Using AlamofireObjectMapper Error - swift

Question

I'm using AlamofireObjectMapper to map object from JSON,But no response retrieved.Here is my code
let articleAPI = "https://news-at.zhihu.com/api/4/news/latest"
Alamofire.request(articleAPI).responseObject {(response: DataResponse<ArticleResponse>) in
let contents = response.result.value
print((contents?.stories?[0].title)!)
the ArticleResponse object looks like the following:
import ObjectMapper
class ArticleResponse: Mappable {
var date: String?
var stories: [ArticleDetailResponse]?
var top_stories: [TopStoriesResponse]?
required init?(map: Map) {
}
func mapping(map: Map) {
date <- map["date"]
stories <- map["stories"]
top_stories <- map["top_stories"]
}
}
class ArticleDetailResponse: Mappable {
var images: [String]?
var type: Int?
var id: Int?
var ga_prefix: String?
var title: String?
var multipic: Bool?
required init?(map: Map) {
}
func mapping(map: Map) {
images <- map["images"]
type <- map["type"]
id <- map["id"]
ga_prefix <- map["ga_prefix"]
title <- map["title"]
multipic <- map["multipic"]
}
}
class TopStoriesResponse: Mappable {
var image: String?
var type: Int?
var id: Int?
var ga_prefix: String?
var title: String?
required init?(map: Map) {
}
func mapping(map: Map) {
image <- map["image"]
type <- map["type"]
id <- map["id"]
ga_prefix <- map["ga_prefix"]
title <- map["title"]
}
}
I am doing this according the AlamofireObjectMapper usage document.In addition to that,Is there anything else have to implement?