网络不可用时:
if (Application.internetReachability== NetworkReachability.NotReachable) //Do Something
使用wifi时:
if (Application.internetReachability == NetworkReachability.ReachableViaLocalAreaNetwork) //Do Something
使用移动网络时:
if (Application.internetReachability == NetworkReachability.ReachableViaCarrierDataNetwork) //Do Something