使用Reachability来查看本机的网络状态.
github地址: Reachability.swift

//declare this property where it won't go out of scope relative to your listener
let reachability = Reachability()!

reachability.whenReachable = { reachability in
    // this is called on a background thread, but UI updates must
    // be on the main thread, like this:
    dispatch_async(dispatch_get_main_queue()) {
        if reachability.isReachableViaWiFi() {
            print("Reachable via WiFi")
        } else {
            print("Reachable via Cellular")
        }
    }
}
reachability.whenUnreachable = { reachability in
    // this is called on a background thread, but UI updates must
    // be on the main thread, like this:
    dispatch_async(dispatch_get_main_queue()) {
        print("Not reachable")
    }
}

do {
    try reachability.startNotifier()
} catch {
    print("Unable to start notifier")
}