检查网络连接状态。

首先仍是用懒汉模式来创建Network这个工具对象。

其次，validNetwork的方法，用到connectivityManager和activeNetInfo这2个原生控件。

public class Network {

    private ConnectivityManager connectivityManager;
    private NetworkInfo activeNetInfo;
    private static Context context;

    // 饿汉式
    private static Network instance = new Network();

    private Network(){}

    public static Network newInstance(Context context){
        context = context;
        return instance;
    }

    /*
    *    check network permission whether is approved
    *    @return Success: [String]connect successfully        Failed: [String] Failed to connect
    */
    private String message;
    public String validNetwork() {
        connectivityManager = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
        activeNetInfo = connectivityManager.getActiveNetworkInfo();
        if (activeNetInfo != null && activeNetInfo.getType() == ConnectivityManager.TYPE_WIFI) {
            message = "己連接網絡";
        } else {
            message = "未連接網絡";
        }
        return message;
    }

}