCMediaConstraints(
      mandatoryConstraints: self.mediaConstrains,
      optionalConstraints: nil)
    self.peerConnection!.offer(for: constrains) { (sdp, error) in
      guard let sdp = sdp else {
        return
      }
      
      self.peerConnection!.setLocalDescription(sdp, completionHandler: { (error) in
        completion(sdp)
      })
    }
  }
  
  func send(sdp rtcSdp: RTCSessionDescription, to person: String) {
    do {
      let dataMessage = try self.encoder.encode(SessionDescription(from: rtcSdp))
      let dict = try JSONSerialization.jsonObject(with: dataMessage, options: .allowFragments) as! [String: Any]
      Firestore.firestore().collection(person).document("sdp").setData(dict) { (err) in
        if let err = err {
          print("Error send sdp: \(err)")
        } else {
          print("Sdp sent!")
        }
      }
    }
    catch {
      debugPrint("Warning: Could not encode sdp: \(error)")
    }
  }