func add()

in firestore/FirestoreSwiftUIExample/ViewModels/RestaurantViewModel.swift [38:80]
37 lines of code
4 McCabe index (conditional complexity)

  func add(review: Review) {
    db.runTransaction({ (transaction, errorPointer) -> Any? in
      let restaurantRef = self.restaurant.reference!
      let restaurantDocument: DocumentSnapshot
      do {
        try restaurantDocument = transaction.getDocument(restaurantRef)
      } catch let fetchError as NSError {
        errorPointer?.pointee = fetchError
        return nil
      }

      guard let ratingCount = restaurantDocument.data()?["numRatings"] as? Int else {
        errorPointer?.pointee = self.getNSError(document: restaurantDocument)
        return nil
      }

      guard let averageRating = restaurantDocument.data()?["avgRating"] as? Float else {
        errorPointer?.pointee = self.getNSError(document: restaurantDocument)
        return nil
      }

      let newAverage = (Float(ratingCount) * averageRating + Float(review.rating))
        / Float(ratingCount + 1)

      transaction.setData([
        "numRatings": ratingCount + 1,
        "avgRating": newAverage,
      ], forDocument: restaurantRef, merge: true)

      let reviewDocument = self.restaurant.ratingsCollection!.document()
      do {
        _ = try transaction.setData(from: review, forDocument: reviewDocument)
      } catch {
        fatalError("Unable to add review: \(error.localizedDescription).")
      }

      return nil
    }) { object, error in
      if let error = error {
        print("Transaction failed: \(error)")
      }
    }
  }