获取Bing每日一图作为登陆背景图片

　　之前使用 幻杀博客提供的必应每日图片API作为网站后台登陆页面的背景图片，一直都很稳定，但今天一大早过来 发现获取不到背景图片了，去到幻杀博客看到挂上了一个公告，因为流量使用的问题，到下月为止API使用不了。这一下，我就懵逼了，之前想偷下懒，反正也在开发阶段，没必要自己写，现在API使用不了了，咋办？没办法只有自己写了。

　　整理下思路：进入登陆界面时，判断下是否有存在今天的图片；如果存在直接返回今天的图片地址给前台；不存在 则异步去下载当天图片，同时向前推十天，一天天检查是否存在图片，存在就立刻返回此图片路径，都没有就返回默认图片路径。

private string GetWallpaper()
        {
            string wallPath = System.Web.Hosting.HostingEnvironment.MapPath("~/") + "wallpaper\\";
            string wall = "/wallpaper/20160715.jpg";
            string temp = wallPath + DateTime.Now.ToString("yyyyMMdd") + ".jpg";
            if (System.IO.File.Exists(temp))
            {
                wall = "/wallpaper/" + DateTime.Now.ToString("yyyyMMdd") + ".jpg";
            }
            else
            {
                //异步开始下载bing图片
                TaskDelegat downloadWall = Task;
                IAsyncResult asyncResult = downloadWall.BeginInvoke(temp, null, null);
                asyncResult.AsyncWaitHandle.WaitOne(0);

                for (int i = 1; i <= 11; i++)
                {
                    temp = wallPath + DateTime.Now.AddDays(0 - i).ToString("yyyyMMdd") + ".jpg";
                    if (System.IO.File.Exists(temp))
                    {
                        wall = "/wallpaper/" + DateTime.Now.AddDays(0 - i).ToString("yyyyMMdd") + ".jpg";
                        break;
                    }
                }
            }
            return wall;
        }

private delegate void TaskDelegat(string path);
        private static void Task(string path)
        {
            try {
                string json = string.Empty;
                //请求bing壁纸链接
                using (var webclient = new WebClient())
                {
                    json = webclient.DownloadString("http://cn.bing.com/HPImageArchive.aspx?format=js&idx=0&n=1");
                }
                Regex urlreg = new Regex(@"http://.*?.jpg");
                string url = "";
                foreach (Match mch in urlreg.Matches(json))
                {
                    url = mch.Value.Trim();
                }
                if (!string.IsNullOrEmpty(url))
                {
                    HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);
                    req.UserAgent = "Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.57 Safari/537.36";
                    req.Headers.Add("Accept-Encoding", "gzip");
                    WebResponse res = req.GetResponse();
                    Stream resStream = res.GetResponseStream();
                    int count = (int)res.ContentLength;
                    int offset = 0;
                    byte[] buf = new byte[count];
                    while (count > 0)
                    {
                        int n = resStream.Read(buf, offset, count);
                        if (n == 0) break;
                        count -= n;
                        offset += n;
                    }
                    FileStream fs = new FileStream(path, FileMode.Create, FileAccess.Write);
                    fs.Write(buf, 0, buf.Length);
                    fs.Flush();
                    fs.Close();
                }
            }
            catch(Exception ex)
            {
                throw ex;
            }
        }