错误：在DownloadManager中找不到符号方法getSystemService（String）

Question

我在asynctask中使用了downloadmanager类，因为我想异步下载listview中的文件。我按照链接中的代码

进行了操作

Show Download progress inside activity using DownloadManager

我把那些代码放在我的AsyncTask中的doInBackground中。

 protected String doInBackground(String... f_url) {
         DownloadManager.Request request = new DownloadManager.Request(Uri.parse(file_url));

         request.setDescription("Testando");
         request.setTitle("Download");
         request.allowScanningByMediaScanner();
         request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
         request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "teste.zip");

          final DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);

         final long downloadId = manager.enqueue(request);

         new Thread(new Runnable() {

             @Override
             public void run() {

                 boolean downloading = true;

                 while (downloading) {

                     DownloadManager.Query q = new DownloadManager.Query();
                     q.setFilterById(downloadId);

                     Cursor cursor = manager.query(q);
                     cursor.moveToFirst();
                     int bytes_downloaded = cursor.getInt(cursor
                             .getColumnIndex(DownloadManager.COLUMN_BYTES_DOWNLOADED_SO_FAR));
                     int bytes_total = cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_TOTAL_SIZE_BYTES));

                     if (cursor.getInt(cursor.getColumnIndex(DownloadManager.COLUMN_STATUS)) == DownloadManager.STATUS_SUCCESSFUL) {
                         downloading = false;
                     }

                     final int dl_progress = (bytes_downloaded *1001 ) / bytes_total;
                     runOnUiThread(new Runnable() {

                         @Override
                         public void run() {
                             publishProgress((int)dl_progress);
                             // mProgressBar.setProgress((int) dl_progress);

                         }
                     });
                 }

             }
         }).start();
         return null;
     }

但我继续收到以下错误

Error:(53, 61) error: cannot find symbol method getSystemService(String)

在这一行

final DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);

我尝试通过此链接解决它

Getting a cannot find symbol : method getSystemService from ant

但我不知道如何在我的上下文中使用它。请帮忙