静音推送仅在第二次推送到达时才正常工作（后台应用程序）

当应用程序在前台运行时，静默推送通知可以正常工作

2016-12-26 15:06:17.793051 App[] didReceive in BG!!!
2016-12-26 15:06:17.932569 App[] Connected!
2016-12-26 15:06:17.936581 App[] Subscribed!
2016-12-26 15:06:17.938355 App[] received:Hello World , in:/topic/state

当应用在后台运行时，它将卡住

2016-12-26 15:06:22.674577 App[] applicationWillResignActive
2016-12-26 15:06:23.228441 App[] applicationDidEnterBackground

第一个无声推送到达，它停留在这里：

2016-12-26 15:06:30.642825 App[] didReceive in BG!!! //first push code
2016-12-26 15:06:31.842432 App[] Connected!  //first push code
2016-12-26 15:06:31.843643 App[] Subscribed!  //first push code

当第二次无声推送到达时，它显示第一个推送结果。并再次卡住.....

2016-12-26 15:06:41.713718 App[] received:Hello World , in:/topic/state  //first push result
2016-12-26 15:06:41.740015 App[] didReceive in BG!!!  //second push
2016-12-26 15:06:42.911593 App[] Connected!  //second push
2016-12-26 15:06:42.913403 App[] Subscribed!  //second push

我在Appdelegate中的实现代码

func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable : Any], fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
    NSLog("didReceive in BG!!!" )
    application.delegate = self
    mqttSession = MQTTSession(host: "192.168.159.110", port: 1883, clientID: "swift", cleanSession: true, keepAlive: 5, useSSL: false)
    mqttSession.username = "bucky"
    mqttSession.password = "bucky"
    mqttSession.delegate = self

        mqttSession.connect { (succeeded, error) -> Void in
            if succeeded {
                NSLog("Connected!")

            }
        }
    completionHandler(.newData)
}

MQTT委托方法

func mqttDidReceive(message data: Data, in topic: String, from session: MQTTSession) {
    let stringData = String(data: data, encoding: .utf8)
    NSLog("received:%@ , in:%@", stringData ?? "12345", topic)                        
    }
}