问如何取消downloadManager发起的下载任务

EN

String url = "URL";
            DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
            request.setDescription("Some descrition");
            request.setTitle("Some title");

            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
                request.allowScanningByMediaScanner();
                request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
            }
            request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, "PDF_demo.ext");

            DownloadManager manager = (DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE);
            if (manager != null) {
                long downloadID = manager.enqueue(request);

            }