UIImage刷新swift3

问题是,它加载图像但不刷新,除非我将viewController更改为其他

DispatchQueue.main.async {
           self.getDataFromUrl(thumbURL,completion: { (data) in
                        let image = UIImage(data: data!)
                        self.cityImageView.image = image
                        print("img refreshed")
              })
           }

 func getDataFromUrl(_ url:string,completion: @escaping ((_ data: Data?) -> Void)) {
        URLSession.shared.dataTask(with: URL(String: url)!,completionHandler: { (data,response,error) in
            if let newData = data {
                completion(newData)
            }
        }) .resume()
    }

所以我得到一个打印出来的img刷新但在前端没有任何变化,我缺少什么？