return array of object with Alamofire

In my app I am using AlamofireObjectMapper. I want to make a method that returns an array of objects. With the help of Alamofire I made a GET request, which gives the response as responseArray. With using void function array listOfType always has values. But when I use non-void function that should return array of object MedicineType, array listOfType is nil. So here is my code.

func getAll() -> [MedicineType] {
  var listOfTypes: [MedicineType]?;
  Alamofire.request(BASE_URL, method:.get)
      .responseArray(keyPath:"value") {(response: DataResponse<[MedicineType]>) in
         if let status = response.response?.statusCode {
             switch(status) {
                case 200:
                    guard response.result.isSuccess else {
                    //error handling
                       return
                    }
                    listOfTypes = response.result.value;
                default:
                    log.error("Error", status);
              }
           }
        }
   return listOfTypes!;
}