socket 在应用进入后台后收不到消息

https://www.cnblogs.com/chengxiaoyu/articles/4968696.html
https://stackoverflow.com/questions/8748874/gcdasyncsocket-background-voip
https://www.jianshu.com/p/bb978ca08c7c
https://stackoverflow.com/questions/8748874/gcdasyncsocket-background-voip
https://www.cnblogs.com/liuhuakun/p/6704762.html
https://blog.csdn.net/jeffasd/article/details/53321446
https://blog.csdn.net/zhangkongzhongyun/article/details/38678137