订阅代码如下 ：

override func viewDidLoad() {
...
query = AVQuery(className: "feedback")
query?.whereKey("channel", equalTo: self.sender.displayName)
query?.findObjectsInBackground{ [weak self] objects, error in
  guard let `self` = self else {return}
  objects?.forEach(){
    ....
  }
}

// 建立同步查询订阅
liveQuery = AVLiveQuery(query: query!)
liveQuery!.delegate = self
liveQuery!.subscribe{ (succeeded, error) in
  if succeeded {
    print(" 实时消息订阅成功")
  }else{
    print("实时消息订阅失败 \(error.localizedDescription)")
}    
...

显示订阅失败，错误码64，错误描述：host is down